Is there a way to test that a default constructor does not exist?

Question

I am writing a test driver for a type this is explicitly supposed to not be default constructable. Is there any way to assert in my test driver that this is the case? I can verify manually via compilation errors, but I want something that will protect against future changes that may misguidedly add a default constructor.

Edit: I'm stuck in an environment with C++03. Keeping that in mind, are there any other options than is_default_constructable?

I don't get it. Why will verification via compilation errors *not* protect you against future changes? — Christian Hackl, Jul 08 '14 at 17:54
@ChristianHackl: It's difficult to automate the test, and check that compilation is failing for the right reason. — Mike Seymour, Jul 08 '14 at 17:56
What I mean is that I can cause an error by trying to use the default constructor to verify one does not exist. I obviously can't leave that error in the code. — GBleaney, Jul 08 '14 at 17:56
@ChristianHackl: He means he could _only_ test it manually, so the test would not be in his automated regression test suite and it would be easy to forget to re-run the manual test for future versions. — Lightness Races in Orbit, Jul 08 '14 at 18:29
@LightnessRacesinOrbit: Well, he could add a meta step to his tests and test compiler invocations. But as Mike Seymour said above, this would prove very complicated. — Christian Hackl, Jul 08 '14 at 18:41

score 11 · Accepted Answer · answered Jul 08 '14 at 17:54

11

You can use static_assert(!std::is_default_constructible<T>::value, "Boo");. Make sure to #include <type_traits>.

answered Jul 08 '14 at 17:54

Kerrek SB

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Makes me really wish I had access to C++11 :(. Is there any way to do this in C++03? – GBleaney Jul 08 '14 at 18:54
@GBleaney: Hm, maybe there's some kind of SFINAE trick with defaulted arguments that might work. Let me try. – Kerrek SB Jul 08 '14 at 18:59
@GBleaney: Try [this question](http://stackoverflow.com/questions/2733377/is-there-a-way-to-test-whether-a-c-class-has-a-default-constructor-other-than/2770326). – Kerrek SB Jul 08 '14 at 19:08

Is there a way to test that a default constructor does not exist?

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