Three kinds of names can be used within a template definition:
The name of the template itself, and names declared within the template itself.
Names dependent on a template-parameter ([temp.dep]).
Names from scopes which are visible within the template definition.
A name used in a template declaration or definition and that is dependent on a template-parameter is assumed not to name a type unless the applicable name lookup finds a type name or the name is qualified by the keyword typename. [ Example:
class X;
template<class T> class Y {
class Z;
void f() {
X* a1; T* a2; Y* a3; Z* a4; typedef typename T::A TA;
TA* a5; typename T::A* a6; T::A* a7; B* a8; }
};
— end example ]
If a specialization of a template is instantiated for a set of template-arguments such that the qualified-id prefixed by typename does not denote a type or a class template, the specialization is ill-formed. The usual qualified name lookup is used to find the qualified-id even in the presence of typename. [ Example:
struct A {
struct X { };
int X;
};
struct B {
struct X { };
};
template<class T> void f(T t) {
typename T::X x;
}
void foo() {
A a;
B b;
f(b); f(a); }
— end example ]
If, for a given set of template arguments, a specialization of a template is instantiated that refers to a qualified-id that denotes a type or a class template, and the qualified-id refers to a member of an unknown specialization, the qualified-id shall either be prefixed by typename or shall be used in a context in which it implicitly names a type as described above. [ Example:
template <class T> void f(int i) {
T::x * i; }
struct Foo {
typedef int x;
};
struct Bar {
static int const x = 5;
};
int main() {
f<Bar>(1); f<Foo>(1); }
— end example ]
Within the definition of a class template or within the definition of a member of a class template following the declarator-id, the keyword typename is not required when referring to the name of a previously declared member of the class template that declares a type or a class template. [ Note: Such names can be found using unqualified name lookup, class member lookup ([class.qual]) into the current instantiation, or class member access expression lookup ([basic.lookup.classref]) when the type of the object expression is the current instantiation ([temp.dep.expr]). — end note ] [ Example:
template<class T> struct A {
typedef int B;
B b; };
— end example ]
Knowing which names are type names allows the syntax of every template to be checked. The program is ill-formed, no diagnostic required, if:
no valid specialization can be generated for a template or a substatement of a constexpr if statement within a template and the template is not instantiated, or
every valid specialization of a variadic template requires an empty template parameter pack, or
a hypothetical instantiation of a template immediately following its definition would be ill-formed due to a construct that does not depend on a template parameter, or
the interpretation of such a construct in the hypothetical instantiation is different from the interpretation of the corresponding construct in any actual instantiation of the template. [ Note: This can happen in situations including the following:
a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is performed, or
lookup for a name in the template definition found a using-declaration, but the lookup in the corresponding scope in the instantiation does not find any declarations because the using-declaration was a pack expansion and the corresponding pack is empty, or
an instantiation uses a default argument or default template argument that had not been defined at the point at which the template was defined, or
constant expression evaluation within the template instantiation uses
the value of a const object of integral or unscoped enumeration type or
the value of a constexpr object or
the value of a reference or
the definition of a constexpr function,
and that entity was not defined when the template was defined, or
a class template specialization or variable template specialization that is specified by a non-dependent simple-template-id is used by the template, and either it is instantiated from a partial specialization that was not defined when the template was defined or it names an explicit specialization that was not declared when the template was defined.
— end note ]
Otherwise, no diagnostic shall be issued for a template for which a valid specialization can be generated. [ Note: If a template is instantiated, errors will be diagnosed according to the other rules in this International Standard. Exactly when these errors are diagnosed is a quality of implementation issue. — end note ] [ Example:
int j;
template<class T> class X {
void f(T t, int i, char* p) {
t = i; p = i; p = j; }
void g(T t) {
+; }
};
template<class... T> struct A {
void operator++(int, T... t); };
template<class... T> union X : T... { }; template<class... T> struct A : T..., T... { };
— end example ]
When looking for the declaration of a name used in a template definition, the usual lookup rules ([basic.lookup.unqual], [basic.lookup.argdep]) are used for non-dependent names. The lookup of names dependent on the template parameters is postponed until the actual template argument is known ([temp.dep]). [ Example:
#include <iostream>
using namespace std;
template<class T> class Set {
T* p;
int cnt;
public:
Set();
Set<T>(const Set<T>&);
void printall() {
for (int i = 0; i<cnt; i++)
cout << p[i] << '\n';
}
};
in the example, i is the local variable i declared in printall, cnt is the member cnt declared in Set, and cout is the standard output stream declared in iostream. However, not every declaration can be found this way; the resolution of some names must be postponed until the actual template-arguments are known. For example, even though the name operator<< is known within the definition of printall() and a declaration of it can be found in <iostream>, the actual declaration of operator<< needed to print p[i] cannot be known until it is known what type T is ([temp.dep]). — end example ]
If a name does not depend on a template-parameter (as defined in [temp.dep]), a declaration (or set of declarations) for that name shall be in scope at the point where the name appears in the template definition; the name is bound to the declaration (or declarations) found at that point and this binding is not affected by declarations that are visible at the point of instantiation. [ Example:
void f(char);
template<class T> void g(T t) {
f(1); f(T(1)); f(t); dd++; }
enum E { e };
void f(E);
double dd;
void h() {
g(e); g('a'); }
— end example ]
[ Note: For purposes of name lookup, default arguments and noexcept-specifiers of function templates and default arguments and noexcept-specifiers of member functions of class templates are considered definitions ([temp.decls]). — end note ]
17.6.1 Locally declared names [temp.local]Within the scope of a class template specialization or partial specialization, when the injected-class-name is used as a type-name, it is equivalent to the template-name followed by the template-arguments of the class template specialization or partial specialization enclosed in <>. [ Example:
template<template<class> class T> class A { };
template<class T> class Y;
template<> class Y<int> {
Y* p; Y<char>* q; A<Y>* a; class B {
template<class> friend class Y; };
};
— end example ]
The injected-class-name of a class template or class template specialization can be used either as a template-name or a type-name wherever it is in scope. [ Example:
template <class T> struct Base {
Base* p;
};
template <class T> struct Derived: public Base<T> {
typename Derived::Base* p; };
template<class T, template<class> class U = T::template Base> struct Third { };
Third<Base<int> > t;
— end example ]
A lookup that finds an injected-class-name ([class.member.lookup]) can result in an ambiguity in certain cases (for example, if it is found in more than one base class). If all of the injected-class-names that are found refer to specializations of the same class template, and if the name is used as a template-name, the reference refers to the class template itself and not a specialization thereof, and is not ambiguous. [ Example:
template <class T> struct Base { };
template <class T> struct Derived: Base<int>, Base<char> {
typename Derived::Base b; typename Derived::Base<double> d; };
— end example ]
When the normal name of the template (i.e., the name from the enclosing scope, not the injected-class-name) is used, it always refers to the class template itself and not a specialization of the template. [ Example:
template<class T> class X {
X* p; X<T>* p2;
X<int>* p3;
::X* p4; };
— end example ]
A template-parameter shall not be redeclared within its scope (including nested scopes). A template-parameter shall not have the same name as the template name. [ Example:
template<class T, int i> class Y {
int T; void f() {
char T; }
};
template<class X> class X;
— end example ]
In the definition of a member of a class template that appears outside of the class template definition, the name of a member of the class template hides the name of a template-parameter of any enclosing class templates (but not a template-parameter of the member if the member is a class or function template). [ Example:
template<class T> struct A {
struct B { /* ... */ };
typedef void C;
void f();
template<class U> void g(U);
};
template<class B> void A<B>::f() {
B b; }
template<class B> template<class C> void A<B>::g(C) {
B b; C c; }
— end example ]
In the definition of a member of a class template that appears outside of the namespace containing the class template definition, the name of a template-parameter hides the name of a member of this namespace. [ Example:
namespace N {
class C { };
template<class T> class B {
void f(T);
};
}
template<class C> void N::B<C>::f(C) {
C b; }
— end example ]
In the definition of a class template or in the definition of a member of such a template that appears outside of the template definition, for each non-dependent base class ([temp.dep.type]), if the name of the base class or the name of a member of the base class is the same as the name of a template-parameter, the base class name or member name hides the template-parameter name. [ Example:
struct A {
struct B { /* ... */ };
int a;
int Y;
};
template<class B, class a> struct X : A {
B b; a b; };
— end example ]
17.6.2 Dependent names [temp.dep]Inside a template, some constructs have semantics which may differ from one instantiation to another. Such a construct depends on the template parameters. In particular, types and expressions may depend on the type and/or value of template parameters (as determined by the template arguments) and this determines the context for name lookup for certain names. An expressions may be type-dependent (that is, its type may depend on a template parameter) or value-dependent (that is, its value when evaluated as a constant expression ([expr.const]) may depend on a template parameter) as described in this subclause. In an expression of the form:
postfix-expression ( expression-listopt )
where the postfix-expression is an unqualified-id, the unqualified-id denotes a dependent name if
any of the expressions in the expression-list is a pack expansion,
any of the expressions or braced-init-lists in the expression-list is type-dependent, or
the unqualified-id is a template-id in which any of the template arguments depends on a template parameter.
If an operand of an operator is a type-dependent expression, the operator also denotes a dependent name. Such names are unbound and are looked up at the point of the template instantiation ([temp.point]) in both the context of the template definition and the context of the point of instantiation.
[ Example:
template<class T> struct X : B<T> {
typename T::A* pa;
void f(B<T>* pb) {
static int i = B<T>::i;
pb->j++;
}
};
the base class name B<T>, the type name T::A, the names B<T>::i and pb->j explicitly depend on the template-parameter. — end example ]
In the definition of a class or class template, the scope of a dependent base class is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member. [ Example:
typedef double A;
template<class T> class B {
typedef int A;
};
template<class T> struct X : B<T> {
A a; };
The type name A in the definition of X<T> binds to the typedef name defined in the global namespace scope, not to the typedef name defined in the base class B<T>. — end example ] [ Example:
struct A {
struct B { /* ... */ };
int a;
int Y;
};
int a;
template<class T> struct Y : T {
struct B { /* ... */ };
B b; void f(int i) { a = i; } Y* p; };
Y<A> ya;
The members A::B, A::a, and A::Y of the template argument A do not affect the binding of names in Y<A>. — end example ]
17.6.2.1 Dependent types [temp.dep.type]A name refers to the current instantiation if it is
in the definition of a class template, a nested class of a class template, a member of a class template, or a member of a nested class of a class template, the injected-class-name of the class template or nested class,
in the definition of a primary class template or a member of a primary class template, the name of the class template followed by the template argument list of the primary template (as described below) enclosed in <> (or an equivalent template alias specialization),
in the definition of a nested class of a class template, the name of the nested class referenced as a member of the current instantiation, or
in the definition of a partial specialization or a member of a partial specialization, the name of the class template followed by the template argument list of the partial specialization enclosed in <> (or an equivalent template alias specialization). If the nth template parameter is a parameter pack, the nth template argument is a pack expansion whose pattern is the name of the parameter pack.
The template argument list of a primary template is a template argument list in which the nth template argument has the value of the nth template parameter of the class template. If the nth template parameter is a template parameter pack, the nth template argument is a pack expansion whose pattern is the name of the template parameter pack.
A template argument that is equivalent to a template parameter (i.e., has the same constant value or the same type as the template parameter) can be used in place of that template parameter in a reference to the current instantiation. In the case of a non-type template argument, the argument must have been given the value of the template parameter and not an expression in which the template parameter appears as a subexpression. [ Example:
template <class T> class A {
A* p1; A<T>* p2; A<T*> p3; ::A<T>* p4; class B {
B* p1; A<T>::B* p2; typename A<T*>::B* p3; };
};
template <class T> class A<T*> {
A<T*>* p1; A<T>* p2; };
template <class T1, class T2, int I> struct B {
B<T1, T2, I>* b1; B<T2, T1, I>* b2; typedef T1 my_T1;
static const int my_I = I;
static const int my_I2 = I+0;
static const int my_I3 = my_I;
B<my_T1, T2, my_I>* b3; B<my_T1, T2, my_I2>* b4; B<my_T1, T2, my_I3>* b5; };
— end example ]
A dependent base class is a base class that is a dependent type and is not the current instantiation. [ Note: A base class can be the current instantiation in the case of a nested class naming an enclosing class as a base. [ Example:
template<class T> struct A {
typedef int M;
struct B {
typedef void M;
struct C;
};
};
template<class T> struct A<T>::B::C : A<T> {
M m; };
— end example ] — end note ]
A name is a member of the current instantiation if it is
An unqualified name that, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof. [ Note: This can only occur when looking up a name in a scope enclosed by the definition of a class template. — end note ]
A qualified-id in which the nested-name-specifier refers to the current instantiation and that, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof. [ Note: If no such member is found, and the current instantiation has any dependent base classes, then the qualified-id is a member of an unknown specialization; see below. — end note ]
An id-expression denoting the member in a class member access expression for which the type of the object expression is the current instantiation, and the id-expression, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof. [ Note: If no such member is found, and the current instantiation has any dependent base classes, then the id-expression is a member of an unknown specialization; see below. — end note ]
[ Example:
template <class T> class A {
static const int i = 5;
int n1[i]; int n2[A::i]; int n3[A<T>::i]; int f();
};
template <class T> int A<T>::f() {
return i; }
— end example ]
A name is a dependent member of the current instantiation if it is a member of the current instantiation that, when looked up, refers to at least one member of a class that is the current instantiation.
A name is a member of an unknown specialization if it is
A qualified-id in which the nested-name-specifier names a dependent type that is not the current instantiation.
A qualified-id in which the nested-name-specifier refers to the current instantiation, the current instantiation has at least one dependent base class, and name lookup of the qualified-id does not find any member of a class that is the current instantiation or a non-dependent base class thereof.
An id-expression denoting the member in a class member access expression in which either
the type of the object expression is the current instantiation, the current instantiation has at least one dependent base class, and name lookup of the id-expression does not find a member of a class that is the current instantiation or a non-dependent base class thereof; or
the type of the object expression is dependent and is not the current instantiation.
If a qualified-id in which the nested-name-specifier refers to the current instantiation is not a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the qualified-id is not instantiated; no diagnostic required. Similarly, if the id-expression in a class member access expression for which the type of the object expression is the current instantiation does not refer to a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the member access expression is not instantiated; no diagnostic required. [ Example:
template<class T> class A {
typedef int type;
void f() {
A<T>::type i; typename A<T>::other j; }
};
— end example ]
If, for a given set of template arguments, a specialization of a template is instantiated that refers to a member of the current instantiation with a qualified-id or class member access expression, the name in the qualified-id or class member access expression is looked up in the template instantiation context. If the result of this lookup differs from the result of name lookup in the template definition context, name lookup is ambiguous. [ Example:
struct A {
int m;
};
struct B {
int m;
};
template<typename T>
struct C : A, T {
int f() { return this->m; } int g() { return m; } };
template int C<B>::f(); template int C<B>::g();
— end example ]
A type is dependent if it is
a template parameter,
a member of an unknown specialization,
a nested class or enumeration that is a dependent member of the current instantiation,
a cv-qualified type where the cv-unqualified type is dependent,
a compound type constructed from any dependent type,
an array type whose element type is dependent or whose bound (if any) is value-dependent,
a function type whose exception specification is value-dependent,
a simple-template-id in which either the template name is a template parameter or any of the template arguments is a dependent type or an expression that is type-dependent or value-dependent or is a pack expansion [ Note: This includes an injected-class-name of a class template used without a template-argument-list. — end note ] , or
denoted by decltype(expression), where expression is type-dependent.
[ Note: Because typedefs do not introduce new types, but instead simply refer to other types, a name that refers to a typedef that is a member of the current instantiation is dependent only if the type referred to is dependent. — end note ]
17.6.2.2 Type-dependent expressions [temp.dep.expr]Except as described below, an expression is type-dependent if any subexpression is type-dependent.
this is type-dependent if the class type of the enclosing member function is dependent ([temp.dep.type]).
An id-expression is type-dependent if it contains
an identifier associated by name lookup with one or more declarations declared with a dependent type,
an identifier associated by name lookup with a non-type template-parameter declared with a type that contains a placeholder type,
an identifier associated by name lookup with one or more declarations of member functions of the current instantiation declared with a return type that contains a placeholder type,
an identifier associated by name lookup with a structured binding declaration whose brace-or-equal-initializer is type-dependent,
the identifier __func__, where any enclosing function is a template, a member of a class template, or a generic lambda,
a template-id that is dependent,
a conversion-function-id that specifies a dependent type, or
a nested-name-specifier or a qualified-id that names a member of an unknown specialization;
or if it names a dependent member of the current instantiation that is a static data member of type “array of unknown bound of T” for some T ([temp.static]). Expressions of the following forms are type-dependent only if the type specified by the type-id, simple-type-specifier or new-type-id is dependent, even if any subexpression is type-dependent:
simple-type-specifier ( expression-listopt ) ::opt new new-placementopt new-type-id new-initializeropt ::opt new new-placementopt ( type-id ) new-initializeropt dynamic_cast < type-id > ( expression ) static_cast < type-id > ( expression ) const_cast < type-id > ( expression ) reinterpret_cast < type-id > ( expression ) ( type-id ) cast-expression
A class member access expression is type-dependent if the expression refers to a member of the current instantiation and the type of the referenced member is dependent, or the class member access expression refers to a member of an unknown specialization. [ Note: In an expression of the form x.y or xp->y the type of the expression is usually the type of the member y of the class of x (or the class pointed to by xp). However, if x or xp refers to a dependent type that is not the current instantiation, the type of y is always dependent. If x or xp refers to a non-dependent type or refers to the current instantiation, the type of y is the type of the class member access expression. — end note ]
A braced-init-list is type-dependent if any element is type-dependent or is a pack expansion.
17.6.2.3 Value-dependent expressions [temp.dep.constexpr]Except as described below, an expression used in a context where a constant expression is required is value-dependent if any subexpression is value-dependent.
An id-expression is value-dependent if:
it is type-dependent,
it is the name of a non-type template parameter,
it names a static data member that is a dependent member of the current instantiation and is not initialized in a member-declarator,
it names a static member function that is a dependent member of the current instantiation, or
it is a constant with literal type and is initialized with an expression that is value-dependent.
Expressions of the following form are value-dependent if the unary-expression or expression is type-dependent or the type-id is dependent:
sizeof unary-expression sizeof ( type-id ) typeid ( expression ) typeid ( type-id ) alignof ( type-id ) noexcept ( expression )
[ Note: For the standard library macro offsetof, see [support.types]. — end note ]
An expression of the form &qualified-id where the qualified-id names a dependent member of the current instantiation is value-dependent. An expression of the form &cast-expression is also value-dependent if evaluating cast-expression as a core constant expression succeeds and the result of the evaluation refers to a templated entity that is an object with static or thread storage duration or a member function.
17.6.3 Non-dependent names [temp.nondep]Non-dependent names used in a template definition are found using the usual name lookup and bound at the point they are used. [ Example:
void g(double);
void h();
template<class T> class Z {
public:
void f() {
g(1); h++; }
};
void g(int);
— end example ]
17.6.4 Dependent name resolution [temp.dep.res]In resolving dependent names, names from the following sources are considered:
Declarations that are visible at the point of definition of the template.
Declarations from namespaces associated with the types of the function arguments both from the instantiation context ([temp.point]) and from the definition context.
For a function template specialization, a member function template specialization, or a specialization for a member function or static data member of a class template, if the specialization is implicitly instantiated because it is referenced from within another template specialization and the context from which it is referenced depends on a template parameter, the point of instantiation of the specialization is the point of instantiation of the enclosing specialization. Otherwise, the point of instantiation for such a specialization immediately follows the namespace scope declaration or definition that refers to the specialization.
If a function template or member function of a class template is called in a way which uses the definition of a default argument of that function template or member function, the point of instantiation of the default argument is the point of instantiation of the function template or member function specialization.
For a noexcept-specifier of a function template specialization or specialization of a member function of a class template, if the noexcept-specifier is implicitly instantiated because it is needed by another template specialization and the context that requires it depends on a template parameter, the point of instantiation of the noexcept-specifier is the point of instantiation of the specialization that requires it. Otherwise, the point of instantiation for such a noexcept-specifier immediately follows the namespace scope declaration or definition that requires the noexcept-specifier.
For a class template specialization, a class member template specialization, or a specialization for a class member of a class template, if the specialization is implicitly instantiated because it is referenced from within another template specialization, if the context from which the specialization is referenced depends on a template parameter, and if the specialization is not instantiated previous to the instantiation of the enclosing template, the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.
If a virtual function is implicitly instantiated, its point of instantiation is immediately following the point of instantiation of its enclosing class template specialization.
An explicit instantiation definition is an instantiation point for the specialization or specializations specified by the explicit instantiation.
The instantiation context of an expression that depends on the template arguments is the set of declarations with external linkage declared prior to the point of instantiation of the template specialization in the same translation unit.
A specialization for a function template, a member function template, or of a member function or static data member of a class template may have multiple points of instantiations within a translation unit, and in addition to the points of instantiation described above, for any such specialization that has a point of instantiation within the translation unit, the end of the translation unit is also considered a point of instantiation. A specialization for a class template has at most one point of instantiation within a translation unit. A specialization for any template may have points of instantiation in multiple translation units. If two different points of instantiation give a template specialization different meanings according to the one-definition rule, the program is ill-formed, no diagnostic required.
17.6.4.2 Candidate functions [temp.dep.candidate]For a function call where the postfix-expression is a dependent name, the candidate functions are found using the usual lookup rules ([basic.lookup.unqual], [basic.lookup.argdep]) except that:
For the part of the lookup using unqualified name lookup, only function declarations from the template definition context are found.
For the part of the lookup using associated namespaces ([basic.lookup.argdep]), only function declarations found in either the template definition context or the template instantiation context are found.
If the call would be ill-formed or would find a better match had the lookup within the associated namespaces considered all the function declarations with external linkage introduced in those namespaces in all translation units, not just considering those declarations found in the template definition and template instantiation contexts, then the program has undefined behavior.
17.6.5 Friend names declared within a class template [temp.inject]Friend classes or functions can be declared within a class template. When a template is instantiated, the names of its friends are treated as if the specialization had been explicitly declared at its point of instantiation.
As with non-template classes, the names of namespace-scope friend functions of a class template specialization are not visible during an ordinary lookup unless explicitly declared at namespace scope ([class.friend]). Such names may be found under the rules for associated classes ([basic.lookup.argdep]).140 [ Example:
template<typename T> struct number {
number(int);
friend number gcd(number x, number y) { return 0; };
};
void g() {
number<double> a(3), b(4);
a = gcd(a,b); b = gcd(3,4); }
— end example ]
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