Recommendations for Choosing Between Functions and Macros
The latest version of this topic can be found at Recommendations for Choosing Between Functions and Macros.
Most Microsoft run-time library routines are compiled or assembled functions, but some routines are implemented as macros. When a header file declares both a function and a macro version of a routine, the macro definition takes precedence, because it always appears after the function declaration. When you invoke a routine that is implemented as both a function and a macro, you can force the compiler to use the function version in two ways:
Enclose the routine name in parentheses.
#include <ctype.h> a = _toupper(a); // Use macro version of toupper. a = (_toupper)(a); // Force compiler to use // function version of toupper."Undefine" the macro definition with the
#undefdirective:#include <ctype.h> #undef _toupper
If you need to choose between a function and a macro implementation of a library routine, consider the following trade-offs:
Speed versus size The main benefit of using macros is faster execution time. During preprocessing, a macro is expanded (replaced by its definition) inline each time it is used. A function definition occurs only once regardless of how many times it is called. Macros may increase code size but do not have the overhead associated with function calls.
Function evaluation A function evaluates to an address; a macro does not. Thus you cannot use a macro name in contexts requiring a pointer. For instance, you can declare a pointer to a function, but not a pointer to a macro.
Type-checking When you declare a function, the compiler can check the argument types. Because you cannot declare a macro, the compiler cannot check macro argument types; although it can check the number of arguments you pass to a macro.