Function call in for loop condition?

Question

If I have function call in for loop condition like this:

for (auto it = s.begin(); it != s.end(), ++it) {}

is it called on each iteration? I expect that yes. Is compiler allow to optimized it away? Are current compilers smart enough to do so? Or am I better using something like following:

for (auto it = s.begin(), auto end = s.end(); it != end; ++it) {}

?

Answer 1

In

for ( auto it = s.begin(); it != s.end(), ++it )

s.begin() is called only once.
s.end() and operator++() (for ++it) are called in each iteration of the loop.

Is compiler allow to optimized it away?

The compiler can optimize away the call to s.end() depending on the compiler, the implementation, and the optimization level. I will be surprised if it can optimize away the operator++() call.

Are current compilers smart enough to do so?

Can't answer that.

Or am I better using something like following:

for (auto it = s.begin(), auto end = s.end(); it != end; ++it) {}

It won't hurt. However, it can be a problem if s is modified in the loop. If s is not modified in the loop, I would recommend using this approach.