P2360R0: Extend init-statement to allow alias-declaration (original) (raw)

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

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-basedfor statements.

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

Rationale

This is an obvious inconsistency. For modern code, it is generally recommended to use _alias-declaration_s 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