Template argument deduction is done by comparing each function template parameter type (call it P) that contains template-parameters that participate in template argument deduction with the type of the corresponding argument of the call (call it A) as described below. If removing references and cv-qualifiers from P gives std::initializer_list<P'> or P'[N] for some P' and N and the argument is a non-empty initializer list ([dcl.init.list]), then deduction is performed instead for each element of the initializer list, taking P' as a function template parameter type and the initializer element as its argument, and in the P'[N] case, if N is a non-type template parameter, N is deduced from the length of the initializer list. Otherwise, an initializer list argument causes the parameter to be considered a non-deduced context ([temp.deduct.type]). [ Example:
template<class T> void f(std::initializer_list<T>);
f({1,2,3}); f({1,"asdf"});
template<class T> void g(T);
g({1,2,3});
template<class T, int N> void h(T const(&)[N]);
h({1,2,3});
template<class T> void j(T const(&)[3]);
j({42});
struct Aggr { int i; int j; };
template<int N> void k(Aggr const(&)[N]);
k({1,2,3}); k({{1},{2},{3}});
template<int M, int N> void m(int const(&)[M][N]);
m({{1,2},{3,4}});
template<class T, int N> void n(T const(&)[N], T);
n({{1},{2},{3}},Aggr());
— end example ] For a function parameter pack that occurs at the end of the parameter-declaration-list, deduction is performed for each remaining argument of the call, taking the type P of the declarator-id of the function parameter pack as the corresponding function template parameter type. Each deduction deduces template arguments for subsequent positions in the template parameter packs expanded by the function parameter pack. When a function parameter pack appears in a non-deduced context ([temp.deduct.type]), the type of that parameter pack is never deduced. [ Example:
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); g(x, y, z); g1(x, y, z); g1<int, int, int>(x, y, z); }
— end example ]
If P is not a reference type:
If A is an array type, the pointer type produced by the array-to-pointer standard conversion is used in place of A for type deduction; otherwise,
If A is a function type, the pointer type produced by the function-to-pointer standard conversion is used in place of A for type deduction; otherwise,
If A is a cv-qualified type, the top-level cv-qualifiers of A's type are ignored for type deduction.
If P is a cv-qualified type, the top-level cv-qualifiers of P's type are ignored for type deduction. If P is a reference type, the type referred to by P is used for type deduction. [ Example:
template<class T> int f(const T&); int n1 = f(5); const int i = 0; int n2 = f(i); template <class T> int g(volatile T&); int n3 = g(i);
— end example ] A forwarding reference is an rvalue reference to a cv-unqualified template parameter that does not represent a template parameter of a class template (during class template argument deduction ([over.match.class.deduct])). If P is a forwarding reference and the argument is an lvalue, the type “lvalue reference to A” is used in place of A for type deduction. [ Example:
template <class T> int f(T&& heisenreference);
template <class T> int g(const T&&);
int i;
int n1 = f(i); int n2 = f(0); int n3 = g(i);
template <class T> struct A {
template <class U>
A(T&&, U&&, int*); A(T&&, int*); };
template <class T> A(T&&, int*) -> A<T>;
int *ip;
A a{i, 0, ip}; A a0{0, 0, ip}; A a2{i, ip};
— end example ]
In general, the deduction process attempts to find template argument values that will make the deduced A identical to A (after the type A is transformed as described above). However, there are three cases that allow a difference:
If the original P is a reference type, the deduced A (i.e., the type referred to by the reference) can be more cv-qualified than the transformed A.
The transformed A can be another pointer or pointer to member type that can be converted to the deduced A via a function pointer conversion and/or qualification conversion.
If P is a class and P has the form simple-template-id, then the transformed A can be a derived class of the deduced A. Likewise, if P is a pointer to a class of the form simple-template-id, the transformed A can be a pointer to a derived class pointed to by the deduced A.
These alternatives are considered only if type deduction would otherwise fail. If they yield more than one possible deduced A, the type deduction fails. [ Note: If a template-parameter is not used in any of the function parameters of a function template, or is used only in a non-deduced context, its corresponding template-argument cannot be deduced from a function call and the template-argument must be explicitly specified. — end note ]
When P is a function type, function pointer type, or pointer to member function type:
If the argument is an overload set containing one or more function templates, the parameter is treated as a non-deduced context.
If the argument is an overload set (not containing function templates), trial argument deduction is attempted using each of the members of the set. If deduction succeeds for only one of the overload set members, that member is used as the argument value for the deduction. If deduction succeeds for more than one member of the overload set the parameter is treated as a non-deduced context.
[ Example:
template <class T> int f(T (*p)(T)); int g(int); int g(char); int i = f(g);
— end example ]
[ Example:
template <class T> int f(T, T (*p)(T)); int g(int); char g(char); int i = f(1, g);
— end example ]
[ Example:
template <class T> int f(T, T (*p)(T)); char g(char); template <class T> T g(T); int i = f(1, g);
— end example ]
If deduction succeeds for all parameters that contain template-parameters that participate in template argument deduction, and all template arguments are explicitly specified, deduced, or obtained from default template arguments, remaining parameters are then compared with the corresponding arguments. For each remaining parameter P with a type that was non-dependent before substitution of any explicitly-specified template arguments, if the corresponding argument A cannot be implicitly converted to P, deduction fails. [ Note: Parameters with dependent types in which no template-parameters participate in template argument deduction, and parameters that became non-dependent due to substitution of explicitly-specified template arguments, will be checked during overload resolution. — end note ] [ Example:
template <class T> struct Z {
typedef typename T::x xx;
};
template <class T> typename Z<T>::xx f(void *, T); template <class T> void f(int, T); struct A {} a;
int main() {
f(1, a); }
— end example ]
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4