ISO/IEC JTC1 SC22 WG21 P2360R0
Author: Jens Maurer
Target audience: EWG
2021-04-13

P2360R0: Extend init-statement to allow alias-declaration

Introduction

C++98 and onwards allowed to declare a local variable in a for loop:

  for (int i = 0; i < 10; ++i)
    /* something */;

The papers

P0305R1 Selection statements with initializer and
P0614R1 Range-based for statements with initializer

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++20 this paper

for (typedef int T; T e : v) /* something */;

for (using T = int; T e : v) /* something */;

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

Rationale

This is an obvious inconsistency. For modern code, it is generally recommended to use alias-declarations in lieu of typedefs (e.g. https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rt-using). 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.

Miscellanea

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.

Wording

Change in 8.1 [stmt.pre] paragraph 1:

init-statement:
    expression-statement
    simple-declaration
    alias-declaration