Conditional inclusion

 

C++

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Technical Specifications

 

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
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inline specifier
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noexcept specifier (C++11)
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throw-expression	try-catch block
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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)
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Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Initializer lists this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
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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++

 

Preprocessor

#if#ifdef#ifndef#else#elif#endif

#define#undef#,## operators

#include__has_include(C++17)

#error

#pragma_Pragma(C++11)

#line

 

The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef and #endif directives.

Syntax

#if expression
#ifdef expression
#ifndef expression
#elif expression
#else
#endif

Explanation

The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif 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, #elif, #else, #ifdef and #ifndef directives control code block until first #elif, #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 and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else or #elif directive (if any) is processed. In the former case, the code block controlled by the #else directive is unconditionally compiled. In the latter case, the #elif 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 and #else directives. The conditional preprocessing block is terminated by #endif directive.

Condition evaluation

#if, #elif

The expression is a constant expression.

If the expression contains unary operators in form defined identifier or defined (identifier), it is evaluated first. The result is 1 if the identifier was defined as a macro name and 0 otherwise.

After macro expansion and evaluation of the defined operator, any identifier which is not a boolean literal, nor currently defined as a macro name, are replaced with the number 0.

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

#ifdef, #ifndef

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).

Example

Run this code

#define ABCD 2
#include <iostream>
 
int main()
{
 
#ifdef ABCD
    std::cout << "1: yes\n";
#else
    std::cout << "1: no\n";
#endif
 
#ifndef ABCD
    std::cout << "2: no1\n";
#elif ABCD == 2
    std::cout << "2: yes\n";
#else
    std::cout << "2: no2\n";
#endif
 
#if !defined(DCBA) && (ABCD < 2*4-3)
    std::cout << "3: yes\n";
#endif
}

Output:

1: yes
2: yes
3: yes

See also

C documentation for Conditional inclusion