13 Templates [temp]

template<class T> class B { /* ... */ };
template<class T> class D : public B<T> { /* ... */ };

void f(void*);
void f(B<int>*);

void g(D<int>* p, D<char>* pp, D<double>* ppp) {
  f(p);             // instantiation of D<int> required: call f(B<int>*)
  B<char>* q = pp;  // instantiation of D<char> required: convert D<char>* to B<char>*
  delete ppp;       // instantiation of D<double> required
}

template<class T> class X;
X<char> ch;         // error: incomplete type X<char>

template<class T>
struct C {
  void f() { T x; }
  void g() = delete;
};
C<void> c;                      // OK, definition of C<void>​::​f is not instantiated at this point
template<> void C<int>::g() { } // error: redefinition of C<int>​::​g

template<class T, class U>
struct Outer {
  template<class X, class Y> struct Inner;
  template<class Y> struct Inner<T, Y>;         // #1a
  template<class Y> struct Inner<T, Y> { };     // #1b; OK: valid redeclaration of #1a
  template<class Y> struct Inner<U, Y> { };     // #2
};

Outer<int, int> outer;                          // error at #2

template<typename T> struct Friendly {
  template<typename U> friend int f(U) { return sizeof(T); }
};
Friendly<char> fc;
Friendly<float> ff;                             // error: produces second definition of f(U)

template<class T> struct Z {
  void f();
  void g();
};

void h() {
  Z<int> a;         // instantiation of class Z<int> required
  Z<char>* p;       // instantiation of class Z<char> not required
  Z<double>* q;     // instantiation of class Z<double> not required

  a.f();            // instantiation of Z<int>​::​f() required
  p->g();           // instantiation of class Z<char> required, and
                    // instantiation of Z<char>​::​g() required
}

template<typename T> constexpr int f() { return T::value; }
template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
template<bool B, typename T> void g(...);
template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
template<bool B, typename T> void h(...);
void x() {
  g<false, int>(0); // OK, B ? f<T>() : 0 is not potentially constant evaluated
  h<false, int>(0); // error, instantiates f<int> even though B evaluates to false and
                    // list-initialization of int from int cannot be narrowing
}

template <class T> struct S {
  operator int();
};

void f(int);
void f(S<int>&);
void f(S<float>);

void g(S<int>& sr) {
  f(sr);            // instantiation of S<int> allowed but not required
                    // instantiation of S<float> allowed but not required
};

namespace N {
  template<class T> class List {
  public:
    T* get();
  };
}

template<class K, class V> class Map {
public:
  N::List<V> lt;
  V get(K);
};

void g(Map<const char*,int>& m) {
  int i = m.get("Nicholas");
}

template<class T> void f(T x, T y = ydef(T()), T z = zdef(T()));

class  A { };

A zdef(A);

void g(A a, A b, A c) {
  f(a, b, c);       // no default argument instantiation
  f(a, b);          // default argument z = zdef(T()) instantiated
  f(a);             // error: ydef is not declared
}

template<class T> class X {
  X<T>* p;          // OK
  X<T*> a;          // implicit generation of X<T> requires
                    // the implicit instantiation of X<T*> which requires
                    // the implicit instantiation of X<T**> which …
};

template<typename T> concept C = sizeof(T) > 2;
template<typename T> concept D = C<T> && sizeof(T) > 4;

template<typename T> struct S {
  S() requires C<T> { }         // #1
  S() requires D<T> { }         // #2
};

S<char> s1;                     // error: no matching constructor
S<char[8]> s2;                  // OK, calls #2

template<typename T> struct S1 {
  template<typename U>
    requires false
  struct Inner1;                // ill-formed, no diagnostic required
};

template<typename T> struct S2 {
  template<typename U>
    requires (sizeof(T[-(int)sizeof(T)]) > 1)
  struct Inner2;                // ill-formed, no diagnostic required
};