Question

Why is callback called once only?

bool callback() {     static bool res = false;     res = !res;     return res; }  int main(int argc, char* argv[]) {     vector<int> x(10);      bool result=false;     for_each(x.begin(),x.end(),var(result)=var(result)||bind(callback));      return 0; } 

Answer 1

The || expression short circuits after the first time bind returns true.

The first time you evaluate

result = result || bind(...)  // result is false at this point 

bind is called, because that’s the only way to determine the value of false || bind(...). Because bind(...) returns true, result is set to true.

Every other time you say

result = result || bind(...)  // result is true at this point 

… the bind(...) expression isn’t evaluated, because it doesn’t matter what it returns; the expression true || anything is always true, and the || expression short circuits.

One way to ensure that bind is always called would be to move it to the left side of the ||, or change the || to an &&, depending on what you are trying to accomplish with result.