CODEWITHSUNDEEP
c++templatescasting
someCoder
someCoder

Reputation: 185

Is it possible to cast template parameters on runtime?

Given the following templated method

template <typename T>
T max(T x, T y)
{
    return (x > y)? x : y;
}

I want the following call to work:

int main()
{
    cout << max(3, 7.0) << std::endl; //compiler error
}

I understand that the compiler fails due to the fact that I'm passing 2 different datatypes while my template only accepts 2 parameters with same type. The easiest solution would be using polymorphism and creating a new template method that uses type T and X instead of 2 types T but I want to ask you if there's a way to make some sort of cast of parameter y to the type of parameter x without modifying the call in main()

Upvotes: 1

Views: 519

Answers (3)

walnut
walnut

Reputation: 22152

It is not exactly clear from your question what type you expect T to be in this scenario, but if you want it to be e.g. the type of the first argument, then you can make the second parameter a non-deduced context:

template <typename T>
T max(T x, std::type_identity<T>::type y)
{
    return (x > y)? x : y;
}

std::type_identity is a C++20 feature but its implementation is really simple:

template<typename T>
struct type_identity {
    using type = T;
};

Everything left of the scope resolution operator in the parameter is non-deduced, so the whole parameter is non-deduced context and will not participate in template argument deduction. Instead normal implicit conversions will be applied in overload resolution.

Upvotes: 1

n314159
n314159

Reputation: 5095

Yes, you can do that by creating another template method:

template<class T, class U>
T max(T x, U y) {
    return max(x, static_cast<T>(y));
}

Note: If it's just about max, use std::max from the STL.

Upvotes: 0

Alex.-
Alex.-

Reputation: 87

that fails because of template's type deduction which fails to deduce between int and double, you could either explicitly specify the template type, use two template parameters or cast it inside to T with static_cast

Upvotes: 1

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