How it works...

The expression for the range-based for loops shown earlier in the How to do it... section is basically syntactic sugar as the compiler transforms it into something else. Before C++17, the code generated by the compiler used to be the following:

    { 
      auto && __range = range_expression; 
      for (auto __begin = begin_expr, __end = end_expr; 
      __begin != __end; ++__begin) { 
        range_declaration = *__begin; 
        loop_statement 
      } 
    }

What begin_expr and end_expr are in this code depends on the type of the range:

• For C-like arrays: __range and __range + __bound (where __bound is the number of elements in the array)
• For a class type with begin and end members (regardless of their type and accessibility): __range.begin() and __range.end().
• For others it is begin(__range) and end(__range) that are determined via argument-dependent lookup.

It is important to note that if a class contains any members (function, data member, or enumerators) called begin or end, regardless of their type and accessibility, they will be picked for begin_expr and end_expr. Therefore, such a class type cannot be used in range-based for loops.

In C++17, the code generated by the compiler is slightly different:

    { 
      auto && __range = range_expression; 
      auto __begin = begin_expr; 
      auto __end = end_expr; 
      for (; __begin != __end; ++__begin) { 
        range_declaration = *__begin; 
        loop_statement 
      } 
    }

The new standard has removed the constraint that the begin expression and end expression must have the same type. The end expression does not need to be an actual iterator, but it has to be able to be compared for inequality with an iterator. A benefit of this is that the range can be delimited by a predicate.