#if, #elif, #else, and #endif directives (C/C++)

Article
01/25/2023

The #if directive, with the #elif, #else, and #endif directives, controls compilation of portions of a source file. If the expression you write (after the #if) has a nonzero value, the line group immediately following the #if directive is kept in the translation unit.

Grammar

conditional :
if-part elif-parts_opt else-part_opt endif-line

if-part :
if-line text

if-line :
#if constant-expression
#ifdef identifier
#ifndef identifier

elif-parts :
elif-line text
elif-parts elif-line text

elif-line :
#elif constant-expression

else-part :
else-line text

else-line :
#else

endif-line :
#endif

Remarks

Each #if directive in a source file must be matched by a closing #endif directive. Any number of #elif directives can appear between the #if and #endif directives, but at most one #else directive is allowed. The #else directive, if present, must be the last directive before #endif.

The #if, #elif, #else, and #endif directives can nest in the text portions of other #if directives. Each nested #else, #elif, or #endif directive belongs to the closest preceding #if directive.

All conditional-compilation directives, such as #if and #ifdef, must match a closing #endif directive before the end of file. Otherwise, an error message is generated. When conditional-compilation directives are contained in include files, they must satisfy the same conditions: There must be no unmatched conditional-compilation directives at the end of the include file.

Macro replacement is done within the part of the line that follows an #elif command, so a macro call can be used in the constant-expression.

The preprocessor selects one of the given occurrences of text for further processing. A block specified in text can be any sequence of text. It can occupy more than one line. Usually text is program text that has meaning to the compiler or the preprocessor.

The preprocessor processes the selected text and passes it to the compiler. If text contains preprocessor directives, the preprocessor carries out those directives. Only text blocks selected by the preprocessor are compiled.

The preprocessor selects a single text item by evaluating the constant expression following each #if or #elif directive until it finds a true (nonzero) constant expression. It selects all text (including other preprocessor directives beginning with #) up to its associated #elif, #else, or #endif.

If all occurrences of constant-expression are false, or if no #elif directives appear, the preprocessor selects the text block after the #else clause. When there's no #else clause, and all instances of constant-expression in the #if block are false, no text block is selected.

The constant-expression is an integer constant expression with these additional restrictions:

Expressions must have integral type and can include only integer constants, character constants, and the defined operator.
The expression can't use sizeof or a type-cast operator.
The target environment may be unable to represent all ranges of integers.
The translation represents type int the same way as type long, and unsigned int the same way as unsigned long.
The translator can translate character constants to a set of code values different from the set for the target environment. To determine the properties of the target environment, use an app built for that environment to check the values of the LIMITS.H macros.
The expression must not query the environment, and must remain insulated from implementation details on the target computer.

Preprocessor operators

defined

The preprocessor operator defined can be used in special constant expressions, as shown by the following syntax:

defined( identifier )
defined identifier

This constant expression is considered true (nonzero) if the identifier is currently defined. Otherwise, the condition is false (0). An identifier defined as empty text is considered defined. The defined operator can be used in an #if and an #elif directive, but nowhere else.

In the following example, the #if and #endif directives control compilation of one of three function calls:

#if defined(CREDIT)
    credit();
#elif defined(DEBIT)
    debit();
#else
    printerror();
#endif

The function call to credit is compiled if the identifier CREDIT is defined. If the identifier DEBIT is defined, the function call to debit is compiled. If neither identifier is defined, the call to printerror is compiled. Both CREDIT and credit are distinct identifiers in C and C++ because their cases are different.

The conditional compilation statements in the following example assume a previously defined symbolic constant named DLEVEL.

#if DLEVEL > 5
    #define SIGNAL  1
    #if STACKUSE == 1
        #define STACK   200
    #else
        #define STACK   100
    #endif
#else
    #define SIGNAL  0
    #if STACKUSE == 1
        #define STACK   100
    #else
        #define STACK   50
    #endif
#endif
#if DLEVEL == 0
    #define STACK 0
#elif DLEVEL == 1
    #define STACK 100
#elif DLEVEL > 5
    display( debugptr );
#else
    #define STACK 200
#endif

The first #if block shows two sets of nested #if, #else, and #endif directives. The first set of directives is processed only if DLEVEL > 5 is true. Otherwise, the statements after #else are processed.

The #elif and #else directives in the second example are used to make one of four choices, based on the value of DLEVEL. The constant STACK is set to 0, 100, or 200, depending on the definition of DLEVEL. If DLEVEL is greater than 5, then the statement

#elif DLEVEL > 5
display(debugptr);

is compiled, and STACK isn't defined.

A common use for conditional compilation is to prevent multiple inclusions of the same header file. In C++, where classes are often defined in header files, constructs like this one can be used to prevent multiple definitions:

/*  EXAMPLE.H - Example header file  */
#if !defined( EXAMPLE_H )
#define EXAMPLE_H

class Example
{
    //...
};

#endif // !defined( EXAMPLE_H )

The preceding code checks to see if the symbolic constant EXAMPLE_H is defined. If so, the file has already been included and doesn't need reprocessing. If not, the constant EXAMPLE_H is defined to mark EXAMPLE.H as already processed.

__has_include

Visual Studio 2017 version 15.3 and later: Determines whether a library header is available for inclusion:

#ifdef __has_include
#  if __has_include(<filesystem>)
#    include <filesystem>
#    define have_filesystem 1
#  elif __has_include(<experimental/filesystem>)
#    include <experimental/filesystem>
#    define have_filesystem 1
#    define experimental_filesystem
#  else
#    define have_filesystem 0
#  endif
#endif