Different behaviour between std deque/vector in MSVCC/g++/icc

Question

I have this very simple piece of code;

#include <deque>
#include <vector>

using namespace std;

class A
{
public:
    A(){};
    ~A(){};
    deque<A> my_array; // vector<A> my_array;
};

int main(void)
{
}

If I compile this code with both g++ and icc/icpc on linux it compiles fine, even with -Wall it gives no warnings. If I swap the deque to a vector the situation is the same.

I would like to build this code on windows using MSVCC (cl) but unfortunately it throws error c2027:

error C2027: use of undefined type 'A'

If however I change the std::deque to a std::vector it compiles fine with Visual Studio 2010.

My question is; is this behaviour to be expected for some reason? If so, why are there differences between compilers or is this a mistake with either g++/icc or MSVCC?

Answer 1

It's undefined behavior (both with std::deque and with std::vector , so whatever an implementation does with it is fine, as far as the standard is concerned. You're instantiating a standard container with an incomplete type.

When compiling with g++, -Wall (and in general, all options starting with -W ) only concern the language. For library issues, you should be compiling with -D_GLIBCXX_CONCEPT_CHECKS -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC as well. (If this causes performance problems, you can remove the last two -D in optimized builds.)

Answer 2

Further to James Kanze's answer I searched around and found this Dr Dobbs article which explains the standard's stance on using containers with incomplete types.

Further it hints at that the reason it works with vector s but not deque s, namely the implementation. A typical vector may be something like

class vector<T> {
    T* buff;
    int size;
    // ... snip
};

which is okay with imcomplete types since we have only a pointer to T but deque could well be implemented in such a way (in VS2010) that it makes use of T by value thus rendering it incompatible with incomplete types.