The preprocessor supports text macro replacement. Function-like text macro replacement is also supported.

[edit]Syntax

[edit]Explanation

[edit]#define directives

The#define directives define theidentifier as macro, that is instruct the compiler to replace most successive occurrences ofidentifier withreplacement-list, which will be additionally processed. Exceptions arise from the rules ofscanning and replacement. If the identifier is already defined as any type of macro, the program is ill-formed unless the definitions are identical.

[edit]Object-like macros

Object-like macros replace every occurrence of definedidentifier withreplacement-list. Version (1) of the#define directive behaves exactly like that.

[edit]Function-like macros

Function-like macros replace each occurrence of definedidentifier withreplacement-list, additionally taking a number of arguments, which then replace corresponding occurrences of any of theparameters in thereplacement-list.

The syntax of a function-like macro invocation is similar to the syntax of a function call: each instance of the macro name followed by a( as the next preprocessing token introduces the sequence of tokens that is replaced by thereplacement-list. The sequence is terminated by the matching) token, skipping intervening matched pairs of left and right parentheses.

For version (2), the number of arguments must be the same as the number of parameters in macro definition. For versions (3,4), the number of arguments must not be less than the number of parameters (not(since C++20) counting...). Otherwise the program is ill-formed. If the identifier is not in functional-notation, i.e. does not have parentheses after itself, it is not replaced at all.

Version (2) of the#define directive defines a simple function-like macro.

Version (3) of the#define directive defines a function-like macro with variable number of arguments. The additional arguments (calledvariable arguments) can be accessed using__VA_ARGS__ identifier, which is then replaced with arguments, supplied with the identifier to be replaced.

Version (4) of the#define directive defines a function-like macro with variable number of arguments, but no regular arguments. The arguments (calledvariable arguments) can be accessed only with__VA_ARGS__ identifier, which is then replaced with arguments, supplied with the identifier to be replaced.

#define F(...) f(0 __VA_OPT__(,) __VA_ARGS__)F(a, b, c)// replaced by f(0, a, b, c)F()// replaced by f(0) #define G(X, ...) f(0, X __VA_OPT__(,) __VA_ARGS__)G(a, b, c)// replaced by f(0, a, b, c)G(a,)// replaced by f(0, a)G(a)// replaced by f(0, a) #define SDEF(sname, ...) S sname __VA_OPT__(= { __VA_ARGS__ })SDEF(foo);// replaced by S foo;SDEF(bar,1,2);// replaced by S bar = { 1, 2 };

Note: if an argument of a function-like macro includes commas that are not protected by matched pairs of left and right parentheses (most commonly found in template argument lists, as inassert(std::is_same_v<int,int>); orBOOST_FOREACH(std::pair<int,int> p, m)), the comma is interpreted as macro argument separator, causing a compilation failure due to argument count mismatch.

[edit]Scanning and Replacement

• Scanning keeps track of macros they replaced. If scan finds text matching such macro, it marks it "to be ignored" (all scans will ignore it). This prevents recursion.
• If scanning found function-like macro, arguments are scanned before put insidereplacement-list. Except# and## operators take argument without scan.
• After macro was replaced, result text is scanned.

Note, it is possible to define pseudo recursive macro:

#define EMPTY#define SCAN(x)     x#define EXAMPLE_()  EXAMPLE#define EXAMPLE(n)  EXAMPLE_ EMPTY()(n-1) (n)EXAMPLE(5)SCAN(EXAMPLE(5))

EXAMPLE_ ()(5 -1) (5)EXAMPLE_ ()(5 -1 -1) (5 -1) (5)

[edit]Reserved macro names

A translation unit thatincludes a standard library header may not#define or#undef names declared in anystandard library header.

A translation unit that uses any part of the standard library is not allowed to#define or#undef names lexically identical to:

• keywords

Otherwise, the behavior is undefined.

[edit]# and## operators

In function-like macros, a# operator before an identifier in thereplacement-list runs the identifier through parameter replacement and encloses the result in quotes, effectively creating a string literal. In addition, the preprocessor adds backslashes to escape the quotes surrounding embedded string literals, if any, and doubles the backslashes within the string as necessary. All leading and trailing whitespace is removed, and any sequence of whitespace in the middle of the text (but not inside embedded string literals) is collapsed to a single space. This operation is called "stringification". If the result of stringification is not a valid string literal, the behavior is undefined.

#define showlist(...) puts(#__VA_ARGS__)showlist();// expands to puts("")showlist(1,"x",int);// expands to puts("1, \"x\", int")

A## operator between any two successive identifiers in thereplacement-list runs parameter replacement on the two identifiers (which are not macro-expanded first) and then concatenates the result. This operation is called "concatenation" or "token pasting". Only tokens that form a valid token together may be pasted: identifiers that form a longer identifier, digits that form a number, or operators+ and= that form a+=. A comment cannot be created by pasting/ and* because comments are removed from text before macro substitution is considered. If the result of concatenation is not a valid token, the behavior is undefined.

Note: some compilers offer an extension that allows## to appear after a comma and before__VA_ARGS__, in which case the## does nothing when the variable arguments are present, but removes the comma when the variable arguments are not present: this makes it possible to define macros such asfprintf(stderr, format,##__VA_ARGS__).This can also be achieved in a standard manner using__VA_OPT__, such asfprintf(stderr, format __VA_OPT__(,) __VA_ARGS__).(since C++20)

[edit]#undef directive

The#undef directive undefines theidentifier, that is cancels previous definition of theidentifier by#define directive. If the identifier does not have associated macro, the directive is ignored.

[edit]Predefined macros

The following macro names are predefined in every translation unit:

The following additional macro names may be predefined by the implementations:

The values of these macros (except for__FILE__ and__LINE__) remain constant throughout the translation unit. Attempts to redefine or undefine these macros result in undefined behavior.

[edit]Example

#include <iostream> // Make function factory and use it#define FUNCTION(name, a) int fun_##name() { return a; } FUNCTION(abcd,12)FUNCTION(fff,2)FUNCTION(qqq,23) #undef FUNCTION#define FUNCTION 34#define OUTPUT(a) std::cout << "output: " #a << '\n' // Using a macro in the definition of a later macro#define WORD "Hello "#define OUTER(...) WORD #__VA_ARGS__ int main(){std::cout<<"abcd: "<< fun_abcd()<<'\n';std::cout<<"fff: "<< fun_fff()<<'\n';std::cout<<"qqq: "<< fun_qqq()<<'\n'; std::cout<< FUNCTION<<'\n';    OUTPUT(million);//note the lack of quotes std::cout<< OUTER(World)<<'\n';std::cout<< OUTER(WORD World)<<'\n';}

abcd: 12fff: 2qqq: 2334output: millionHello WorldHello WORD World

[edit]Defect reports

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

[edit]See also

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