A variable not detected as not used

Question

I am using g++ 4.3.0 to compile this example :

#include 

int main()
{
  std::vector< int > a;
  int b;
}

If I compile

Answer 1

a is not a built-in type. You are actually calling the constructor of std::vector and assigning the result to a. The compiler sees this as usage because the constructor could have side effects.