Why does this c++ template need reference?

Question

I try to print the type using specilization, but this doesn't work.

template<typename T>
struct print_type {
    static constexpr char const value[] = "unknown";
};

template<>
struct print_type<void> {
    static constexpr char const value[] = "void";
};

template<>
struct print_type<int> {
    static constexpr char const value[] = "int";
};

int main() {
    cout <<  print_type<void>::value << endl;
}

The compiler shows:

Undefined symbols for architecture x86_64:
  "print_type<void>::value", referenced from:
      _main in main.cpp.o
ld: symbol(s) not found for architecture x86_64