More C++ Idioms/Multi-statement Macro

Multi-statement Macro

Intent

To write a multi-statement (multi-line) macro.

Motivation

Sometimes it is useful to group two or more statements into a macro and call them like a function call. Usually, an inline function should be the first preference but things such as debug macros are almost invariably macros rather than function calls. Grouping multiple statements into one macro in a naive way could lead to compilation errors, which are not obvious at first look. For example,

#define MACRO(X,Y) { statement1; statement2; }

would fail in an if statement if a semi-colon is appended at the end.

if (cond)
   MACRO(10,20); // Compiler error here because of the semi-colon.
else
   statement3;

The above statement expands to

if (cond)
   { statement1; statement2; }; // Compiler error here because of the semi-colon.
else
   statement3;

giving a compiler error. Therefore, people came up with a widely used idiom for multi-statement macros, which is based on a do-while loop.

Solution and Sample Code

Here is an example of the multi-statement macro idiom.

#define MACRO(arg1, arg2) do {  \
  /* declarations, if any */    \
  statement1;                   \
  statement2;                   \
  /* ... */                     \
  } while(0)	/* (no trailing ; ) */

When the caller appends a semicolon, this expansion becomes a single statement regardless of the context. Optimizing compilers usually remove any dead tests, such as while(0). This idiom is not useful when macro is used as a parameter to a function call. Moreover, this idiom allows return statement.

func(MACRO(10,20)); // Syntax error here.

Known Uses

ACE_NEW_RETURN, ACE_NEW_NORETURN macros in Adaptive Communication Environment (ACE).

#define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) \
      do { POINTER = new (ACE_nothrow) CONSTRUCTOR; \
      if (POINTER == 0) { errno = ENOMEM; return RET_VAL; } \
    } while (0)

References

• What's the best way to write a multi-statement macro?