模板 Class 类型和功能的专业化添加

Question

I have been learning templates and templates specializations. I want to be able to add two template parameters together. I was able to do this with basic types, but could this be done with functions as well with the use of a lambda function and a class specialization.

The following code with not compile:

template<class R>
class Adder
{
public:
    template<R A, R B>
    static R Add() { return A + B; }
protected:
};

template<class R, class ...Args>
class Adder<std::function<R(Args...)>>
{
public:
    template<std::function<R(Args...)> A, std::function<R(Args...)> B>
    static std::function<R(Args...)> Add()
    {
        std::function<R(Args...)> Func = [](Args...) -> R { return A(Args...) + B(Args...); };
        return Func;
    }
protected:
};


double Test1(double x)
{
    return x;
}

int main()
{

    int a = Adder<int>::Add<2,2>();

    std::function<double(double)> Func = Adder<std::function<double(double)>>::Add<Test1, Test1>();

    return 0;
}

I get the following errors:

error C2993: 'std::function<double (double)>': is not a valid type for non-type template parameter 'A'
message : 'function<double __cdecl(double)>' is not a literal class type
message : see reference to class template instantiation 'Adder<std::function<double (double)>>' being compiled
error C2993: 'std::function<double (double)>': is not a valid type for non-type template parameter 'B'
message : 'function<double __cdecl(double)>' is not a literal class type
error C2672: 'Adder<std::function<double (double)>>::Add': no matching overloaded function found
error C2993: 'std::function<double (double)>': is not a valid type for non-type template parameter 'A'
error C2993: 'std::function<double (double)>': is not a valid type for non-type template parameter 'B'

Based on my limited mad man experience I would think this class would be possible, but I wasn't able to find much information on this specific topic. I'm sure the answer is in the error messages.

Answer 1

This will compile and works as expected... Adds the Types or Functions.

template<class R>
class Adder
{
public:
    template<R A, R B>
    static R Add() { return A + B; }
protected:
};

template<class R, class ...Args>
class Adder<std::function<R(Args...)>>
{
public:
    template<R Func1(Args...), R Func2(Args...)>
    static std::function<R(Args...)> Add()
    {
        return [](Args... Vars) -> R { return Func1(Vars...) + Func2(Vars...); };
    }
protected:
};

int Test1(int x)
{
    return x;
}

int main()
{
    int a1 = Adder<int>::Add<1, 1>();
    std::function<int(int)> F1 = Adder<std::function<int(int)>>::Add<Test1, Test1>();
    int V1 = F1(1);

    return 0;
}