11

If you compile this program with a C++11 compiler, the vector is not moved out of the function.

#include <vector> using namespace std; vector<int> create(bool cond) { vector<int> a(1); vector<int> b(2); return cond ? a : b; } int main() { vector<int> v = create(true); return 0; }

If you return the instance like this, it is moved.

if(cond) return a; else return b;

Here is a demo on ideone.

I tried it with gcc 4.7.0 and MSVC10. Both behave the same way.
My guess why this happens is this:
The ternary operators type is an lvalue because it is evaluated before return statement is executed. At this point a and b are not yet xvalues (soon to expire).
Is this explanation correct?

Is this a defect in the standard?
This is clearly not the intended behaviour and a very common case in my opinion.

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2
  • "If the second and third operands are glvalues of the same value category and have the same type, the result is of that type and value category [...]" §5.16/4 Commented Aug 26, 2012 at 18:28
  • Neither example involves rvalues or xvalues. But it's an interesting question why a copy is done instead of a move. Commented Aug 26, 2012 at 18:31

2 Answers 2

Reset to default
8

Here are the relevant Standard quotes:

12.8 paragraph 32:

Copy elision is permitted in the following circumstances [...]

  • 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 throwing, with conditions]
  • [when the source is a temporary, with conditions]
  • [when catching by value, with conditions]

paragraph 33:

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. If overload resolution fails, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object's type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue. [Note: This two-stage overload resolution must be performed regardless of whether copy elision will occur. It determines the constructor to be called if elision is not performed, and the selected constructor must be accessible even if the call is elided. - end note]

Since the expression in return (cond ? a : b); is not a simple variable name, it's not eligible for copy elision or rvalue treatment. Maybe a bit unfortunate, but it's easy to imagine stretching the example a little bit further at a time until you create a headache of an expectation for compiler implementations.

You can of course get around all this by explicitly saying to std::move the return value when you know it's safe.

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2 Comments

So are you saying that paragraph 33 is the only thing in the standard by which an lvalue can be moved to a return value, and hence the fact that paragraph 33 doesn't apply to this return statement (on account of it not being eligible for elision and not being a function parameter), means that it can't be moved? Not that I disbelieve this, I'm just not sure I've followed the reasoning.
@SteveJessop: That sounds right. Without 12.8p32-33, the only correct behavior would be to copy to the return value. With p32 and without p33, the implementation can choose between copying or eliding the copy. With both p32 and p33 (reality), the implementation can choose instead between moving or eliding the move. The implementation never gets to choose between copying and moving: overload resolution and this particular exception spell out which happens if not elided.
8

This will fix it

return cond ? std::move(a) : std::move(b);

Consider the ternary operator as a function, like your code is

return ternary(cond, a, b);

The parameters won't be moved implicitly, you need to make it explicit.

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3 Comments

return std::move( cond ? a : b ); also works. Is that equivalent?
@Walter, this is a correct answer. I think. Is there an error in aschepler's answer?
Doesn't this impede NRVO?

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