Default constructor prevents from calling emplace_back

Question

It seems that adding a default constructor prevents from calling emplace_back and produces the error message: "static assertion failed: type is not assignable" (gcc 5.3 with -std=c++14). Here is a simple code that illustrates the issue:

class A {
public:
    int a;
    A() = default;
    A(int a) {
        this->a = a;
    }
    A(A const & a) = delete;
    A& operator =(A const & a) = delete;
    A(A && a) = default;
    A& operator =(A && a) = default;
};

int main() {

    A a(4);
    std::vector<A> vec;
    vec.emplace_back(std::move(a)); // Error: type is not assignable
    return 0;
}

When removing the default constructor, the error goes away! Also, if the default constructor is defined (even if it does nothing), the error also goes away:

class A {
public:
    int a;
    A() {
    }
    A(int a) {
        this->a = a;
    }
    A(A const & a) = delete;
    A& operator =(A const & a) = delete;
    A(A && a) = default;
    A& operator =(A && a) = default;
};

int main() {

    A b;
    A a(4);
    std::vector<A> vec;
    vec.emplace_back(std::move(a)); // Error gone
    return 0;
}

It seems that "A() = default;" is what is causing the problem. Is this normal behaviour on part of the compiler or is it a bug?

Answer 1

It's a libstdc++ bug (edit: reported as bug 69478).

Briefly, libstdc++'s std::vector, as relevant here, uses std::uninitialized_copy (paired with move iterators) to move elements on reallocation, which is reduced to std::copy if the type is trivial and the iterators' reference types are assignable (i.e., the assignment operator that would conceptually be used is usable).

Then, the std::copy for pointers to trivial types (or in our case, a move_iterator wrapping a pointer) is in turn optimized into a call to memmove coupled with a check for is_copy_assignable. Of course, that check is wrong in this case, since the uninitialized_copy, paired with move iterators, only requires the thing to be move constructible.

When you don't have a default constructor or if the default constructor is user-defined, then the class isn't trivial, so you don't hit the code path that triggers this bug.