Dependent names

Inside the definition of a template (both class template and function template

template<typename T>
struct X : B<T> // “B<T>” is dependent on T
{
    typename T::A* pa; // “T::A” is dependent on T
                       // (see below for the meaning of this use of “typename”)
 
    void f(B<T>* pb)
    {
        static int i = B<T>::i; // “B<T>::i” is dependent on T
        pb->j++; // “pb->j” is dependent on T
    }
};

Name lookup and binding are different for dependent names and non-dependent names.

Binding rules

Non-dependent names are looked up and bound at the point of template definition. This binding holds even if at the point of template instantiation there is a better match:

#include <iostream>
 
void g(double) { std::cout << "g(double)\n"; }
 
template<class T>
struct S
{
    void f() const
    {
        g(1); // “g” is a non-dependent name, bound now
    }
};
 
void g(int) { std::cout << "g(int)\n"; }
 
int main()
{
    g(1);  // calls g(int)
 
    S<int> s;
    s.f(); // calls g(double)
}

If the meaning of a non-dependent name changes between the definition context and the point of instantiation of a specialization of the template, the program is ill-formed, no diagnostic required. This is possible in the following situations:

• a type used in a non-dependent name is incomplete at the point of definition but complete at the point of instantiation

• an instantiation uses a default argument or default template argument that had not been defined at the point of definition
• a constant expression at the point of instantiation uses the value of a const object of integral or unscoped enum type, the value of a constexpr object, the value of a reference, or the definition of a constexpr function(since C++11), and that object/reference/function(since C++11)
• the template uses a non-dependent class template specialization or variable template specialization (since C++14)

Binding of dependent names is postponed until lookup takes place.

Lookup rules

The lookup of a dependent name used in a template is postponed until the template arguments are known, at which time

• non-ADL lookup examines function declarations with external linkage that are visible from the template definition context
• ADL examines function declarations with external linkage that are visible from either the template definition context or the template instantiation context

(in other words, adding a new function declaration after template definition does not make it visible, except via ADL).

The purpose of this rule is to help guard against violations of the ODR for template instantiations:

// an external library
namespace E
{
    template<typename T>
    void writeObject(const T& t)
    {
        std::cout << "Value = " << t << '\n';
    }
}
 
// translation unit 1:
// Programmer 1 wants to allow E::writeObject to work with vector<int>
namespace P1
{
    std::ostream& operator<<(std::ostream& os, const std::vector<int>& v)
    {
        for (int n : v)
            os << n << ' ';
        return os;
    }
 
    void doSomething()
    {
        std::vector<int> v;
        E::writeObject(v); // Error: will not find P1::operator<<
    }
}
 
// translation unit 2:
// Programmer 2 wants to allow E::writeObject to work with vector<int>
namespace P2
{
    std::ostream& operator<<(std::ostream& os, const std::vector<int>& v)
    {
        for (int n : v)
            os << n << ':';
        return os << "[]";
    }
 
    void doSomethingElse()
    {
        std::vector<int> v;
        E::writeObject(v); // Error: will not find P2::operator<<
    }
}

In the above example, if non-ADL lookup for operator<< were allowed from the instantiation context, the instantiation of E::writeObject<vector<int>> would have two different definitions: one using P1::operator<< and one using P2::operator<<

To make ADL examine a user-defined namespace, either std::vector

namespace P1
{
    // if C is a class defined in the P1 namespace
    std::ostream& operator<<(std::ostream& os, const std::vector<C>& v)
    {
        for (C n : v)
            os << n;
        return os;
    }
 
    void doSomething()
    {
        std::vector<C> v;
        E::writeObject(v); // OK: instantiates writeObject(std::vector<P1::C>)
                           //     which finds P1::operator<< via ADL
    }
}

Note: this rule makes it impractical to overload operators for standard library types:

#include <iostream>
#include <iterator>
#include <utility>
#include <vector>
 
// Bad idea: operator in global namespace, but its arguments are in std::
std::ostream& operator<<(std::ostream& os, std::pair<int, double> p)
{
    return os << p.first << ',' << p.second;
}
 
int main()
{
    typedef std::pair<int, double> elem_t;
    std::vector<elem_t> v(10);
    std::cout << v[0] << '\n'; // OK, ordinary lookup finds ::operator<<
    std::copy(v.begin(), v.end(),
              std::ostream_iterator<elem_t>(std::cout, " "));
    // Error: both ordinary lookup from the point of definition of
    // std::ostream_iterator and ADL will only consider the std namespace,
    // and will find many overloads of std::operator<<, so the lookup will be done.
    // Overload resolution will then fail to find operator<< for elem_t
    // in the set found by the lookup.
}

Note: limited lookup (but not binding) of dependent names also takes place at template definition time, as needed to distinguish them from non-dependent names and also to determine whether they are members of the current instantiation or members of unknown specialization. The information obtained by this lookup can be used to detect errors, see below.

Dependent types

The following types are dependent types :

• template parameter
• a member of an unknown specialization (see below)
• a nested class/enum that is a dependent member of unknown specialization (see below)
• a cv-qualified version of a dependent type
• a compound type constructed from a 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 template-id where either

• the template name is a template parameter, or
• any of template arguments is type-dependent, or value-dependent, or is a pack expansion(since C++11) (even if the template-id is used without its argument list, as injected-class-name

Note: a typedef member of a current instantiation is only dependent when the type it refers to is.

Type-dependent expressions

The following expressions are type-dependent :

• an expression whose any subexpression is a type-dependent expression
• this, if the class is a dependent type.
• an identifier expression that is not a concept-id and(since C++20)

• contains an identifier for which name lookup finds at least one dependent declaration
• contains a dependent template-id

• contains the name of conversion function to a dependent type
• contains a nested name specifier or qualified-id that is a member of unknown specialization
• names a dependent member of the current instantiation which is a static data member of type "array of unknown bound"

• any cast expression to a dependent type
• new expression that creates an object of a dependent type
• member access expression that refers to a member of the current instantiation whose type is dependent
• member access expression that refers to a member of unknown specialization

The following expressions are never type-dependent because the types of these expressions cannot be:

• literals
• pseudo-destructor calls
• sizeof

• throw
• typeid
• delete

Value-dependent expressions

The following expressions are value-dependent :

• an expression used in context where constant expression is required, and whose any subexpression is value-dependent
• an identifier expression that satisfies any of the following conditions:

• It is type-dependent.
• It is a 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.
• It names a static member function that is a dependent member of the current instantiation.
• It is a constant with a integer or enumeration(until C++11) literal(since C++11)

• the following expressions where the operand is a type-dependent expression:

• sizeof
• typeid

• the following expressions where the operand is a dependent type-id:

• sizeof
• typeid

• the following expressions where the target type is dependent or the operand is a type-dependent expression:

• C-style cast
• static_cast
• const_cast
• reinterpret_cast
• dynamic_cast

• function-style cast expression where the target type is dependent or a value-dependent expression is enclosed by parenthesesor braces(since C++11)

• address-of expression where the argument is a qualified identifier that names a dependent member of the current instantiation
• address-of expression where the argument is any expression which, evaluated as a core constant expression, refers to a templated entity that is an object with static or thread storage(since C++11)

Dependent names

Current instantiation

Within a class template definition (including its member functions and nested classes) some names may be deduced to refer to the current instantiation. This allows certain errors to be detected at the point of definition, rather than instantiation, and removes the requirement on the typename and template

Only the following names can refer to the current instantiation:

• 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 template argument list (or an equivalent alias template specialization) for the primary template where each argument is equivalent (defined below) to its corresponding parameter.
• in the definition of a nested class of a class template:
  • the name of the nested class used as a member of the current instantiation
• in the definition of a class template partial specialization or a member of a class template partial specialization:
  • the name of the class template followed by template argument list for the partial specialization, where each argument is equivalent to its corresponding parameter
• in the definition of a templated function:
  • the name of a local class

A template argument is equivalent to a template parameter if

• for a type parameter, the template argument denotes the same type as the template parameter.
• for a non-type parameter, the template argument is an identifier

• it has the same type as the template parameter (ignoring cv-qualification) and
• its initializer consists of a single identifier that names the template parameter or, recursively, such a variable.

template<class T>
class A
{
    A* p1;      // A is the current instantiation
    A<T>* p2;   // A<T> is the current instantiation
    ::A<T>* p4; // ::A<T> is the current instantiation
    A<T*> p3;   // A<T*> is not the current instantiation
 
    class B
    {
        B* p1;                 // B is the current instantiation
        A<T>::B* p2;           // A<T>::B is the current instantiation
        typename A<T*>::B* p3; // A<T*>::B is not the current instantiation
    };
};
 
template<class T>
class A<T*>
{
    A<T*>* p1; // A<T*> is the current instantiation
    A<T>* p2;  // A<T> is not the current instantiation
};
 
template<int I>
struct B
{
    static const int my_I = I;
    static const int my_I2 = I + 0;
    static const int my_I3 = my_I;
    static const long my_I4 = I;
    static const int my_I5 = (I);
 
    B<my_I>* b1;  // B<my_I> is the current instantiation:
                  //   my_I has the same type as I,
                  //   and it is initialized with only I
    B<my_I2>* b2; // B<my_I2> is not the current instantiation:
                  //   I + 0 is not a single identifier
    B<my_I3>* b3; // B<my_I3> is the current instantiation:
                  //   my_I3 has the same type as I,
                  //   and it is initialized with only my_I (which is equivalent to I)
    B<my_I4>* b4; // B<my_I4> is not the current instantiation:
                  //   the type of my_I4 (long) is not the same as the type of I (int)
    B<my_I5>* b5; // B<my_I5> is not the current instantiation:
                  //   (I) is not a single identifier
};

Note that a base class can be the current instantiation if a nested class derives from its enclosing class template. Base classes that are dependent types but are not the current instantiation are dependent base classes:

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; // OK, A<T>::M
};

A name is classified as a member of the current instantiation if it is

• an unqualified name that is found by unqualified lookup in the current instantiation or in its non-dependent base.
• qualified name, if the qualifier (the name to the left of ::
• a name used in a class member access expression (y in x.y or xp->y), where the object expression (x or *xp

template<class T>
class A
{
    static const int i = 5;
 
    int n1[i];       // i refers to a member of the current instantiation
    int n2[A::i];    // A::i refers to a member of the current instantiation
    int n3[A<T>::i]; // A<T>::i refers to a member of the current instantiation
 
    int f();
};
 
template<class T>
int A<T>::f()
{
    return i; // i refers to a member of the current instantiation
}

Members of the current instantiation may be both dependent and non-dependent.

If the lookup of a member of current instantiation gives a different result between the point of instantiation and the point of definition, the lookup is ambiguous. Note however that when a member name is used, it is not automatically converted to a class member access expression, only explicit member access expressions indicate members of current instantiation:

struct A { int m; };
struct B { int m; };
 
template<typename T>
struct C : A, T
{
    int f() { return this->m; } // finds A::m in the template definition context
    int g() { return m; }       // finds A::m in the template definition context
};
 
template int C<B>::f(); // error: finds both A::m and B::m
 
template int C<B>::g(); // OK: transformation to class member access syntax
                        // does not occur in the template definition context

Unknown specializations

Within a template definition, certain names are deduced to belong to an unknown specialization, in particular,

• a qualified name, if any name that appears to the left of :: is a dependent type that is not a member of the current instantiation
• a qualified name
• a name of a member in a class member access expression (the y in x.y or xp->y), if the type of the object expression (x or *xp
• a name of a member in a class member access expression (the y in x.y or xp->y), if the type of the object expression (x or *xp

template<typename T>
struct Base {};
 
template<typename T>
struct Derived : Base<T>
{
    void f()
    {
        // Derived<T> refers to current instantiation
        // there is no “unknown_type” in the current instantiation
        // but there is a dependent base (Base<T>)
        // Therefore, “unknown_type” is a member of unknown specialization
        typename Derived<T>::unknown_type z;
    }
};
 
template<>
struct Base<int> // this specialization provides it
{
    typedef int unknown_type;
};

This classification allows the following errors to be detected at the point of template definition (rather than instantiation):

• If any template definition has a qualified name

template<class T>
class A
{
    typedef int type;
 
    void f()
    {
        A<T>::type i; // OK: “type” is a member of the current instantiation
        typename A<T>::other j; // Error:
 
        // “other” is not a member of the current instantiation
        // and it is not a member of an unknown specialization
        // because A<T> (which names the current instantiation),
        // has no dependent bases for “other” to hide in.
    }
};

• If any template definition has a member access expression where the object expression is the current instantiation, but the name is neither a member of current instantiation nor a member of unknown specialization, the program is ill-formed even if the template is never instantiated.

Members of unknown specialization are always dependent, and are looked up and bound at the point of instantiation as all dependent names (see above)

The typename disambiguator for dependent names

In a declaration or a definition of a template, including alias template, a name that is not a member of the current instantiation and is dependent on a template parameter is not considered to be a type unless the keyword typename

#include <iostream>
#include <vector>
 
int p = 1;
 
template<typename T>
void foo(const std::vector<T> &v)
{
    // std::vector<T>::const_iterator is a dependent name,
    typename std::vector<T>::const_iterator it = v.begin();
 
    // without “typename”, the following is parsed as multiplication
    // of the type-dependent data member “const_iterator”
    // and some variable “p”. Since there is a global “p” visible
    // at this point, this template definition compiles.
    std::vector<T>::const_iterator* p;
 
    typedef typename std::vector<T>::const_iterator iter_t;
    iter_t * p2; // “iter_t” is a dependent name, but it is known to be a type name
}
 
template<typename T>
struct S
{
    typedef int value_t; // member of current instantiation
 
    void f()
    {
        S<T>::value_t n{}; // S<T> is dependent, but “typename” not needed
        std::cout << n << '\n';
    }
};
 
int main()
{
    std::vector<int> v;
    foo(v); // template instantiation fails: there is no member variable
            // called “const_iterator” in the type std::vector<int>
    S<int>().f();
}

The keyword typename may only be used in this way before qualified names (e.g. T::x

Usual qualified name lookup is used for the identifier prefixed by typename. Unlike the case with elaborated type specifier

struct A // A has a nested variable X and a nested type struct X
{
    struct X {};
    int X;
};
 
struct B
{
    struct X {}; // B has a nested type struct X
};
 
template<class T>
void f(T t)
{
    typename T::X x;
}
 
void foo()
{
    A a;
    B b;
    f(b); // OK: instantiates f<B>, T::X refers to B::X
    f(a); // error: cannot instantiate f<A>:
          // because qualified name lookup for A::X finds the data member
}

The keyword typename can be used even outside of templates.

#include <vector>
 
int main()
{
    // Both OK (after resolving CWG 382)
    typedef typename std::vector<int>::const_iterator iter_t;
    typename std::vector<int> v;
}

The template disambiguator for dependent names

Similarly, in a template definition, a dependent name that is not a member of the current instantiation is not considered to be a template name unless the disambiguation keyword template

template<typename T>
struct S
{
    template<typename U>
    void foo() {}
};
 
template<typename T>
void bar()
{
    S<T> s;
    s.foo<T>();          // error: < parsed as less than operator
    s.template foo<T>(); // OK
}

The keyword template may only be used in this way after operators :: (scope resolution), -> (member access through pointer), and .

• T::template foo<X>();
• s.template foo<X>();
• this- > template foo<X> ( ) ;
• typename T:: template iterator< int > :: value_type v;

As is the case with typename, the template

Even if the name to the left of :: refers to a namespace, the template disambiguator is allowed:

template<typename>
struct S {};
 
::template S<void> q; // allowed, but unnecessary

template<int>
struct A { int value; };
 
template<class T>
void f(T t)
{
    t.A<0>::value; // Ordinary lookup of A finds a class template.
                   // A<0>::value names member of class A<0>
    // t.A < 0;    // Error: “<” is treated as the start of template argument list
}

Keywords

template, typename

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.