Question

12.8 Copying and moving class objects [class.copy] §31 and §32 say:

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

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 to select the constructor for the copy is first performed as if the object were designated by an rvalue.

Hence we can write:

unique_ptr<int> make_answer()
{
    unique_ptr<int> result(new int(42));
    return result;   // lvalue is implicitly treated as rvalue
}

However, I noticed that g++ 4.6.3 also accepts lvalues that are not names, for example:

    return (result);
    return *&result;
    return true ? result : result;

By contrast, return rand() ? result : result; does not work. Is the compiler’s optimizer interfering with the language semantics? As I interpret the standard, return (result); should not compile, because (result) is not a name, but a parenthesized expression. Am I right or wrong?

Answer 1

Regarding parenthesized expressions [√]

You are wrong when talking about parenthesized expressions and that it shouldn’t be able to trigger a move when being returned and containing only the name of a moveable object.

5.1.1/1      General      [expr.prim.general]

A parenthesized expression is a primary expression whose type and
value are identical to those of the enclosed expression. The
presence of parentheses does not affect whether the expression is an lvalue. The parenthesized expression can be used in exactly the same contexts as those where the enclosed expression can be used, and with the same meaning, except as otherwise indicated.

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Regarding the constexpr conditional operator [╳]

^{The way I interpret the standard in regards to constant-expressions and he coditional operator is that the use of return true ? result : result is well-behaved since it is a constant expression and therefore equivalent to return result;}

I have now gone through the standard more carefully and nowhere does it say that a constant conditional-expression is the same as if only the "returned" expression would have been written.

true ? <expr1> : <expr2>; // this is not the same as just writing <expr1>;

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Regarding return *&result; [╳]

In C99 it is explicitly stated that *&result is the exact equivalent of having written result instead, this is not the case in the C++ specification.

Though we can all agree on that using *&result will indeed yield the same lvalue as result, but according to the standard *&result (of course) isn’t an expression where "the expression is the name of a non-volatile automatic object".

Sure, the expression contains an appropriate name, but it’s not just only that.

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To sum things up…

return result; // #1, OK

return (result);                  // as described earlier, OK
return true ? result : result;    // as described earlier, ill-formed
return rand () ? result : result; // as described earlier, ill-formed
return *&result;                  // as described earlier, ill-formed