C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 35:
35.1 What's the idea behind templates?
35.2 What's the syntax / semantics for a "class template"?
35.3 What's the syntax / semantics for a "function template"?
35.4 How do I explicitly select which version of a function template should get called?
35.5 What is a "parameterized type"?
35.6 What is "genericity"?
35.7 My template function does something special when the template type T is int or std::string; how do I write my template so it uses the special code when T is one of those specific types?
35.8 Huh? Can you provide an example of template specialization that doesn't use foo and bar?
35.9 But most of the code in my template function is the same; is there some way to get the benefits of template specialization without duplicating all that source code?
35.10 All those templates and template specializations must slow down my program, right?
35.11 So templates are overloading, right?
35.12 Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
35.13 How can I avoid linker errors with my template functions? Updated!
35.14 How does the C++ keyword export help with template linker errors? Updated!
35.15 How can I avoid linker errors with my template classes? Updated!
35.16 Why do I get linker errors when I use template friends?
35.17 How can any human hope to understand these overly verbose template-based error messages?
35.18 Why am I getting errors when my template-derived-class uses a nested type it inherits from its template-base-class?
35.19 Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?
35.20 Can the previous problem hurt me silently? Is it possible that the compiler will silently generate the wrong code?
35.21 How can I create a container-template that allows my users to supply the type of the underlying container that actually stores the values?
35.22 Follow-up to previous: can I pass in the underlying structure and the element-type separately?
35.23 Related: all those proxies must negatively reflect on the speed of my program. Don't they?
[35.7] My template function does something special when the template type T is int or std::string; how do I write my template so it uses the special code when T is one of those specific types?

Before showing how to do this, let's make sure you're not shooting yourself in the foot. Does the function's behavior appear different to your users? In other words, is the observable behavior different in some substantive way? If so, you're probably shooting yourself in the foot and you will probably confuse your users — you're probably better off using different functions with different names — don't use templates, don't use overloading. For example, if the code for int inserts something into a container and sorts the result, but the code for std::string removes something from a container and does not sort the result, those two functions ought not to be an overloaded pair — their observable behavior is different so they ought to have different names.

However if the function's observable behavior is consistent for all the T types with the differences limited to implementation details, then you can proceed. Let's proceed with an example of this (conceptual only; not C++):

template<typename T>
void foo(T const& x)
{
  switch (typeof(T)) {   conceptual only; not C++
    case int:
      ...   implementation details when T is int
      break;

    case std::string:
      ...   implementation details when T is std::string
      break;

    default:
      ...   implementation details when T is neither int nor std::string
      break;
  }
}
One way to implement the above is via template specialization. Instead of a switch-statement, you end up breaking up the code into separate functions. The first function is the default case — the code to be used when T is anything other than int or std::string:
template<typename T>
void foo(T const& x)
{
  ...   implementation details when T is neither int nor std::string
}
Next are the two specializations, first for the int case...
template<>
void foo<int>(int const& x)
{
  ...   implementation details when T is int
}
...and next for the std::string case...
template<>
void foo<std::string>(std::string const& x)
{
  ...   implementation details when T is std::string
}
That's it; you're done. The compiler will automagically select the correct specialization when it sees which T you are using.