Conditional inclusion

2023-07-11 13:13| 来源: 网络整理| 查看: 265

C++ Compiler support Freestanding and hosted Language Standard library Standard library headers Named requirements Feature test macros (C++20) Language support library Concepts library (C++20) Metaprogramming library (C++11) Diagnostics library General utilities library Strings library Containers library Iterators library Ranges library (C++20) Algorithms library Numerics library Localizations library Input/output library Filesystem library (C++17) Regular expressions library (C++11) Concurrency support library (C++11) Technical specifications Symbols index External libraries [edit] C++ language General topics Preprocessor Comments Keywords Escape sequences Flow control Conditional execution statements if switch Iteration statements (loops) for range-for (C++11) while do-while Jump statements continue - break goto - return Functions Function declaration Lambda function expression inline specifier Dynamic exception specifications (until C++17*) noexcept specifier (C++11) Exceptions throw-expression try-catch block Namespaces Namespace declaration Namespace aliases Types Fundamental types Enumeration types Function types Class/struct types Union types Specifiers decltype (C++11) auto (C++11) alignas (C++11) const/volatile constexpr (C++11) Storage duration specifiers Initialization Default initialization Value initialization Zero initialization Copy initialization Direct initialization Aggregate initialization List initialization (C++11) Constant initialization Reference initialization Expressions Value categories Order of evaluation Operators Operator precedence Alternative representations Literals Boolean - Integer - Floating-point Character - String - nullptr (C++11) User-defined (C++11) Utilities Attributes (C++11) Types typedef declaration Type alias declaration (C++11) Casts Implicit conversions - Explicit conversions static_cast - dynamic_cast const_cast - reinterpret_cast Memory allocation new expression delete expression Classes Class declaration Constructors this pointer Access specifiers friend specifier Class-specific function properties Virtual function override specifier (C++11) final specifier (C++11) explicit (C++11) static Special member functions Default constructor Copy constructor Move constructor (C++11) Copy assignment Move assignment (C++11) Destructor Templates Class template Function template Template specialization Parameter packs (C++11) Miscellaneous Inline assembly History of C++ [edit] Preprocessor #if#ifdef#ifndef#else#elif#elifdef#elifndef#endif(C++23)(C++23) #define#undef#,## operators #include__has_include(C++17) #error#warning(C++23) #pragma_Pragma(C++11) #line [edit]

The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef, #elifdef, #elifndef (since C++23), and #endif directives.

Contents 1 Syntax 2 Explanation 3 Condition evaluation 3.1 #if, #elif 3.2 Combined directives 4 Notes 5 Example 6 Defect reports 7 See also [edit] Syntax #if expression #ifdef identifier #ifndef identifier #elif expression #elifdef identifier (since C++23) #elifndef identifier (since C++23) #else #endif [edit] Explanation

The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif, #elifdef, or #elifndef (since C++23) directives, then optionally includes at most one #else directive and is terminated with #endif directive. Any inner conditional preprocessing blocks are processed separately.

Each of #if, #ifdef, #ifndef, #elif, #elifdef, #elifndef (since C++23), and #else directives control the code block until the first #elif, #elifdef, #elifndef (since C++23), #else, #endif directive not belonging to any inner conditional preprocessing blocks.

#if, #ifdef and #ifndef directives test the specified condition (see below) and if it evaluates to true, compiles the controlled code block. In that case subsequent #else, #elifdef, #elifndef, (since C++23) and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else, #elifdef, #elifndef, (since C++23) or #elif directive (if any) is processed. If the subsequent directive is #else, the code block controlled by the #else directive is unconditionally compiled. Otherwise, the #elif, #elifdef, or #elifndef (since C++23) directive acts as if it was #if directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent #elif, #elifdef, #elifndef, (since C++23) and #else directives. The conditional preprocessing block is terminated by #endif directive.

[edit] Condition evaluation [edit] #if, #elif

The expression may contain:

unary operators in form defined identifier or defined (identifier). The result is 1 if the identifier was defined as a macro name, otherwise the result is 0. __has_include and __has_cpp_attribute (since C++20) are treated as if they were the names of defined macros in this context. (since C++17) (since C++17) __has_include expressions, which detects whether a header or source file exists. (since C++20) __has_cpp_attribute expressions, which detects whether a given attribute token is supported and its supported version.

After all macro expansion and evaluation of defined, __has_include (since C++17), and __has_cpp_attribute (since C++20) expressions, any identifier which is not a boolean literal is replaced with the number 0 (this includes identifiers that are lexically keywords, but not alternative tokens like and).

Then the expression is evaluated as an integral constant expression.

If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise.

Note: Until the resolution of CWG issue 1955, #if cond1 ... #elif cond2 is different from #if cond1 ... #else followed by #if cond2 because if cond1 is true, the second #if is skipped and cond2 does not need to be well-formed, while #elif's cond2 must be a valid expression. As of CWG 1955, #elif that leads the skipped code block is also skipped.

[edit] Combined directives

Checks if the identifier was defined as a macro name.

#ifdef identifier is essentially equivalent to #if defined identifier.

#ifndef identifier is essentially equivalent to #if !defined identifier.

#elifdef identifier is essentially equivalent to #elif defined identifier.

#elifndef identifier is essentially equivalent to #elif !defined identifier.

(since C++23) [edit] Notes

While #elifdef and #elifndef directives target C++23, implementations are encouraged to backport them to the older language modes as conforming extensions.

[edit] Example Run this code #define ABCD 2 #include int main() { #ifdef ABCD std::cout

【本文地址】

公司简介

联系我们