How is the comma operator being used here? [duplicate]

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C++ Comma Operator
Uses of C comma operator

I am not new to C++, but this is the first time I see the following code:

int a=0;
int b=(a=2,a+1);

That is C++ code. Can you tell me what is going on here? And how variable b gets value 3?

Answer 1

This code is equivalent to this:

int a = 2 ; 
int b = a + 1 ;

First expression to the left of comma gets evaluated, then the one to its right. The result of the right most expression is stored in the variable to the left of = sign.

Look up the comma operator for more details.

http://en.wikipedia.org/wiki/Comma_operator

Answer 2

(a = 2, a + 1); return 3 because in general case operator (a, b) return b, and calculation in (a, b) started from right to left. So, in your case, (a = 2, a + 1) return a + 1, and after operator a = 2 was executed a + 1 return 3.