why this works: C++ last statement as result of expression [duplicate]

Question

I found strange macros in a driver implementation that I can't explain to myself. Simplified example is:

cout << ({int i=0; while(i<10) {++i;} i;}) << endl;

It will output 10.

But why this does expression become an rvalue at all? It seems to work in C and in C++.

Can someone explain me? Pointing to keywords and to reference will be great.

Answer 1

The is a GCC extension:

A compound statement enclosed in parentheses may appear as an expression in GNU C.

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct.