Template parameters

Every template is parameterized by one or more template parameters.

Each parameter in template-parameter-list (see template declaration syntax) belongs to one of the following categories:

• constant template parameter
• type template parameter
• template template parameter

[edit] Constant template parameter

Also known as non-type template parameter (see below).

A structural type is one of the following types (optionally cv-qualified, the qualifiers are ignored):

• lvalue reference type (to object or to function);
• an integral type;
• a pointer type (to object or to function);
• a pointer to member type (to member object or to member function);
• an enumeration type;

Array and function types may be written in a template declaration, but they are automatically replaced by pointer to object and pointer to function as appropriate.

When the name of a constant template parameter is used in an expression within the body of the class template, it is an unmodifiable prvalue unless its type was an lvalue reference type, or unless its type is a class type(since C++20).

A template parameter of the form class Foo is not an unnamed constant template parameter of type Foo, even if otherwise class Foo is an elaborated type specifier and class Foo x; declares x to be of type Foo.

struct A
{
    friend bool operator==(const A&, const A&) = default;
};
 
template<A a>
void f()
{
    &a;                       // OK
    const A& ra = a, &rb = a; // Both bound to the same template parameter object
    assert(&ra == &rb);       // passes
}

[edit] Type template parameter

template<class T>
class My_vector { /* ... */ };

template<class T = void>
struct My_op_functor { /* ... */ };

template<typename... Ts>
class My_tuple { /* ... */ };

template<My_concept T>
class My_constrained_vector { /* ... */ };

template<My_concept T = void>
class My_constrained_op_functor { /* ... */ };

template<My_concept... Ts>
class My_constrained_tuple { /* ... */ };

The name of the parameter is optional:

// Declarations of the templates shown above:
template<class>
class My_vector;
template<class = void>
struct My_op_functor;
template<typename...>
class My_tuple;

In the body of the template declaration, the name of a type parameter is a typedef-name which aliases the type supplied when the template is instantiated.

template<typename T>
concept C1 = true;
template<typename... Ts> // variadic concept
concept C2 = true;
template<typename T, typename U>
concept C3 = true;
 
template<C1 T>         struct s1; // constraint-expression is C1<T>
template<C1... T>      struct s2; // constraint-expression is (C1<T> && ...)
template<C2... T>      struct s3; // constraint-expression is (C2<T> && ...)
template<C3<int> T>    struct s4; // constraint-expression is C3<T, int>
template<C3<int>... T> struct s5; // constraint-expression is (C3<T, int> && ...)

[edit] Template template parameter

In the body of the template declaration, the name of this parameter is a template-name (and needs arguments to be instantiated).

template<typename T>
class my_array {};
 
// two type template parameters and one template template parameter:
template<typename K, typename V, template<typename> typename C = my_array>
class Map
{
    C<K> key;
    C<V> value;
};

[edit] Name resolution for template parameters

The name of a template parameter is not allowed to be redeclared within its scope (including nested scopes). A template parameter is not allowed to have the same name as the template name.

template<class T, int N>
class Y
{
    int T;      // error: template parameter redeclared
    void f()
    {
        char T; // error: template parameter redeclared
    }
};
 
template<class X>
class X; // error: template parameter redeclared

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.

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; // A's B, not the template parameter
}
 
template<class B>
template<class C>
void A<B>::g(C)
{
    B b; // A's B, not the template parameter
    C c; // the template parameter C, not A's C
}

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.

namespace N
{
    class C {};
 
    template<class T>
    class B
    {
        void f(T);
    };
}
 
template<class C>
void N::B<C>::f(C)
{
    C b; // C is the template parameter, not N::C
}

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, 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.

struct A
{
    struct B {};
    int C;
    int Y;
};
 
template<class B, class C>
struct X : A
{
    B b; // A's B
    C b; // error: A's C isn't a type name
};

[edit] Default template arguments

Default template arguments are specified in the parameter lists after the = sign. Defaults can be specified for any kind of template parameter (type, constant, or template), but not to parameter packs(since C++11).

If the default is specified for a template parameter of a primary class template, primary variable template,(since C++14) or alias template, each subsequent template parameter must have a default argument, except the very last one may be a template parameter pack(since C++11). In a function template, there are no restrictions on the parameters that follow a default, and a parameter pack may be followed by more type parameters only if they have defaults or can be deduced from the function arguments(since C++11).

Default parameters are not allowed

• in the out-of-class definition of a member of a class template (they have to be provided in the declaration inside the class body). Note that member templates of non-template classes can use default parameters in their out-of-class definitions (see GCC bug 53856)
• in friend class template declarations

Default template arguments that appear in the declarations are merged similarly to default function arguments:

template<typename T1, typename T2 = int> class A;
template<typename T1 = int, typename T2> class A;
 
// the above is the same as the following:
template<typename T1 = int, typename T2 = int> class A;

But the same parameter cannot be given default arguments twice in the same scope:

template<typename T = int> class X;
template<typename T = int> class X {}; // error

When parsing a default template argument for a constant template parameter, the first non-nested > is taken as the end of the template parameter list rather than a greater-than operator:

template<int i = 3 > 4>   // syntax error
class X { /* ... */ };
 
template<int i = (3 > 4)> // OK
class Y { /* ... */ };

The template parameter lists of template template parameters can have their own default arguments, which are only in effect where the template template parameter itself is in scope:

// class template, with a type template parameter with a default
template<typename T = float>
struct B {};
 
// template template parameter T has a parameter list, which
// consists of one type template parameter with a default
template<template<typename = float> typename T>
struct A
{
    void f();
    void g();
};
 
// out-of-body member function template definitions
 
template<template<typename TT> class T>
void A<T>::f()
{
    T<> t; // error: TT has no default in scope
}
 
template<template<typename TT = char> class T>
void A<T>::g()
{
    T<> t; // OK: t is T<char>
}

Member access for the names used in a default template parameter is checked at the declaration, not at the point of use:

class B {};
 
template<typename T>
class C
{
protected:
    typedef T TT;
};
 
template<typename U, typename V = typename U::TT>
class D: public U {};
 
D<C<B>>* d; // error: C::TT is protected

template<typename T, typename U = int>
struct S {};
 
S<bool>* p; // The default argument for U is instantiated at this point
            // the type of p is S<bool, int>*

[edit] Notes

Before C++26, constant template parameter were called non-type template parameter in the standard wording. The terminology was changed by P2841R6 / PR#7587.

template<typename>
concept C = true;
 
template<C,     // type parameter 
         C auto // constant parameter
        >
struct S{};
 
S<int, 0> s;

[edit] Examples

#include <array>
#include <iostream>
#include <numeric>
 
// simple constant template parameter
template<int N>
struct S { int a[N]; };
 
template<const char*>
struct S2 {};
 
// complicated constant example
template
<
    char c,             // integral type
    int (&ra)[5],       // lvalue reference to object (of array type)
    int (*pf)(int),     // pointer to function
    int (S<10>::*a)[10] // pointer to member object (of type int[10])
>
struct Complicated
{
    // calls the function selected at compile time
    // and stores the result in the array selected at compile time
    void foo(char base)
    {
        ra[4] = pf(c - base);
    }
};
 
//  S2<"fail"> s2;        // error: string literal cannot be used
    char okay[] = "okay"; // static object with linkage
//  S2<&okay[0]> s3;      // error: array element has no linkage
    S2<okay> s4;          // works
 
int a[5];
int f(int n) { return n; }
 
// C++20: NTTP can be a literal class type
template<std::array arr>
constexpr
auto sum() { return std::accumulate(arr.cbegin(), arr.cend(), 0); }
 
// C++20: class template arguments are deduced at the call site
static_assert(sum<std::array<double, 8>{3, 1, 4, 1, 5, 9, 2, 6}>() == 31.0);
// C++20: NTTP argument deduction and CTAD
static_assert(sum<std::array{2, 7, 1, 8, 2, 8}>() == 28);
 
int main()
{
    S<10> s; // s.a is an array of 10 int
    s.a[9] = 4;
 
    Complicated<'2', a, f, &S<10>::a> c;
    c.foo('0');
 
    std::cout << s.a[9] << a[4] << '\n';
}

42

[edit] Defect reports

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