temporary lifetime in range-for expression

Question

Consider a simple class A that can be used as a range:

struct A { 
    ~A() { std::cout << \"~A \"; }

    const char* begin() const {
            


        

Answer 1

The reason the lifetime of the temporary is not extended is how the standard defines range-based for loops in

6.5.4 The range-based for statement [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
   }
}

Note that auto && __range = range-init; would extend the lifetime of a temporary returned from range-init, but it does not extend the lifetime of nested temporaries inside of range-init.

This is IMHO a very unfortunate definition and was even discussed as Defect Report 900. It seems to be the only part of the standard where a reference is implicitly bound to extend the lifetime of an expressions result without extending the lifetime of nested temporaries.

The solution is to store a copy in the wrapper - which often defeats the purpose of the wrapper.