std::apply to constructor in a templated function

Question

I am writing generic code, and I need to call the constructor of a generic template parameter T with a generic variadic tuple of arguments:

T& init_and_return(ArgsTuple& args)
{
    m_data = std::apply(&T::T, args); // here compiler complains
    return m_data;
}

In my main, T will be a type called A. Compiler is saying "no member named T in A".

How can I refer to the constructor of T in a generic way?

You can't get a pointer to a constructor, whether the type is a template parameter or not. — interjay, Jul 04 '19 at 16:42
@interjay any workaround to use std::apply with constructor? — nyarlathotep108, Jul 04 '19 at 16:43
Here is how to construct an object from args tuple https://stackoverflow.com/questions/14897527/constructor-arguments-from-tuple — Renat, Jul 04 '19 at 16:43
`std::apply([](auto&&... args2) { return T{std::forward(args2)...}; }, args);` would be sufficient, but Yakk´s answer is even better. — super, Jul 04 '19 at 16:52

Yakk - Adam Nevraumont · Accepted Answer · 2019-07-04T17:25:46.600

15

The constructor is not a function or a method like other methods are -- it is special, and you cannot take its address. Personally I think it should be possible, but it isn't.

The C++ standard has make from tuple, which does what you want.

m_data = std::make_from_tuple<T>(args);

edited Jul 04 '19 at 17:25

answered Jul 04 '19 at 16:45

Yakk - Adam Nevraumont

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Could you please explain how constructor is not a function or method? – SolidMercury Jul 04 '19 at 17:03
`[class.mem.special]` states that "*The default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, and destructor are **special member functions**."*. That kinda contradicts with what you said, doesn't it? – Fureeish Jul 04 '19 at 17:06
3

@Fureeish Sure, that is technically correct, which is the best kind of correct. – Yakk - Adam Nevraumont Jul 04 '19 at 17:25

std::apply to constructor in a templated function

1 Answers1