C++ - What is the purpose of function template specialization? When to use it?

Question

Learning C++, came upon function templates. The chapter mentioned template specialization.

1. template <> void foo<int>(int);

2. void foo( int );

Why specialize when you can use the second? I thought templates were suppose to generalize. What's the point of specializing a function for a specific data type when you can just use a regular function?

Obviously, template specialization exists for a reason. When should it be used? I read Sutter's "Why not Specialize..." article but I need more of a layman's version since I'm just learning this stuff.

Answer 1

The main difference is that in the first case you are providing the compiler with an implementation for the particular type, while in the second you are providing an unrelated non-templated function.

If you always let the compiler infer the types, non-templated functions will be preferred by the compiler over a template, and the compiler will call the free function instead of the template, so providing a non-templated function that matches the arguments will have the same effect of specializations in most cases.

On the other hand, if at any place you provide the template argument (instead of letting the compiler infer), then it will just call the generic template and probably produce unexpected results:

template <typename T> void f(T) { 
   std::cout << "generic" << std::endl; 
}
void f(int) { 
   std::cout << "f(int)" << std::endl; 
}
int main() {
   int x = 0;
   double d = 0.0;
   f(d); // generic
   f(x); // f(int)
   f<int>(x); // generic !! maybe not what you want
   f<int>(d); // generic (same as above)
}

If you had provided an specialization for int of the template, the last two calls would call that specialization and not the generic template.