ISO/IEC JTC1 SC22 WG21 P2360R0
Author: Jens Maurer
Target audience: EWG

P2360R0: Extend init-statement to allow alias-declaration


C++98 and onwards allowed to declare a local variable in a for loop:
  for (int i = 0; i < 10; ++i)
    /* something */;
The papers extended this facility such that additional variables can be declared in if, switch, and range-based for statements.

Grammatically, this is an init-statement, which allows expressions and variable declarations as well as typedefs, but (inconsistently) not alias-declarations. This paper proposes to also allow alias-declarations.

C++20this paper
  for (typedef int T; T e : v)
    /* something */;
  for (using T = int; T e : v)
    /* something */;


This is an obvious inconsistency. For modern code, it is generally recommended to use alias-declarations in lieu of typedefs (e.g. The example shown in the introduction is the only case where this fails.

The alternative would be to prohibit typedefs as an init-statement, but while unusual, they seem useful in some situations. After all, the argument of a reduced scope for variables introduced in an init-statement applies to typedefs as well.


No feature test macro is proposed. If a user cares about backward compatibility, a typedef should be written unconditionally. The argument for a #error check seems weak, since the compiler error message for an unrecognized alias-declaration is unconditional and should be clear.

This has not been implemented.


Change in 8.1 [stmt.pre] paragraph 1: