Anonymous functions using GCC statement expressions

by

This declaration (at block scope):

int (*max)(int, int) =
    ({
    int __fn__ (int x, int y) { return x > y ? x : y; }
    __fn__;
    });

is not C but is valid GNU C.

It makes use of two gcc extensions:

  1. nested functions
  2. statement expressions

Both nested functions (defining a function inside a compound statement) and statement expressions (({}), basically a block that yields a value) are not permitted in C and come from GNU C.

In a statement expression, the last expression statement is the value of the construct. This is why the nested function __fn__ appears as an expression statement at the end of the statement expression. A function designator (__fn__ in the last expression statement) in a expression is converted to a pointer to a function by the usual conversions. This is the value used to initialize the function pointer max.

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