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The #define Directive

Updated: March 2009

You can use the #define directive to give a meaningful name to a constant in your program. The two forms of the syntax are:

#define identifier token-stringopt

#define identifier( identifieropt, ... , identifieropt ) token-stringopt

The #define directive substitutes token-string for all subsequent occurrences of an identifier in the source file. The identifier is replaced only when it forms a token. (See C++ Tokens in the C++ Language Reference.) For instance, identifier is not replaced if it appears in a comment, within a string, or as part of a longer identifier.

A #define without a token-string removes occurrences of identifier from the source file. The identifier remains defined and can be tested using the #if defined and #ifdef directives.

The token-string argument consists of a series of tokens, such as keywords, constants, or complete statements. One or more white-space characters must separate token-string from identifier. This white space is not considered part of the substituted text, nor is any white space following the last token of the text.

Formal parameter names appear in token-string to mark the places where actual values are substituted. Each parameter name can appear more than once in token-string, and the names can appear in any order. The number of arguments in the call must match the number of parameters in the macro definition. Liberal use of parentheses ensures that complicated actual arguments are interpreted correctly.

The second syntax form allows the creation of function-like macros. This form accepts an optional list of parameters that must appear in parentheses. References to the identifier after the original definition replace each occurrence of identifier( identifieropt, ..., identifieropt ) with a version of the token-string argument that has actual arguments substituted for formal parameters.

The formal parameters in the list are separated by commas. Each name in the list must be unique, and the list must be enclosed in parentheses. No spaces can separate identifier and the opening parenthesis. Use line concatenation — place a backslash (\) immediately before the newline character — for long directives on multiple source lines. The scope of a formal parameter name extends to the new line that ends token-string.

When a macro has been defined in the second syntax form, subsequent textual instances followed by an argument list constitute a macro call. The actual arguments following an instance of identifier in the source file are matched to the corresponding formal parameters in the macro definition. Each formal parameter in token-string that is not preceded by a stringizing (#), charizing (#@), or token-pasting (##) operator, or not followed by a ## operator, is replaced by the corresponding actual argument. Any macros in the actual argument are expanded before the directive replaces the formal parameter. (The operators are described in Preprocessor Operators.)

The following examples of macros with arguments illustrate the second form of the #define syntax:

// Macro to define cursor lines 
#define CURSOR(top, bottom) (((top) << 8) | (bottom))

// Macro to get a random integer with a specified range 
#define getrandom(min, max) \
    ((rand()%(int)(((max) + 1)-(min)))+ (min))

Arguments with side effects sometimes cause macros to produce unexpected results. A given formal parameter may appear more than once in token-string. If that formal parameter is replaced by an expression with side effects, the expression, with its side effects, may be evaluated more than once. (See the examples under Token-Pasting Operator (##).)

The #undef directive causes an identifier's preprocessor definition to be forgotten. See The #undef Directive for more information.

If the name of the macro being defined occurs in token-string (even as a result of another macro expansion), it is not expanded.

A second #define for a macro with the same name generates an error unless the second token sequence is identical to the first.

Microsoft Specific

Microsoft C/C++ lets you redefine a macro if the new definition is syntactically identical to the original definition. In other words, the two definitions can have different parameter names. This behavior differs from ANSI C, which requires that the two definitions be lexically identical.

For example, the following two macros are identical except for the parameter names. ANSI C does not allow such a redefinition, but Microsoft C/C++ compiles it without error.

#define multiply( f1, f2 ) ( f1 * f2 )
#define multiply( a1, a2 ) ( a1 * a2 )

On the other hand, the following two macros are not identical and will generate a warning in Microsoft C/C++.

#define multiply( f1, f2 ) ( f1 * f2 )
#define multiply( a1, a2 ) ( b1 * b2 )

END Microsoft Specific

This example illustrates the #define directive:

#define WIDTH       80
#define LENGTH      ( WIDTH + 10 )

The first statement defines the identifier WIDTH as the integer constant 80 and defines LENGTH in terms of WIDTH and the integer constant 10. Each occurrence of LENGTH is replaced by (WIDTH + 10). In turn, each occurrence of WIDTH + 10 is replaced by the expression (80 + 10). The parentheses around WIDTH + 10 are important because they control the interpretation in statements such as the following:

var = LENGTH * 20;

After the preprocessing stage the statement becomes:

var = ( 80 + 10 ) * 20;

which evaluates to 1800. Without parentheses, the result is:

var = 80 + 10 * 20;

which evaluates to 280.

Microsoft Specific

Defining macros and constants with the /D compiler option has the same effect as using a #define preprocessing directive at the beginning of your file. Up to 30 macros can be defined with the /D option.

END Microsoft Specific




March 2009

Revised section on macro redefinition.

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