Should (in C++11) std::vector::resize(size_type) work for the default constructible value_type int[4]?

Question

In C++11, there are two versions of std::vector::resize():

void resize( size_type count );
void resize( size_type count, const value_type& v         


        

Answer 1

The requirement (23.3.6.3:10) on vector.resize(n) being well-formed is that T should be CopyInsertable, i.e. that the following should be well-formed (23.2.1:13):

allocator_traits::construct(m, p, v);

where A is the allocator type of the vector, m is the allocator, p is of type T * and v is of type T.

As you can discover from 20.6.8.2:5, this is invalid for array types in the general case as it is equivalent to calling

::new(static_cast(p))block(v);

which is invalid for array types (arrays cannot be initialized by parentheses).

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Actually, you're correct that g++ has a bug; it should always be possible to work around the issue with CopyInsertable by providing an appropriate allocator, but g++ fails to allow this:

#include 

template struct ArrayAllocator: std::allocator {
    void construct(T (*p)[n], T (&v)[n]) {
        for (int i = 0; i < n; ++i)
            ::new(static_cast(p + i)) T{v[i]};
    }
};

int main() {
    std::vector> c;
    c.resize(100);  // fails

    typedef ArrayAllocator A;
    A m;
    int (*p)[4] = 0, v[4];
    std::allocator_traits::construct(m, p, v); // works
}

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Another bug is in the standard itself; 20.9.4.3:3 specifies std::is_default_constructible as equivalent to std::is_constructible, where 20.9.4.3:6 specifies std::is_constructible as the well-formedness criterion on T t(std::declval()...), which is valid for array types (as @Johannes Schaub-litb points out, array types can be initialised with (zero-pack-expansion)). However, 17.6.3.1:2 requires for DefaultConstructible in addition that T() be well-formed, which is not the case for an array type T but is not checked by std::is_default_constructible.