17 Templates [temp]

template<class T> void f(std::initializer_list<T>);
f({1,2,3});                     // T deduced to int
f({1,"asdf"});                  // error: T deduced to both int and const char*

template<class T> void g(T);
g({1,2,3});                     // error: no argument deduced for T

template<class T, int N> void h(T const(&)[N]);
h({1,2,3});                     // T deduced to int, N deduced to 3

template<class T> void j(T const(&)[3]);
j({42});                        // T deduced to int, array bound not considered

struct Aggr { int i; int j; };
template<int N> void k(Aggr const(&)[N]);
k({1,2,3});                     // error: deduction fails, no conversion from int to Aggr
k({{1},{2},{3}});               // OK, N deduced to 3

template<int M, int N> void m(int const(&)[M][N]);
m({{1,2},{3,4}});               // M and N both deduced to 2

template<class T, int N> void n(T const(&)[N], T);
n({{1},{2},{3}},Aggr());        // OK, T is Aggr, N is 3

template<class ... Types> void f(Types& ...);
template<class T1, class ... Types> void g(T1, Types ...);
template<class T1, class ... Types> void g1(Types ..., T1);

void h(int x, float& y) {
  const int z = x;
  f(x, y, z);                   // Types is deduced to int, float, const int
  g(x, y, z);                   // T1 is deduced to int; Types is deduced to float, int
  g1(x, y, z);                  // error: Types is not deduced
  g1<int, int, int>(x, y, z);   // OK, no deduction occurs
}

template<class T> int f(const T&);
int n1 = f(5);                  // calls f<int>(const int&)
const int i = 0;
int n2 = f(i);                  // calls f<int>(const int&)
template <class T> int g(volatile T&);
int n3 = g(i);                  // calls g<const int>(const volatile int&)

template <class T> int f(T&& heisenreference);
template <class T> int g(const T&&);
int i;
int n1 = f(i);                  // calls f<int&>(int&)
int n2 = f(0);                  // calls f<int>(int&&)
int n3 = g(i);                  // error: would call g<int>(const int&&), which
                                // would bind an rvalue reference to an lvalue

template <class T> struct A {
  template <class U>
    A(T&&, U&&, int*);          // #1: T&& is not a forwarding reference.
                                // U&& is a forwarding reference.
  A(T&&, int*);                 // #2
};

template <class T> A(T&&, int*) -> A<T>;    // #3: T&& is a forwarding reference.

int *ip;
A a{i, 0, ip};                  // error: cannot deduce from #1
A a0{0, 0, ip};                 // uses #1 to deduce A<int> and #1 to initialize
A a2{i, ip};                    // uses #3 to deduce A<int&> and #2 to initialize

// Only one function of an overload set matches the call so the function parameter is a deduced context.
template <class T> int f(T (*p)(T));
int g(int);
int g(char);
int i = f(g);       // calls f(int (*)(int))

// Ambiguous deduction causes the second function parameter to be a non-deduced context.
template <class T> int f(T, T (*p)(T));
int g(int);
char g(char);
int i = f(1, g);    // calls f(int, int (*)(int))

// The overload set contains a template, causing the second function parameter to be a non-deduced context.
template <class T> int f(T, T (*p)(T));
char g(char);
template <class T> T g(T);
int i = f(1, g);    // calls f(int, int (*)(int))

  template <class T> struct Z {
    typedef typename T::x xx;
  };
  template <class T> typename Z<T>::xx f(void *, T);    // #1
  template <class T> void f(int, T);                    // #2
  struct A {} a;
  int main() {
    f(1, a);        // OK, deduction fails for #1 because there is no conversion from int to void*
  }