neulab/tldr huggingface dataset:
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior).  This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions.
-fpermitted-flt-eval-methods=style ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type.  These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of "FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written expecting the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11.  The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.
-aux-info filename Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files.  This option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character).  In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.
-fallow-parameterless-variadic-functions Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments.  This is only supported for C as this construct is allowed by C++.
-fno-asm Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers.  You can use the keywords "__asm__", "__inline__" and "__typeof__" instead.  -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard keywords.  You may want to use the -fno-gnu-keywords flag instead, which has the same effect.  In C99 mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99.
-fno-builtin -fno-builtin-function Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy loops.  The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.  In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function.  For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled.  function must not begin with __builtin_.  If a function is named that is not built-in in this version of GCC, this option is ignored.  There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n)          __builtin_abs ((n)) #define strcpy(d, s)    __builtin_strcpy ((d), (s))
-fgimple Enable parsing of function definitions marked with "__GIMPLE".  This is an experimental feature that allows unit testing of GIMPLE passes.
-fhosted Assert that compilation targets a hosted environment.  This implies -fbuiltin.  A hosted environment is one in which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly everything except a kernel.  This is equivalent to -fno-freestanding.
-ffreestanding Assert that compilation targets a freestanding environment.  This implies -fno-builtin.  A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at "main".  The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fopenacc Enable handling of OpenACC directives "#pragma acc" in C/C++ and "!$acc" in Fortran.  When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>.  This option implies -pthread, and thus is only supported on targets that have support for -pthread.
-fopenacc-dim=geom Specify default compute dimensions for parallel offload regions that do not explicitly specify.  The geom value is a triple of ':'-separated sizes, in order 'gang', 'worker' and, 'vector'.  A size can be omitted, to use a target-specific default value.
-fopenmp Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran.  When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>.  This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd.
-fopenmp-simd Enable handling of OpenMP's SIMD directives with "#pragma omp" in C/C++ and "!$omp" in Fortran. Other OpenMP directives are ignored.
-fgnu-tm When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009).  This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes.  Please note that not all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non- call exceptions (-fnon-call-exceptions).
-fms-extensions Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW; struct ABC { UOW UOW; };
Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets except for x86 targets using ms-abi.
-fplan9-extensions Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef.    This is only supported for C, not C++.
-fcond-mismatch Allow conditional expressions with mismatched types in the second and third arguments.  The value of such an expression is void.  This option is not supported for C++.
-flax-vector-conversions Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types.  This option should not be used for new code.
-funsigned-char Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be.  It is either like "unsigned char" by default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the signedness of an object.  But many programs have been written to use plain "char" and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for.  This option, and its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its behavior is always just like one of those two.
-fsigned-char Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char.  Likewise, the option -fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields -funsigned-bitfields -fno-signed-bitfields -fno-unsigned-bitfields These options control whether a bit-field is signed or unsigned, when the declaration does not use either "signed" or "unsigned".  By default, such a bit-field is signed, because this is consistent: the basic integer types such as "int" are signed types.
-fsso-struct=endianness Set the default scalar storage order of structures and unions to the specified endianness.  The accepted values are big-endian, little- endian and native for the native endianness of the target (the default).  This option is not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target.
Options Controlling C++ Dialect This section describes the command-line options that are only meaningful for C++ programs.  You can also use most of the GNU compiler options regardless of what language your program is in.  For example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Some options for compiling C programs, such as -std, are also relevant for C++ programs.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n Use version n of the C++ ABI.  The default is version 0.
Version 0 refers to the version conforming most closely to the C++ ABI specification.  Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names.  For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence.  It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors.  It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.
Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
Version 15, which first appeared in G++ 11, changes the mangling of "__alignof__" to be distinct from that of "alignof", and dependent operator names.
See also -Wabi.
-fabi-compat-version=n On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name.  This switch specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility).  If another ABI version is explicitly selected, this defaults to 0.  For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases.  If this option is provided along with -Wabi (without the version), the version from this option is used for the warning.
-fno-access-control Turn off all access checking.  This switch is mainly useful for working around bugs in the access control code.
-faligned-new Enable support for C++17 "new" of types that require more alignment than "void* ::operator new(std::size_t)" provides.  A numeric argument such as "-faligned-new=32" can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of "alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
-fchar8_t -fno-char8_t Enable support for "char8_t" as adopted for C++20.  This includes the addition of a new "char8_t" fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new "__cpp_char8_t" and "__cpp_lib_char8_t" feature test macros.
This option enables functions to be overloaded for ordinary and UTF-8 strings:
int f(const char *);    // #1 int f(const char8_t *); // #2 int v1 = f("text");     // Calls #1 int v2 = f(u8"text");   // Calls #2
and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t); int v3 = u8'x'_udl1; int operator""_udl2(const char8_t*, std::size_t); int v4 = u8"text"_udl2; template<typename T, T...> int operator""_udl3(); int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards.  For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified.
char ca[] = u8"xx";     // error: char-array initialized from wide //        string const char *cp = u8"xx";// error: invalid conversion from //        `const char8_t*' to `const char*' int f(const char*); auto v = f(u8"xx");     // error: invalid conversion from //        `const char8_t*' to `const char*' std::string s{u8"xx"};  // error: no matching function for call to //        `std::basic_string<char>::basic_string()' using namespace std::literals; s = u8"xx"s;            // error: conversion from //        `basic_string<char8_t>' to non-scalar //        type `basic_string<char>' requested
-fcheck-new Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated.  This check is normally unnecessary because the C++ standard specifies that "operator new" only returns 0 if it is declared "throw()", in which case the compiler always checks the return value even without this option.  In all other cases, when "operator new" has a non-empty exception specification, memory exhaustion is signalled by throwing "std::bad_alloc".  See also new (nothrow).
-fconcepts -fconcepts-ts Below -std=c++20, -fconcepts enables support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015).
With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on.  But the standard specification of Concepts differs significantly from the TS, so some constructs that were allowed in the TS but didn't make it into the standard can still be enabled by -fconcepts-ts.
-fconstexpr-depth=n Set the maximum nested evaluation depth for C++11 constexpr functions to n.  A limit is needed to detect endless recursion during constant expression evaluation.  The minimum specified by the standard is 512.
-fconstexpr-cache-depth=n Set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n.  This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations).  The default is 8.  Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you.
-fconstexpr-loop-limit=n Set the maximum number of iterations for a loop in C++14 constexpr functions to n.  A limit is needed to detect infinite loops during constant expression evaluation.  The default is 262144 (1<<18).
-fconstexpr-ops-limit=n Set the maximum number of operations during a single constexpr evaluation.  Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long.  The default is 33554432 (1<<25).
-fcoroutines Enable support for the C++ coroutines extension (experimental).
-fno-elide-constructors The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type.  Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.  This option also causes G++ to call trivial member functions which otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.
-fno-enforce-eh-specs Don't generate code to check for violation of exception specifications at run time.  This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining "NDEBUG".  This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.
-fextern-tls-init -fno-extern-tls-init The C++11 and OpenMP standards allow "thread_local" and "threadprivate" variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization.  When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn't actually need dynamic initialization.  If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is -fextern-tls-init.  On targets that do not support symbol aliases, the default is -fno-extern-tls-init.
-fno-gnu-keywords Do not recognize "typeof" as a keyword, so that code can use this word as an identifier.  You can use the keyword "__typeof__" instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
-fno-implicit-templates Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time.
-fno-implicit-inline-templates Don't emit code for implicit instantiations of inline templates, either.  The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.
-fno-implement-inlines To save space, do not emit out-of-line copies of inline functions controlled by "#pragma implementation".  This causes linker errors if these functions are not inlined everywhere they are called.
-fmodules-ts -fno-modules-ts Enable support for C++20 modules   The -fno-modules-ts is usually not needed, as that is the default.  Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version.
-fmodule-header -fmodule-header=user -fmodule-header=system Compile a header file to create an importable header unit.
-fmodule-implicit-inline Member functions defined in their class definitions are not implicitly inline for modular code.  This is different to traditional C++ behavior, for good reasons.  However, it may result in a difficulty during code porting.  This option makes such function definitions implicitly inline.  It does however generate an ABI incompatibility, so you must use it everywhere or nowhere.  (Such definitions outside of a named module remain implicitly inline, regardless.)
-fno-module-lazy Disable lazy module importing and module mapper creation.
-fmodule-mapper=[hostname]:port[?ident] -fmodule-mapper=|program[?ident] args... -fmodule-mapper==socket[?ident] -fmodule-mapper=<>[inout][?ident] -fmodule-mapper=<in>out[?ident] -fmodule-mapper=file[?ident] An oracle to query for module name to filename mappings.  If unspecified the CXX_MODULE_MAPPER environment variable is used, and if that is unset, an in-process default is provided.
-fmodule-only Only emit the Compiled Module Interface, inhibiting any object file.
-fms-extensions Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non- standard syntax.
-fnew-inheriting-ctors Enable the P0136 adjustment to the semantics of C++11 constructor inheritance.  This is part of C++17 but also considered to be a Defect Report against C++11 and C++14.  This flag is enabled by default unless -fabi-version=10 or lower is specified.
-fnew-ttp-matching Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters.  This flag is enabled by default for -std=c++17.
-fno-nonansi-builtins Disable built-in declarations of functions that are not mandated by ANSI/ISO C.  These include "ffs", "alloca", "_exit", "index", "bzero", "conjf", and other related functions.
-fnothrow-opt Treat a "throw()" exception specification as if it were a "noexcept" specification to reduce or eliminate the text size overhead relative to a function with no exception specification.  If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away.  The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to "terminate" rather than "unexpected".
-fno-operator-names Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags Disable diagnostics that the standard says a compiler does not need to issue.  Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive Downgrade some diagnostics about nonconformant code from errors to warnings.  Thus, using -fpermissive allows some nonconforming code to compile.
-fno-pretty-templates When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. "void f(T) [with T = int]" rather than "void f(int)") so that it's clear which template is involved.  When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template.  If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them.
-fno-rtti Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features ("dynamic_cast" and "typeid").  If you don't use those parts of the language, you can save some space by using this flag.  Note that exception handling uses the same information, but G++ generates it as needed. The "dynamic_cast" operator can still be used for casts that do not require run-time type information, i.e. casts to "void *" or to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with -fno-rtti may not work.  For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti.
-fsized-deallocation Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14.  This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster.  Enabled by default under -std=c++14 and above.  The flag -Wsized-deallocation warns about places that might want to add a definition.
-fstrict-enums Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators).  This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.
-fstrong-eval-order Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17.  Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17.
-ftemplate-backtrace-limit=n Set the maximum number of template instantiation notes for a single warning or error to n.  The default value is 10.
-ftemplate-depth=n Set the maximum instantiation depth for template classes to n.  A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation.  ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11).  The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.
-fno-threadsafe-statics Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics.  You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
-fuse-cxa-atexit Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit" function.  This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr Don't use the "__cxa_get_exception_ptr" runtime routine.  This causes "std::uncaught_exception" to be incorrect, but is necessary if the runtime routine is not available.
-fvisibility-inlines-hidden This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility ("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO.  Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method.  For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility.  Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary.
-fvisibility-ms-compat This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1.  It sets the default visibility to "hidden", like -fvisibility=hidden.
2.  Types, but not their members, are not hidden by default.
3.  The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.
In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible.  Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal.  When this flag is given, it is a violation of the ODR to define types with the same name differently.
-fno-weak Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available.  This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits.  This option may be removed in a future release of G++.
-fext-numeric-literals (C++ and Objective-C++ only) Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions.  When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14.  This option is off by default for ISO C++11 onwards (-std=c++11, ...).
-nostdinc++ Do not search for header files in the standard directories specific to C++, but do still search the other standard directories.  (This option is used when building the C++ library.)
-flang-info-include-translate -flang-info-include-translate-not -flang-info-include-translate=header Inform of include translation events.  The first will note accepted include translations, the second will note declined include translations.  The header form will inform of include translations relating to that specific header.  If header is of the form "user" or "<system>" it will be resolved to a specific user or system header using the include path.
-flang-info-module-cmi -flang-info-module-cmi=module Inform of Compiled Module Interface pathnames.  The first will note all read CMI pathnames.  The module form will not reading a specific module's CMI.  module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in "<>" or "").
-stdlib=libstdc++,libc++ When G++ is configured to support this option, it allows specification of alternate C++ runtime libraries.  Two options are available: libstdc++ (the default, native C++ runtime for G++) and libc++ which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards).  The option switches G++ to use the headers from the specified library and to emit "-lstdc++" or "-lc++" respectively, when a C++ runtime is required for linking.
In addition, these warning options have meanings only for C++ programs:
-Wabi-tag (C++ and Objective-C++ only) Warn when a type with an ABI tag is used in a context that does not have that ABI tag.  See C++ Attributes for more information about ABI tags.
-Wcomma-subscript (C++ and Objective-C++ only) Warn about uses of a comma expression within a subscripting expression.  This usage was deprecated in C++20.  However, a comma expression wrapped in "( )" is not deprecated.  Example:
void f(int *a, int b, int c) { a[b,c];     // deprecated a[(b,c)];   // OK }
Enabled by default with -std=c++20.
-Wctad-maybe-unsupported (C++ and Objective-C++ only) Warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides.  This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended.  Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD.  This warning can be suppressed with the following pattern:
struct allow_ctad_t; // any name works template <typename T> struct S { S(T) { } }; S(allow_ctad_t) -> S<void>; // guide with incomplete parameter type will never be considered
-Wctor-dtor-privacy (C++ and Objective-C++ only) Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.  Also warn if there are no non- private methods, and there's at least one private member function that isn't a constructor or destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only) Warn when "delete" is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor.  This warning is enabled by -Wall.
-Wdeprecated-copy (C++ and Objective-C++ only) Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up.  This warning is enabled by -Wextra.  With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor.
-Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type.  This conversion was deprecated in C++20.  For example:
enum E1 { e }; enum E2 { f }; int k = f - e;
-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20.  In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion.
-Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only) Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type.  This conversion was deprecated in C++20.  For example:
enum E1 { e }; enum E2 { f }; bool b = e <= 3.7;
-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20.  In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion.
-Wno-init-list-lifetime (C++ and Objective-C++ only) Do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers.  Since the underlying array for an "initializer_list" is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime.  For example:
*   If a function returns a temporary "initializer_list", or a local "initializer_list" variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
*   If a new-expression creates an "initializer_list", the array only lives until the end of the enclosing full-expression, so the "initializer_list" in the heap has a dangling pointer.
*   When an "initializer_list" variable is assigned from a brace- enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the "initializer_list" variable has a dangling pointer.
// li's initial underlying array lives as long as li std::initializer_list<int> li = { 1,2,3 }; // assignment changes li to point to a temporary array li = { 4, 5 }; // now the temporary is gone and li has a dangling pointer int i = li.begin()[0] // undefined behavior
*   When a list constructor stores the "begin" pointer from the "initializer_list" argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary "initializer_list", the pointer is left dangling by the end of the variable declaration statement.
-Winvalid-imported-macros Verify all imported macro definitions are valid at the end of compilation.  This is not enabled by default, as it requires additional processing to determine.  It may be useful when preparing sets of header-units to ensure consistent macros.
-Wno-literal-suffix (C++ and Objective-C++ only) Do not warn when a string or character literal is followed by a ud- suffix which does not begin with an underscore.  As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from "<inttypes.h>".  For example:
#define __STDC_FORMAT_MACROS #include <inttypes.h> #include <stdio.h>
int main() { int64_t i64 = 123; printf("My int64: %" PRId64"\n", i64); }
In this case, "PRId64" is treated as a separate preprocessing token.
This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.
These warnings are enabled by default.
-Wno-narrowing (C++ and Objective-C++ only) For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard.  A narrowing conversion from a constant produces an error, and a narrowing conversion from a non- constant produces a warning, but -Wno-narrowing suppresses the diagnostic.  Note that this does not affect the meaning of well- formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only) Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. "throw()" or "noexcept") but is known by the compiler to never throw an exception.
-Wnoexcept-type (C++ and Objective-C++ only) Warn if the C++17 feature making "noexcept" part of a function type changes the mangled name of a symbol relative to C++14.  Enabled by -Wabi and -Wc++17-compat.
template <class T> void f(T t) { t(); }; void g() noexcept; void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls "f<void(*)()noexcept>".
-Wclass-memaccess (C++ and Objective-C++ only) Warn when the destination of a call to a raw memory function such as "memset" or "memcpy" is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers.  Modifying the representation of such objects may violate invariants maintained by member functions of the class.  For example, the call to "memset" below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed.  The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.
std::string str = "abc"; memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall.  Explicitly casting the pointer to the class object to "void *" or to a type that can be safely accessed by the raw memory function suppresses the warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only) Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified.
-Wregister (C++ and Objective-C++ only) Warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit Register Variables extension.  The use of the "register" keyword as storage class specifier has been deprecated in C++11 and removed in C++17.  Enabled by default with -std=c++17.
-Wreorder (C++ and Objective-C++ only) Warn when the order of member initializers given in the code does not match the order in which they must be executed.  For instance:
struct A { int i; int j; A(): j (0), i (1) { } };
The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the members, emitting a warning to that effect.  This warning is enabled by -Wall.
-Wno-pessimizing-move (C++ and Objective-C++ only) This warning warns when a call to "std::move" prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.
struct T { ... }; T fn() { T t; ... return std::move (t); }
But in this example, the "std::move" call prevents copy elision.
This warning is enabled by -Wall.
-Wno-redundant-move (C++ and Objective-C++ only) This warning warns about redundant calls to "std::move"; that is, when a move operation would have been performed even without the "std::move" call.  This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:
struct T { ... }; T fn(T t) { ... return std::move (t); }
Here, the "std::move" call is redundant.  Because G++ implements Core Issue 1579, another example is:
struct T { // convertible to U ... }; struct U { ... }; U fn() { T t; ... return std::move (t); }
In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue.
This warning is enabled by -Wextra.
-Wrange-loop-construct (C++ and Objective-C++ only) This warning warns when a C++ range-based for-loop is creating an unnecessary copy.  This can happen when the range declaration is not a reference, but probably should be.  For example:
struct S { char arr[128]; }; void fn () { S arr[5]; for (const auto x : arr) { ... } }
It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes.
This warning also warns when a loop variable in a range-based for- loop is initialized with a value of a different type resulting in a copy.  For example:
void fn() { int arr[10]; for (const double &x : arr) { ... } }
In the example above, in every iteration of the loop a temporary value of type "double" is created and destroyed, to which the reference "const double &" is bound.
-Wredundant-tags (C++ and Objective-C++ only) Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity.  For example:
struct foo; struct foo *p;   // warn that keyword struct can be eliminated
On the other hand, in this example there is no warning:
struct foo; void foo ();   // "hides" struct foo void bar (struct foo&);  // no warning, keyword struct is necessary
-Wno-subobject-linkage (C++ and Objective-C++ only) Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage.  If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit.  If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well.  The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions.  -Wsubobject-linkage is enabled by default.
-Weffc++ (C++ and Objective-C++ only) Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
*   Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
*   Prefer initialization to assignment in constructors.
*   Have "operator=" return a reference to *this.
*   Don't try to return a reference when you must return an object.
*   Distinguish between prefix and postfix forms of increment and decrement operators.
*   Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations.  However, the check is extended to warn about the lack of virtual destructor in accessible non- polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.
-Wno-exceptions (C++ and Objective-C++ only) Disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers.
-Wstrict-null-sentinel (C++ and Objective-C++ only) Warn about the use of an uncasted "NULL" as sentinel.  When compiling only with GCC this is a valid sentinel, as "NULL" is defined to "__null".  Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer.  But this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only) Disable warnings when non-template friend functions are declared within a template.  In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default.
-Wold-style-cast (C++ and Objective-C++ only) Warn if an old-style (C-style) cast to a non-void type is used within a C++ program.  The new-style casts ("dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast") are less vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only) Warn when a function declaration hides virtual functions from a base class.  For example, in:
struct A { virtual void f(); };