class std::string_view in C++17 - GeeksforGeeks

18 Dec, 2021
The std::string has some demerits, one of the most common situations is constant strings. Below is the program that demonstrates the problem that occurs in dealing with constant strings with std::string:
Program 1:
C++
// C++ program to demonstrate the// problem occurred in string#include <iostream>#include <string>using namespace std;  // Driver Codeint main(){    char str_1[]{ "Hello !!, GeeksforGeeks" };      string str_2{ str_1 };    string str_3{ str_2 };      // Print the string    cout << str_1 << '\n'         << str_2 << '\n'         << str_3 << '\n';      return 0;}
  Output:
Explanation: The output is the same as expected. But, in order to view “Hello !!, GeeksforGeeks” twice the std::string performs overheads on the memory twice. But here the task was to read the string (“Hello !!, GeeksforGeeks”), and no write operation is required on it. So just to display a string why assign memory multiple times. In order to deal with the strings more efficiently, C++17 proposed std::string_view() which provides the view of pre-defined char str[] without creating a new object to the memory.
Drawback of std::string: In the above example, the same string str_1 is printed twice by assigning two different string variables. So for both the variables str_2 and str_3, the memory was allocated using the static memory allocation, and that cause overhead on our memory twice.
Benefits of std::string_view: 
Light and Cheaper: The std::string_view is a very light, cheaper and is mainly used to provide the view of the string. Whenever the string_view is created there is no need to copy the string in the manner as done in the above example that was inefficient and was causing overhead on the memory. It makes the copying process of the string quite efficient and never creates any copy of the string when the modification is being made in the viewed string the made changes have appeared in the std::string_view.
Better Performance: The std::string_view is better than the const std::string& because it removes the constraint of having a std::string object at the very beginning of the string as std::string_view is composed of two elements first one is const char* that points to the starting position of the array and the second is size.
Supports crucial function: The std::string_view supports mostly all crucial function that is applied over the std::string like substr, compare, find, overloaded comparison operators (e.g., ==, <, >, !=). So in most of the cases, it removes the constraint of having a std::string object declaration when our preference is read-only.
std::string_view: C++17 library has proposed a standard type of string (std::string_view) which is different from the usual std::string. 
The std::string_view provides a lightweight object that offers read-only access to a string or a part of a string using an interface similar to the interface of std::_string and merely refers to the contiguous char sequence. Unlike std::string, which keeps its own copy of the string, It also provides a view of a string that is defined elsewhere in the source code.
it is composed of two members: a const char* that points to the start of the char array, and the _size. It is a non-owning reference to a string in itself.
It is defined in the header(#include <string_view>), and std::string_view class template is as follows:
template<class CharT, class Traits = std::char_traits<CharT>> class basic_string_view;
The class template string_view explains about an object that can refer to a constant contiguous sequence of char’s  or array of char’s -like objects with the first element of the sequence at position zero.
Below is the exact version of the above source code using std::string_view:
Program 2: 
C++
// C++ program to implement// the above approach#include <iostream>using namespace std;#include <string_view>  // Driver codeint main(){    // View the text "hello", which is    // stored in the binary    string_view str_1{ "Hello !!, GeeksforGeeks" };      // View of the same "hello"    string_view str_2{ str_1 };      // View of the same "hello"    string_view str_3{ str_2 };      std::cout << str_1 << '\n' << str_2 << '\n' << str_3 << '\n';    return 0;}
Output-
Explanation: The output would be the same as above, but no more copies of the string “Hello !!, GeeksforGeeks” are created in the Memory.
Parameters In std::string_view:
Char Type:The char type defines the type of characters that are stored in the string_view. The C++ Standard Library provides the following typedefs for specializations of the template described above.
string_view for elements of type charwstring_view, for wchar_tu16string_view for char16_tu32string_view for char32_t
string_view for elements of type char
wstring_view, for wchar_t
u16string_view for char16_t
u32string_view for char32_t
Below is the C++ program to illustrate the Char Type:
Program 3:
C++
// C++ program for the above approach#include <iostream>#include <string>#include <string_view>using namespace std;  string_view sur_name(wstring_view x){    return x.substr(6);}  // Driver Codeint main(){    // Basic_string_view<wchar_t> wstr    // (L"Madhav_Mohan"); both are equivalent    const wchar_t* str1{ L"Madhav_Mohan" };      cout << your_name(str1);      return 0;}
 Output- 
Example Of std::string_view: Below examples, show that most of the string-based functions can be used with std::string_view as used str.compare(), str.back(), str.cend(), and str.at() functions.
Program 4: 
C++
// C++ program to implement// the above approach#include <iostream>#include <string_view>using namespace std;  string to_alpha(int r){    if (r < 0)        return " less than ";    else if (r == 0)        return " equal to ";    else        return " greater than ";}  // Driver codeint main(){    char str{ "Muddy" };    char str_1{ "Maddy" };      string_view str_2{ str_1 };      // This line prints Maddy    cout << str_2 << '\n';      // Change 'M' to 'D' in arr    str_1[0] = 'D';      // This line prints Daddy    cout << str_2 << '\n';      cout << str_2.at(2);    cout << str_2.back();    cout << " is last char of str_2 is y " << (str_2.back() == 's');    cout << " is last char of str_2 is y " << boolalpha << (str_2.back() == 's');      // In above statement boolalpha is    // used to  give the boolean result    // in True or False      int cmp{ str_1.compare(str_2) };    cout << to_alpha(cmp};int cmp_1 = str.compare(1, 4, str_2);cout << "last three characters of str are" << to_alpha(cmp_1) << "str_2.\n";return 0;}
 Output: 
Modification of std::string_view: In C++20 some new functions have been added like str.remove_suffix(), str.remove_suffix() using these functions can modify the std::string_view. Like can remove the selective suffix or prefixes of the given string.
Program 5: 
C++
// C++ program to implement// the above approach#include <iostream>#include <iterator>#include <string_view>using namespace std;  // Driver codeint main(){    string_view str_1{ "Maddy" };    cout << str_1 << '\n';      string_view str_2{ "Daddy" };    cout << str_2 << '\n';      string_view var{ "Hello !!, GeeksforGeeks" };    cout << var << '\n';      cout << var.starts_with("Hi !!") << '\n';    cout << var.ends_with("GeeksforGeeks !!") << '\n';      // Remove the first characters.    str_1.remove_prefix(1);    cout << str_1 << '\n';      // Remove the last 2 characters.    str_1.remove_suffix(2);    std::cout << str_1 << '\n';      return 0;}
Output: 
Below is the example of some other operations that can be performed with std::string_view:
Program 6: 
C++
// C++ program to implement the// above approach#include <array>#include <cstddef>#include <iostream>#include <stdexcept>#include <string>#include <string_view>using namespace std;  // Driver codeint main(){    constexpr string_view str{ "Daddy" };    constexpr string_view str1{ "Maddy" };    string str2{};      // Copy data of str1 into str2    str1.copy(str2.data(), 4);      // This line prints Madddy    cout << str2.data() << '\n';      if ((str.compare(str2)) != 0)        cout << str << " "             << "doesn't = to"             << " " << str1 << '\n';    cout << str1.starts_with("Mad");    cout << '\n'         << str.starts_with("Pad") << '\n';    cout << str1.ends_with("ddt");    cout << '\n'         << str.ends_with("ddy") << '\n';      // Checks whether the given string contains    // the given substring or character    constexpr string_view s1{ "Hello! Madhav" };      // Position of the first character that    // match first    constexpr string_view str3{ "Hi! Maddy" };      size_t found = str3.find(str1);    cout << found << '\n';      // Find the last occurrence of a substring    cout << '\n'         << str3.rfind('d');      // Gives the first time occured char's position    cout << '\n'         << str3.find_first_of("d");      // Gives the last time occured char's position    cout << '\n'         << str3.find_last_of("d");      // Finds the first char that's not equal to    // any of the char(s) in the given string    cout << '\n'         << str3.find_last_not_of(str, 6);      // It givesIndex of first unmatched character    // when successful or string::npos if no such    // character found. It searches for the first    // char that doesn't match any of the char(s)    // that has given in the argument    cout << '\n'         << str3.find_last_not_of(str, 4);      return 0;}
Output:
Non-null Terminated Strings: Unlike C-strings and std::string which need a string terminator (‘\0’) at the end of a string, std::string_view doesn’t need null terminators to mark the end of the string. Because it keeps the track of the string length. 
Below is the C++ program to implement the above approach:
Program 7:
C++
// C++ program to implement// the above approach#include <iostream>  // For std::size#include <iterator>#include <string_view>using namespace std;  // Driver codeint main(){    // No null-terminator.    char name[]{ 'M', 'a', 'd',                 'h', 'a', 'v' };      // Here name isn't null-terminated.    // We need to pass the length manually.    // Because name is an array, we can    // use std::size to get its length.    string_view str_1{ \name, size(name) };      // This is safe and cout knows how to    // print std::string_views.    cout << str_1 << '\n';      return 0;}
Output: 
Conversion Of std::string_view Into std::string: Explicit conversion methods need to be used because implicit conversion won’t help here.
Below is the C++ program to implement the conversion of std::string_view to std::string:
Program 8: 
C++
// C++ program to implement// the above approach#include <iostream>#include <string>#include <string_view>using namespace std;  void print(string s){    cout << s << '\n';}  // Driver codeint main(){    string_view str_1{ "Madhav" };    str_1.remove_suffix(3);      // compile error: won't implicitly    // convert    // print(str_1);      // explicit conversion    string str_2{ str_1 };      print(str_2);      // It'll work    print(static_cast<string>(str_1));      return 0;}
Output:
 Conversion Of std::string_view To C-style String: Functions such as strlen(), these functions need a C-style string in order to be used. So, whenever there is a need to convert a std::string_view to a C-style string, it can be done by first converting to a std::string.
Below is the C++ program to implement the above approach:
Program 9:
C++
// C++ program to implement// the above approach#include <cstring>#include <iostream>#include <string>#include <string_view>  // Driver codeint main(){    string_view str_1{ "Madhav" };    str_1.remove_suffix(3);      // Create a std::string from    // the std::string_view    string str_2{ str_1 };      // A Null_terminated C-style string.    auto Null_Terminated{ str_2.c_str() };      // Pass the null-terminated string    // to the function that we want to use.    cout << str_2 << " has " << strlen(Null_Terminated) << " letter's\n";      return 0;}
Output:
 Scoping Issues With std::string_view: The std::string_view provides a view, so it remains independent of a string of which it provides a view, and it projects the view of the string unless the viewed string is in scope but in case if the viewed strings scope goes out then the std::string_view has nothing that remains to project so it shows undefined behavior. std::string_view always needs a string literal or a string that has been created by a std::string within the scope in order to project the view of string, but as soon as scope dies std::string_view cannot create any strings on its own because it’s just only a view.
Below is the example to implement the above approach:
Program 10:
C++
// C++ program to implement// the above approach#include <iostream>#include <string>#include <string_view>using namespace std;  string_view ask_name(){    cout << "Write your surname?\n";      // A std::string is needed, because    // std::cin needs to modify it.    string str_1{};    cin >> str_1;      // Pass str_1 to std::string_view    string_view str_2{ str_1 };      cout << str_2 << '\n';      // str_1 dies, and so does the    // string that str created.    return str_2;}  // Driver codeint main(){    std::string_view str_2{ ask_name() };      // str_2 is observing a string that    // already died.      // Undefined behavior you'll observe    cout << your surname is << str_2 << '\n';      return 0;}
Output:
Execution Of std::string_view with data() Function: The data() function writes the characters of the string into an array. It returns a pointer to the array, obtained from the conversion of string to the array. Its Return type is not a valid C-string as no ‘\0’ character gets appended at the end of the array.
Below is the C++ program to implement the above approach:
Program 11:
C++
// C++ program to implement// the above approach#include <cstring>#include <iostream>#include <string>using namespace std;#include <string_view>  // Driver codeint main(){    string str{ "Madhav_Mohan" };    string_view str1{ str.data() };    cout << str1 << endl;    string_view str2{ str1.substr(6) };    string_view str3{ str2 };    cout << str3;    return 0;}
Output:
Non-member functions:
   operator==
   operator!=                                    (has been removed in C++20)
   operator<                                     (has been removed in C++20)
   operator>                                     (has been removed in C++20)
   operator<=                                    (has been removed in C++20)
   operator>=                                    (has been removed in C++20)
   operator<=>                                   (has newly been included C++20)
Note: Std::string_view run only in C++17 or in above versions only. There is a number of functions that are incorporated with std::string_view some of them are new(C++20) and some of them are old. Here below I am referencing the web_address where you can get them too into deep.
 
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