c++ - Why are multiple statements in ternary operator not executed

I am puzzled by the following:

#include 

int main()
{
  bool a = true;
  int nb = 1;
  int nb2 = 2;
  a ? nb++, nb2++ : nb--, nb2--;
  std::cout << " (nb,nb2) = (" << nb << "," << nb2 << ")";
}

Result:

(nb,nb2) = (2,2) 

Why is nb2 not equal to 3?

Answer

Because of operators priority. Your expression evaluates as

((a) ? (nb++, nb2++) : nb--), nb2--;

Operator , (comma) is the last thing to process. And this example would not compile at all but

The expression in the middle of the conditional operator (between ? and :) is parsed as if parenthesized.

See C++ Operator Precedence for details.