C++ - Invoking a constructor template explicitly inside a template class?

Question

I have a class template called ScalarClamped. An instance of this class has a user-determined value range, and whenever a value assigned to it is out of it's bounds, the value will be clamped into the user-determined value range, hence the name "ScalarClamped":

ScalarClamped<float> scalar__(75, 0, 100); // Parameters: current value (75), minimum allowed value (0), maximum allowed value(100).
scalar__ += 50;
std::cout << scalar__.value() << std::endl; // Output: 100.
scalar -= 150;
std::cout << scalar__.value() << std::endl; // Output: 0.

When implementing member function operator+, I ran into a problem. Here's an implementation of operator+ for one overload:

template<typename T>
ScalarClamped<T> ScalarClamped<T>::operator+(ScalarClamped const& scalar_clamped_){
    return ScalarClamped<T>::ScalarClamped<T&&, T const&, T const&>(_m_tValue + scalar_clamped_._m_tValue, _m_tValueMin, _m_tValueMax);
}

As you can see, I'm trying to invoke a template constructor. Here's it's implementation (work in progress, may contain bugs as well):

template<typename T>
template<typename TypeValue, typename TypeMin, typename TypeMax>
ScalarClamped<T>::ScalarClamped(TypeValue value_, TypeMin min_, TypeMax max_):
    // Initialization list:
    _m_tValue((std::is_lvalue_reference<TypeValue>::value) ? value_ : std::move(value_)),
    _m_tValueMax((std::is_lvalue_reference<TypeMax>::value) ? max_ : std::move(max_)),
    _m_tValueMin((std::is_lvalue_reference<TypeMin>::value) ? min_ : std::move(min_))
    // Function body:
    {
        Algorithm::clamp<T&, T const&, T const&>(_m_tValue, _m_tValueMin, _m_tValueMax);
    }

The main issue for me is that I'm unable to correctly call this constructor template. I get the following error message (shortened):

... error: dependent-name 'ScalarClamped<T>::ScalarClamped<T&&, const T&, const T&>' is parsed as a non-type, but instantiation yields a type|

Obviously I'm trying to invoke it the wrong way. How to invoke my constructor template appropriately?

Answer 1

It is impossible to provide explicit template arguments to constructor templates.

The types must be inferred:

template<typename T>
template<typename TypeValue, typename TypeMin, typename TypeMax>
ScalarClamped<T>::ScalarClamped(TypeValue&& value_, TypeMin&& min_, TypeMax&& max_):
    // Initialization list:
    _m_tValue(std::forward<TypeValue>(value_)),
    _m_tValueMax(std::forward<TypeMax>(max_)),
    _m_tValueMin(std::forward<TypeMin>(min_))
    // Function body:
    {
        Algorithm::clamp<TypeValue, TypeMin, TypeMax>(_m_tValue, _m_tValueMin, _m_tValueMax);
    }

template<typename T>
ScalarClamped<T>
ScalarClamped<T>::operator+(ScalarClamped const& scalar_clamped_) const
{
    return ScalarClamped<T>::ScalarClamped(
        _m_tValue + scalar_clamped_._m_tValue,
        _m_tValueMin,
        _m_tValueMax);
}