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C++17: Add __has_include operator (#112)

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Arsenic
2022-03-19 21:40:40 +05:30
committed by GitHub
parent cb50e8e5a6
commit bd4cb84e80
2 changed files with 76 additions and 0 deletions
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38
CPP17.md
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@@ -18,6 +18,7 @@ C++17 includes the following new language features:
- [utf-8 character literals](#utf-8-character-literals)
- [direct-list-initialization of enums](#direct-list-initialization-of-enums)
- [fallthrough, nodiscard, maybe_unused attributes](#fallthrough-nodiscard-maybe_unused-attributes)
- [\_\_has\_include](#\_\_has\_include)
C++17 includes the following new library features:
- [std::variant](#stdvariant)
@@ -309,6 +310,43 @@ void my_callback(std::string msg, [[maybe_unused]] bool error) {
}
```
### \_\_has\_include
`__has_include (operand)` operator may be used in `#if` and `#elif` expressions to check whether a header or source file (`operand`) is available for inclusion or not.
One use case of this would be using two libraries that work the same way, using the backup/experimental one if the preferred one is not found on the system.
```c++
#ifdef __has_include
# if __has_include(<optional>)
# include <optional>
# define have_optional 1
# elif __has_include(<experimental/optional>)
# include <experimental/optional>
# define have_optional 1
# define experimental_optional
# else
# define have_optional 0
# endif
#endif
```
It can also be used to include headers existing under different names or locations on various platforms, without knowing which platform the program is running on, OpenGL headers are a good example for this which are located in `OpenGL\` directory on macOS and `GL\` on other platforms.
```c++
#ifdef __has_include
# if __has_include(<OpenGL/gl.h>)
# include <OpenGL/gl.h>
# include <OpenGL/glu.h>
# elif __has_include(<GL/gl.h>)
# include <GL/gl.h>
# include <GL/glu.h>
# else
# error No suitable OpenGL headers found.
# endif
#endif
```
## C++17 Library Features
### std::variant

38
README.md
View File

@@ -49,6 +49,7 @@ C++17 includes the following new language features:
- [utf-8 character literals](#utf-8-character-literals)
- [direct-list-initialization of enums](#direct-list-initialization-of-enums)
- [fallthrough, nodiscard, maybe_unused attributes](#fallthrough-nodiscard-maybe_unused-attributes)
- [\_\_has\_include](#\_\_has\_include)
C++17 includes the following new library features:
- [std::variant](#stdvariant)
@@ -960,6 +961,43 @@ void my_callback(std::string msg, [[maybe_unused]] bool error) {
}
```
### \_\_has\_include
`__has_include (operand)` operator may be used in `#if` and `#elif` expressions to check whether a header or source file (`operand`) is available for inclusion or not.
One use case of this would be using two libraries that work the same way, using the backup/experimental one if the preferred one is not found on the system.
```c++
#ifdef __has_include
# if __has_include(<optional>)
# include <optional>
# define have_optional 1
# elif __has_include(<experimental/optional>)
# include <experimental/optional>
# define have_optional 1
# define experimental_optional
# else
# define have_optional 0
# endif
#endif
```
It can also be used to include headers existing under different names or locations on various platforms, without knowing which platform the program is running on, OpenGL headers are a good example for this which are located in `OpenGL\` directory on macOS and `GL\` on other platforms.
```c++
#ifdef __has_include
# if __has_include(<OpenGL/gl.h>)
# include <OpenGL/gl.h>
# include <OpenGL/glu.h>
# elif __has_include(<GL/gl.h>)
# include <GL/gl.h>
# include <GL/glu.h>
# else
# error No suitable OpenGL headers found.
# endif
#endif
```
## C++17 Library Features
### std::variant