19.15 Statements and Declarations in Expressions

A block enclosed in parentheses can be used as an expression in GNU C. This provides a way to use local variables, loops and switches within an expression. We call it a statement expression.

Recall that a block is a sequence of statements surrounded by braces. In this construct, parentheses go around the braces. For example:

({ int y = foo (); int z;
   if (y > 0) z = y;
   else z = - y;
   z; })

is a valid (though slightly more complex than necessary) expression for the absolute value of foo ().

The last statement in the block should be an expression statement; an expression followed by a semicolon, that is. The value of this expression serves as the value of statement expression. If the last statement is anything else, the statement expression’s value is void.

This feature is mainly useful in making macro definitions compute each operand exactly once. See Using __auto_type for Local Variables.

Statement expressions are not allowed in expressions that must be constant, such as the value for an enumerator, the width of a bit-field, or the initial value of a static variable.

Jumping into a statement expression—with goto, or using a switch statement outside the statement expression—is an error. With a computed goto (see Labels as Values), the compiler can’t detect the error, but it still won’t work.

Jumping out of a statement expression is permitted, but since subexpressions in C are not computed in a strict order, it is unpredictable which other subexpressions will have been computed by then. For example,

  foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz();

calls foo and bar1 before it jumps, and never calls baz, but may or may not call bar2. If bar2 does get called, that occurs after foo and before bar1.