'new auto' in C++ constructor

Question

Can C++ somehow accept this use of 'auto'?:

class A {
    public:
        A(): m_member(new auto)
        {
                [...]
        }

    private:
        BoringToTypeType *m_member;
}

The purpose is to take advantage of 'auto' by simplifying the member element initialisation in A's constructor. As it is, the code raises the following error:

new expression for type 'auto' requires a constructor argument.

Answer 1

new auto(...) deduces the type of the resultant pointer from the expression passed inside (...). In your particular case there's nothing that can be deduced.

You have a few options:

• m_member(new auto(x)), where x is an expression of type BoringToTypeType.

• m_member(new std::remove_pointer_t<decltype(m_member)>), which is most certainly not an improvement over BoringToTypeType.

If you don't mind defining an additional helper function, here's an alternative solution:

template <typename T, typename... TArgs>
auto newer(T*, TArgs&&... args)
{
    return new T(std::forward<TArgs>(args)...);
} 

class A 
{
    public:
        A(): m_member(newer(m_member, 12))
        {

        }

    private:
        int *m_member;
};

In this case T is used purely for type deduction purposes. m_member has to be repeated twice, but you avoid typing out its type this way.

Simple tests on godbolt.org show that newer does not incur any additional overhead.