Question

i am trying to learn templates in c++. Can someone please explain why sq() works and add() does not? How can add() be fixed? What is use of non type templates?

template <typename T>
inline T sq (const T& x) 
{
    return x*x;
}

template <int*>
inline int add(int* x)
{
   return (*x)*2;
}



int main() {
int x = 2;
cout<<"Square(2): "<<sq(2)<<" Add(2): "<<add(&x)<<endl;
return 0;
}

Even after modifying the above example as below, it would still Not work

template <typename T>
inline T sq (const T& x) 
{
    return x*x;
}

template <int>
inline int add(int x)
{
   return x+x;
}



int main() {
    cout<<"Square(2): "<<sq(2)<<" Add(2): "<<add(2)<<endl;
    return 0;
}

Answer 1

I’m not quite sure what you’re asking, but you could use a non-type template parameter to, for example, define a function template that adds any compile-time constant to its argument:

template <int N>
inline int add(int x)
{
    return x + N;
}

int main()
{
    std::cout << add<3>(2) << std::endl; // prints 5
    std::cout << add<4>(2) << std::endl; // prints 6
}

To answer your specific question about why sq(2) compiles but add(&x) doesn’t: type parameters of function templates can be deduced from the function arguments. So sq(2) is equivalent to sq<int>(2). Non-type parameters can’t be deduced, so you have to provide one. In the case of a pointer parameter, the argument must be a pointer to a variable with external linkage, so the following should compile:

int global;

int main() {
    int local = 2;
    std::cout << add<&global>(&local) << std::endl;
}