Performance issue for vector::size() in a loop in C++

Question

In the following code:

std::vector var;
for (int i = 0; i < var.size(); i++);

Is the

Answer 1

I think that if the compiler can conclusively deduce that the variable var is not modified inside the "loop body"

for(int i=0; i< var.size();i++) { 
    // loop body
}

then the above may be transposed to something equivalent of

const size_t var_size = var.size();
for( int i = 0; i < var_size; i++ ) { 
    // loop body
}

However, I am not absolutely sure, so comments are welcome :)

Also,

• In most situations, the size() member function is inlined, so the issue does not warrant worrying

• The concern is perhaps equally applicable to the end() which is always used for iterator based looping, i.e. it != container.end()

• Please consider using size_t or vector::size_type for the type of i [See Steve Jessop's comment below.]