Can the compiler skip evaluating the left operand of a comma operator?

Question

Compiler optimization can sometimes overlook the evaluation of certain statements that are of no value. However, does this apply to the comma operator?

The following code runs without errors in ideone , but I expected it to work.

#include <iostream> int main() { int x = (1/0, 2); std::cout << x << std::endl; } 

The program crashes if I change the statement to int x = 1/0;

Answer 1

Compiler optimization uses the As-if rule.

As-if rule

Allows any code transformations that do not alter the program’s observed behavior.

So, the compiler can optimize this. Check out the following modified sample :

 #include <iostream> int main() { int y = 1; int x = (y=1/0, 2); std::cout << x << std::endl; //std::cout << y << std::endl; } 

Commenting out the last line compiles and correctly executes this code when uncommenting, which gives the expected undefined behavior.

As @jogojapan correctly points out,
It is important to note that compiler optimization is not guaranteed by the standard, and division by zero is undefined behavior. So this code has undefined behavior. Whether the observed behavior is observed due to the fact that the compiler optimizes division by zero or due to undefined behavior, which we will never know about. Technically, this behavior is undefined anyway.