Why did the range based 'for' loop specification change in C++17?

user_700825 0
Viewed 5359

I was looking over some ugly code (that was modifying the underlying sequence while iterating), and to explore the definition of the range-based for loop, I went to cppreference.

There I noticed something strange:

The range based for loop changed in C++17, but I do not see the reason for the change, and the code looks the same to me (just "refactored"). So the old one was:

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

The new one is

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

Why was this change made, and does it make any legal C++14 programs exhibit undefined behavior (UB) in C++17?

c++17 language-lawyer for-loop c++
2 Answers

Using

auto __begin = begin_expr, __end = end_expr;

requires both begin_expr and end_expr to return the same type. This means you cannot have a sentinel iterator type that is different from the beginning type. Using

auto __begin = begin_expr ;
auto __end = end_expr ;

fixes that issue while proving full backwards compatibility with C++14.

user_43424980

It is explained later in the "notes":

As of C++17, the types of the begin_expr and the end_expr do not have to be the same ...

and you can't have that with:

auto __begin = begin_expr, __end = end_expr;
user_856960
Related