# C++ Programming/Compiler/Linker/Libraries
## Libraries

Libraries allow existing code to be reused in a program. Libraries are like programs except that instead of relying on main to do the work you call the specific functions provided by the library to do the work. Functions provide the interface between the program being written and the library being used. This interface is called Application Programming Interface or API.
Libraries should and tend to be domain specific as to permit greater mobility across applications, and provide extended specialization. Libraries that are not, are often header only distribution, intended for static linking as to permit the compiler and the application, only to use the needed bits of code.

### What is an API?

To a programmer, an operating system is defined by its API. API stands for Application Programming Interface. An API encompasses all the function calls that an application program can communicate with the hardware or the operating system, or any other application that provides a set of interfaces to the programmer (i.e.: a library), as well as definitions of associated data types and structures. Most APIs are defined on the application Software Development Kit (SDK) for program development.
In simple terms the API can be considered as the interface through which the user (or user programs) will be able interact with the operating system, hardware or other programs to make them to perform a task that may also result in obtaining a result message.

### Can an API be called a framework?

No, a framework may provide an API, but a framework is more than a simple API. By default a framework also defines how the code is written, it is a set of solutions, even classes, that as a group addresses the handling of a limited set of related problems and provides not only an API but a default functionality, well designed frameworks enable its interchangeability for a similar framework, striving to provides the same API.
As seen in the File organization Section, compiled libraries consists in C++ headers files that are included by the preprocessor and binary library files which are used by the linker to generate the resulting compilation. For a dynamically linked library, only the loading code is added to the compilation that uses them, the actual loading of the library is done in the memory at run-time.
Programs can make use of libraries in two forms, as static or dynamic depending on how the programmer decides to distribute its code or even due to the licensing used by third party libraries, the static and dynamic libraries section of this book will cover in depth this subject.

### Third party libraries

Additional functionality that goes beyond the standard libraries (like garbage collection) are available (often free) by third party libraries, but remember that third party libraries do not necessarily provide the same ubiquitous cross-platform functionality or an API style conformant with as standard libraries. The main motivation for their existence is to avoid having to reinvent the wheel and to make efforts converge; too much energy has been spent by generations of programmers to write safe and "portable" code.
There are several libraries a programmer is expected to know about or have at least a passing idea of what they are. Time, consistency and extended references will make a few libraries pop-out from the rest. One notable example is the highly respected collection of Boost libraries that we will examine ahead.

#### Licensing on third party libraries

The programmer may also be limited by the requirements of the license used on external libraries that he has no direct control, for instance the use of the GNU General Public License (GNU GPL) code in closed source applications isn't permitted to address this issue the FSF provides an alternative in the form of the GNU LGPL license that permits such uses but only in the dynamically linked form, this is mirrored by several other legal requirements a programmer must attend and comply to.
Libraries come in two forms, either in source form or in compiled/binary form. Libraries in source-form must first be compiled before they can be included in another project. This will transform the libraries' cpp-files into a lib-file. If a program must be recompiled to run with a new version of a library, but does not need any further changes, the library is said to be source compatible. If a program does not need to be modified and recompiled to use a new version of a library, the library is then classified as being binary compatible.

### Static and Dynamic Libraries

Advantages of using static binaries:
* Simplification of program distribution (fewer files).
* Code simplification (no version checks as required in dynamic libraries).
* Will only compile the code that is used.
Disadvantages of using static binaries:
* Waste of resources: Generates larger binaries, since the library is compiled into the executable. Wastes memory as the library cannot be shared (in memory) between processes (depending on the operating system).
* Program will not benefit from bug fixes or extensions in the libraries without being recompiled.

#### Binary/Source Compatibility of libraries

A library is said to be binary compatible if the program that dynamically links to an earlier version of that library, continues to work using another versions of the same library. If a recompilation of the program is needed for it to run with each new version the library is said to be source compatible.
Producing binary compatible libraries is beneficial for distribution but harder to maintain by the programmer. It is often seen as a better solution to do static linking, if the library is only source compatible, since it will not cause problems to the end-user.
Binary compatibility saves a lot of trouble and is a signal that the library reached a status of stability. It makes it easier to distribute software for a certain platform. Without ensuring binary compatibility between releases, people will be forced to offer statically linked binaries.

#### header-only libraries

Another distinction that is commonly made about libraries are on how they are distributed (regarding structure and use). A library that is contained only on header files is considered header-only library. Often this means that they are simpler and easy to use, however this will not be the ideal solution for complex code, it will not only hamper readability but result in larger compile times. Also depending on the compiler and it's optimizing capabilities (or options) can, due to the resulting inlining, generate larger binaries. This may not be as important in libraries mostly implemented with templates. Header-only libraries will always contain the source code to the implementation, commercial is rare.

### Example: Configuring MS Visual C++ to use external libraries

The Boost library is used as example library.
Considering you already have decompressed and have the binary part of the Boost library built. There the steps which have to be performed:

#### Include directory

Set up the include directory. This is the directory that contains the header files (.h/hpp), which describes the library interface:

#### Library directory

Set up the library directory. This is the directory that contains the pre-compiled library files (.lib):

#### Library files

Enter library filenames in additional dependencies for the libraries to use:
Some libraries (such as e.g. Boost) use auto-linking to automate the process of selecting library files for linking, based on which header-files are included. Manual selection of library filenames are not required for such libraries if your compiler supports auto-linking.

#### Dynamic libraries

In case of dynamically loaded (.dll) libraries, one also has to place the DLL-files either in the same folder as the executable, or in the system PATH.

#### Run-time library

The libraries also have to be compiled with the same run-time library as the one used in your project. Many libraries therefore come in different editions, depending on whether they are compiled for single- or multi-threaded runtime and debug or release runtime, as well as whether they contain debug symbols or not.