Question

Reading various questions here on Stack Overflow about C++ iterators and performance**, I started wondering if for(auto& elem : container) gets “expanded” by the compiler into the best possible version? (Kind of like auto, which the compiler infers into the right type right away and is therefore never slower and sometimes faster).

** For example, does it matter if you write

for(iterator it = container.begin(), eit = container.end(); it != eit; ++it)

or

for(iterator it = container.begin(); it != container.end(); ++it)

for non-invalidating containers?

Answer 1

The Standard is your friend, see [stmt.ranged]/1

For a range-based for statement of the form

for ( for-range-declaration : expression ) statement

let range-init be equivalent to the expression surrounded by parentheses

( expression )

and for a range-based for statement of the form

for ( for-range-declaration : braced-init-list ) statement

let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

{
  auto && __range = range-init;
  for ( auto __begin = begin-expr,
             __end = end-expr;
        __begin != __end;
        ++__begin )
  {
    for-range-declaration = *__begin;
    statement
  }
}

So yes, the Standard guarantees that the best possible form is achieved.

And for a number of containers, such as vector, it is undefined behavior to modify (insert/erase) them during this iteration.