E.4. #define Preprocessing Directive: Macros

[Note: This section is included for the benefit of C++ programmers who will need to work with C legacy code. In C++, macros can often be replaced by templates and inline functions.] A macro is an operation defined in a #define preprocessing directive. As with symbolic constants, the macro-identifier is replaced with the replacement-text before the program is compiled. Macros may be defined with or without arguments. A macro without arguments is processed like a symbolic constant. In a macro with arguments, the arguments are substituted in the replacement-text, then the macro is expanded—i.e., the replacement-text replaces the macro-identifier and argument list in the program. There is no data type checking for macro arguments. A macro is used simply for text substitution.

Consider the following macro definition with one argument for the area of a circle:

Wherever CIRCLE_AREA( y ) appears in the file, the value of y is substituted for x in the replacement text, the symbolic constant PI is replaced by its value (defined previously) and the macro is expanded in the program. For example, the statement

is expanded to

Because the expression consists only of constants, at compile time the value of the expression can be evaluated, and the result is assigned to area at runtime. The parentheses around each x in the replacement text and around the entire expression force the proper order of evaluation when the macro argument is an expression. For example, the statement

is expanded to

which evaluates correctly, because the parentheses force the proper order of evaluation. If the parentheses are omitted, the macro expansion is

which evaluates incorrectly as

because of the rules of operator precedence.

Macro CIRCLE_AREA could be defined as a function. Function circleArea, as in

performs the same calculation as CIRCLE_AREA, but the overhead of a function call is associated with function circleArea. The advantages of CIRCLE_AREA are that macros insert code directly in the program—avoiding function overhead—and the program remains readable because CIRCLE_AREA is defined separately and named meaningfully. A disadvantage is that its argument is evaluated twice. Also, every time a macro appears in a program, the macro is expanded. If the macro is large, this produces an increase in program size. Thus, there is a trade-off between execution speed and program size (if disk space is low). Note that inline functions (see Chapter 6) are preferred to obtain the performance of macros and the software engineering benefits of functions.

The following is a macro definition with two arguments for the area of a rectangle:

Wherever RECTANGLE_AREA( a, b ) appears in the program, the values of a and b are substituted in the macro replacement text, and the macro is expanded in place of the macro name. For example, the statement

is expanded to

The value of the expression is evaluated and assigned to variable rectArea.

The replacement text for a macro or symbolic constant is normally any text on the line after the identifier in the #define directive. If the replacement text for a macro or symbolic constant is longer than the remainder of the line, a backslash () must be placed at the end of each line of the macro (except the last line), indicating that the replacement text continues on the next line.

Symbolic constants and macros can be discarded using the #undef preprocessing directive. Directive #undef “undefines” a symbolic constant or macro name. The scope of a symbolic constant or macro is from its definition until it is either undefined with #undef or the end of the file is reached. Once undefined, a name can be redefined with #define.

Note that expressions with side effects (e.g., variable values are modified) should not be passed to a macro, because macro arguments may be evaluated more than once.