3

I thought that you could only create a reference or pointer member to a forward-declared class. However, I was surprised to discover this works:

#include <vector>

struct Donkey;

struct Cage
{
    std::vector<Donkey> donkeys;
};

struct Donkey
{
};

int main()
{
    Cage c;
}

http://ideone.com/EP0zKR

How come std::vector can be defined with a forward-declared class? Is this standard?

Neil Kirk
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    Because the vector basically contains a pointer to the type (and size/capacity and maybe some other stuff), but the important part here is the pointer. You don't need the complete definition of a type to declare a pointer of the type, that's why it works. – Some programmer dude Mar 25 '15 at 17:27
  • internally std::vector has a pointer to donkey, so you're still only defining a reference or pointer member to a forward declared class. – Robinson Mar 25 '15 at 17:27
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    Template type parameters don't generally have to be complete types. However, this particular template requires that your type be complete, and your program is ill-formed. – Kerrek SB Mar 25 '15 at 17:29
  • Possible duplicate? https://stackoverflow.com/questions/18672135/why-c-containers-dont-allow-incomplete-types – Praetorian Mar 25 '15 at 17:30
  • @Praetorian: How is "why does vector allow its element to be an incomplete type" a _duplicate_ of "why does vector **not** allow its element to be an incomplete type"? lol. It could not be more opposite. – Lightness Races in Orbit Mar 25 '15 at 17:35
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    @LightnessRacesinOrbit Well I guess it does answer my question! – Neil Kirk Mar 25 '15 at 17:36
  • @LightnessRacesinOrbit "Opposite" only because Neil made an incorrect assumption and worded his question accordingly. I didn't use the dupe hammer because of that difference in wording. Quite clearly even you agree that the answer to that question answers this one too :) – Praetorian Mar 25 '15 at 17:48
  • @Praetorian: Yeah but I don't always agree that similar answers = duplicate question :) – Lightness Races in Orbit Mar 25 '15 at 17:49

2 Answers2

5

Actually, you can't.

Just because your program compiles (which is down to facts of the underlying implementation) does not mean it is valid.

There are times other than declaring a T* or a T& at which you may use a forward declaration; it's just that this is not one of them.

Community
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Lightness Races in Orbit
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  • Note that this is no longer true in C++17. [\[`std::vector`\] (but not its members) can be instantiated with an incomplete element type if the allocator satisfies the allocator completeness requirements](http://en.cppreference.com/w/cpp/container/vector) and [The default allocator satisfies allocator completeness requirements](http://en.cppreference.com/w/cpp/memory/allocator). – nwp Nov 14 '17 at 16:08
  • @nwp: Interesting. Without doing a deep dive on that change I'd be worried about relying on this before I'd cleared up in my head when the destructor (a member!) is and is not instantiated. – Lightness Races in Orbit Nov 15 '17 at 17:09
0

It's undefined behavior.

[C++14/§17.6.4.8] 1 In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ standard library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.

[C++14/§17.6.4.8] 2 In particular, the effects are undefined in the following cases: [...]

  • if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for that component.

Furthermore, an incomplete type doesn't meet the allocator requirements:

[C++14/§17.6.3.5] 9 An allocator may constrain the types on which it can be instantiated and the arguments for which its construct member may be called. If a type cannot be used with a particular allocator, the allocator class or the call to construct may fail to instantiate.

Because your class has a definition by the time the vector is instantiated, it's fine. Be on the lookout for this proposal which will change allocator requirements, N4056 Minimal incomplete type support for standard containers

user4713343
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