c++ - comma operator in if condition

Question

int a = 1, b = 0;

if(a, b)
   printf("success
");
else
   printf("fail
");

if(b, a)
   printf("success
");
else
   printf("fail");

This is a cpp file and I got the output in Visual Studio 2010 as

fail
success

Why this behavior? Could you please explain?

Answer 1

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

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

In your first if:

if (a, b)

a is evaluated first and discarded, b is evaluated second and returned as 0. So this condition is false.

In your second if:

if (b, a)

b is evaluated first and discarded, a is evaluated second and returned as 1. So this condition is true.

If there are more than two operands, the last expression will be returned.

If you want both conditions to be true, you should use the && operator:

if (a && b)