This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1148. Copy elision and move construction of function parameters

[Voted into the WP at the November, 2010 meeting.]

It is unclear whether copy elision is permitted when returning a parameter of class type. If not, it should still be possible to move, rather than copy, the return value.

Suggested resolution: Amend paragraph 34 to explicitly exclude function parameters from copy elision. Amend paragraph 35 to include function parameters as eligible for move-construction.

Proposed resolution (September, 2010):

1. Change 11.4.5.3 [class.copy.ctor] paragraph 34 bullets 1 and 2 as follows:

• in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function's return value

• in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object (14.2 [except.throw]) can be omitted by constructing the automatic object directly into the exception object

Change 11.4.5.3 [class.copy.ctor] paragraph 35 as follows:

When the criteria for elision of a copy operation are met, or would be met save for the fact that the source object is a function parameter, and the object to be copied is designated by an lvalue, overload resolution...