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);

Answer 1

One case for template specialization which is not possible with overloading is for template meta-programming. The following is real code from a library that provides some of it services at compile time.

namespace internal{namespace os{
    template  std::ostream& get();

    struct stdout{};
    struct stderr{};

    template <> inline std::ostream& get() { return std::cout; }
    template <> inline std::ostream& get() { return std::cerr; }
}}

// define a specialization for os::get()
#define DEFINE_FILE(ofs_name,filename)\
    namespace internal{namespace os{                        \
        struct ofs_name{                                    \
            std::ofstream ofs;                              \
            ofs_name(){ ofs.open(filename);}                        \
            ~ofs_name(){ ofs.close(); delete this; }                    \
        };                                          \
        template <> inline std::ostream& get(){ return (new ofs_name())->ofs; }   \
    }}                                              \
    using internal::os::ofs_name;