Question

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template<> in c++

I have seen template<> in c++ code.

Is this valid syntax? If it is, what does it mean?

Answer 1

It’s used whenever you explicitly specialize a template (class or function template) or a member of a class template. The first set of examples use this class template and members:

template<typename T>
struct A {
  void f();

  template<typename U>
  void g();

  struct B {
    void h();
  };

  template<typename U>
  struct C {
    void h();
  };
};

---

Specializing and defining members

// define A<T>::f() */
template<typename T>
void A<T>::f() { 
}

// specialize member A<T>::f() for T = int */
template<>
void A<int>::f() {
}

// specialize member A<T>::g() for T = float
template<>
template<typename T>
void A<float>::g() { 
}

// specialize member A<T>::g for T = float and
// U = int
template<>
template<>
void A<float>::g<int>() {
}


// specialize A<T>::B for T = int
template<>
struct A<int>::B {
  /* different members may appear here! */
  void i();
};

/* defining A<int>::B::i. This is not a specialization, 
 * hence no template<>! */
void A<int>::B::i() {
}

/* specialize A<T>::C for T = int */
template<>
template<typename U>
struct A<int>::C {
  /* different members may appear here! */
  void i();
};

/* defining A<int>::C<U>::i. This is not a specialization, 
 * BUT WE STILL NEED template<>.
 * That's because some of the nested templates are still unspecialized.
 */
template<>
template<typename U>
void A<int>::C<U>::i() {
}

---

Example with specializing a non-member class template

template<typename T>
struct C {
  void h();
};

/* explicitly specialize 'C' */
template<>
struct C<int> {
  /* different members may appear here */ 
  void h(int);
};

/* define void C<int>::h(int). Not a specialization, hence no template<>! */
void C<int>::h(int) { 
}

/* define void C<T>::h(). */
template<typename T>
void C<T>::h() {
}