Question

I would like a template class to automatically pick it’s parameter based on the type of class passed to the constructor.
objects already have types.

InputA a;
InputA a2;
InputB b;

these types should be understood by constructors

out<>(a) out; 

is

out<InputA>(a) out;

.

out<>(a2) out;

is

out<InputA>(a2) out; 

.

out<>(b) out;

is

out<InputB>(b) out;

it seems redundant to define the template argument (the policy) when this can be extracted from the type of the input parameter.

Answer 1

This is the same problem that happend with std::pair.
The standard workaround for this problem is to define a template factory (like std::make_pair()).

e.g.

template<class policy>
out<policy> make_out(policy p)
{
    return out<policy>(p);
}

auto o = make_out(a);