Step 7 of 8

Template specialization

Sometimes one particular type needs different behavior from the general template. A full specialization provides a replacement definition for exactly that type, and the compiler picks the most specific match.

#include <iostream>
#include <string>

template <typename T>
struct Unit {
    static std::string symbol() { return ""; }
};

template <>
struct Unit<double> {
    static std::string symbol() { return " m"; }
};

template <>
struct Unit<std::string> {
    static std::string symbol() { return " (label)"; }
};

template <typename T>
void print_value(const T& v) {
    std::cout << v << Unit<T>::symbol() << "\n";
}

int main() {
    print_value(3.5);
    print_value(std::string("door"));
    print_value(7);
}

3.5 m
door (label)
7

How it works

• The general template is written first. Every specialization comes after it.
• template <> with empty brackets means "all the parameters are already filled in". Then struct Unit<double> names the exact type being specialized.
• A specialization is a completely separate definition. It doesn't inherit anything from the general version, so you must write every member you need.
• print_value uses Unit<T>::symbol(). For int there's no specialization, so the general version is used and returns an empty string.
• static member functions are called on the class itself, with ::, no object needed.

Where you'll see this

This is how the standard library lets you plug your own types into its templates. For example, to use your own struct as a key in std::unordered_map, you specialize std::hash<YourType>.

Your turn: add specializations of TypeName for double ("double"), bool ("bool") and std::string ("string").

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