Difference between revisions of "cpp/preprocessor/include"

Latest revision as of 04:25, 9 July 2023

Includes other source file into current source file at the line immediately after the directive.

[edit] Syntax

[edit] Explanation

__has_include can be expanded in the expression of #if and #elif. It is treated as a defined macro by #ifdef, #ifndef, #elifdef, #elifndef(since C++23) and defined but cannot be used anywhere else.

[edit] Notes

Typical implementations search only standard include directories for syntax (1). The standard C++ library and the standard C library are implicitly included in these standard include directories. The standard include directories usually can be controlled by the user through compiler options.

The intent of syntax (2) is to search for the files that are not controlled by the implementation. Typical implementations first search the directory where the current file resides then falls back to (1).

When a file is included, it is processed by translation phases 1-4, which may include, recursively, expansion of the nested #include directives, up to an implementation-defined nesting limit. To avoid repeated inclusion of the same file and endless recursion when a file includes itself, perhaps transitively, header guards are commonly used: the entire header is wrapped in

#ifndef FOO_H_INCLUDED /* any name uniquely mapped to file name */
#define FOO_H_INCLUDED
// contents of the file are here
#endif

Many compilers also implement the non-standard pragma #pragma once with similar effects: it disables processing of a file if the same file (where file identity is determined in OS-specific way) has already been included.

A sequence of characters that resembles an escape sequence in q-char-sequence or h-char-sequence might result in an error, be interpreted as the character corresponding to the escape sequence, or have a completely different meaning, depending on the implementation.

A __has_include result of 1 only means that a header or source file with the specified name exists. It does not mean that the header or source file, when included, would not cause an error or would contain anything useful. For example, on a C++ implementation that supports both C++14 and C++17 modes (and provides __has_include in its C++14 mode as a conforming extension), __has_include(<optional>) may be 1 in C++14 mode, but actually #include <optional> may cause an error.

[edit] Example

#if __has_include(<optional>)
#  include <optional>
#  define has_optional 1
   template<class T> using optional_t = std::optional<T>;
#elif __has_include(<experimental/optional>)
#  include <experimental/optional>
#  define has_optional -1
   template<class T> using optional_t = std::experimental::optional<T>;
#else
#  define has_optional 0
#  include <utility>
 
template<class V>
class optional_t
{
    V v_{}; bool has_{false};
public:
    optional_t() = default;
    optional_t(V&& v) : v_(v), has_{true} {}
    V value_or(V&& alt) const& { return has_ ? v_ : alt; }
    /*...*/
};
#endif
 
#include <iostream>
 
int main()
{
    if (has_optional > 0)
        std::cout << "<optional> is present\n";
    else if (has_optional < 0)
        std::cout << "<experimental/optional> is present\n";
    else
        std::cout << "<optional> is not present\n";
 
    optional_t<int> op;
    std::cout << "op = " << op.value_or(-1) << '\n';
    op = 42;
    std::cout << "op = " << op.value_or(-1) << '\n';
}

<optional> is present
op = -1
op = 42

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also