Template parameters and template arguments

Every template is parametrized by one or more template parameters, indicated in the parameter-list of the template declaration syntax:


`template` `<` parameter-list `>` declaration	(1)

Each parameter in parameter-list may be:

a non-type template parameter;
a type template parameter;
a template template parameter.

Non-type template parameter


type name(optional)	(1)

type name(optional) `=` default	(2)

type `...` name(optional)	(3)	(since C++11)

`auto` name	(4)	(since C++17)

1) A non-type template parameter with an optional name.

2) A non-type template parameter with an optional name and a default value.

3) A non-type template parameter pack with an optional name.

4) A non-type template parameter with a deduced type (note, it may also be auto ** or any other type that includes the placeholder auto )

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

std::nullptr_t (since C++11);
integral type;
lvalue reference type (to object or to function);
pointer type (to object or to function);
pointer to member type (to member object or to member function);
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 non-type 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.

A template parameter of the form class Foo is not an unnamed non-type 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.

The type of a non-type template parameter may be deduced if it includes the placeholder keyword auto:

template<auto n> struct B { /* ... */ };
B<5> b1;   // OK: non-type template parameter type is int
B<'a'> b2; // OK: non-type template parameter type is char
B<2.5> b3; // error: non-type template parameter type cannot be double

(since C++17)

Type template parameter


`typename` name(optional)	(1)

`class` name(optional)	(2)

`typename\|class` name(optional) `=` default	(3)

`typename\|class` `...` name(optional)	(4)	(since C++11)

1) A type template parameter with an optional name.

2) Exactly the same as 1).

3) A type template parameter with an optional name and a default.

4) A type template parameter pack with an optional name.

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.

There is no difference between the keywords class and typename in a type template parameter declaration.

Template template parameter


`template` `<` parameter-list `>` `typename(C++17)\|class` name(optional)	(1)

`template` `<` parameter-list `>` `typename(C++17)\|class` name(optional) `=` default	(2)

`template` `<` parameter-list `>` `typename(C++17)\|class` `...` name(optional)	(3)	(since C++11)

1) A template template parameter with an optional name.

2) A template template parameter with an optional name and a default.

3) A template template parameter pack with an optional name.

Unlike type template parameter declaration, template template parameter declaration can only use the keyword class and not typename.

(until C++17)

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;
};

Template arguments

In order for a template to be instantiated, every template parameter (type, non-type, or template) must be replaced by a corresponding template argument. For class templates, the arguments are either explicitly provided, deduced from the initializer, (since C++17) or defaulted. For function templates, the arguments are explicitly provided, deduced from the context, or defaulted.

If an argument can be interpreted as a both a type-id and an expression, it is always interpreted as a type-id, even if the corresponding template parameter is non-type:

template<class T> void f(); // #1
template<int I> void f(); // #2
void g() {
    f<int()>(); // "int()" is both a type and an expression,
                // calls #1 because it is interpreted as a type
}

Template non-type arguments

The following limitations apply when instantiating templates that have non-type template parameters:

For integral and arithmetic types, the template argument provided during instantiation must be a converted constant expression of the template parameter's type (so certain implicit conversion applies).
For pointers to objects, the template arguments have to designate the address of an object with static storage duration and a linkage (either internal or external), or a constant expression that evaluates to the appropriate null pointer or std::nullptr_t value.
For pointers to functions, the valid arguments are pointers to functions with linkage (or constant expressions that evaluate to null pointer values).
For lvalue reference parameters, the argument provided at instantiation cannot be a temporary, an unnamed lvalue, or a named lvalue with no linkage (in other words, the argument must have linkage).
For pointers to members, the argument has to be a pointer to member expressed as &Class::Member or a constant expression that evaluates to null pointer or std::nullptr_t value.

In particular, this implies that string literals, addresses of array elements, and addresses of non-static members cannot be used as template arguments to instantiate templates whose corresponding non-type template parameters are pointers to objects.

(until C++17)

The template argument that can be used with a non-type template parameter can be any converted constant expression of the type of the template parameter.

template<const int* pci> struct X {};
int ai[10];
X<ai> xi;  // ok: array to pointer conversion and cv-qualification conversion
 
struct Y {};
template<const Y& b> struct Z {};
Y y;
Z<y> z;  // ok: no conversion
 
template<int (&pa)[5]> struct W {};
int b[5];
W<b> w; // ok: no conversion
 
void f(char);
void f(int);
template<void (*pf)(int)> struct A {};
A<&f> a; // ok: overload resolution selects f(int)

The only exceptions are that non-type template parameters of reference and pointer type cannot refer to/be the address of

a subobject (including non-static class member, base subobject, or array element);
a temporary object (including one created during reference initialization);
a string literal;
the result of typeid;
or the predefined variable __func__.

template<class T, const char* p> class X {};
X<int, "Studebaker"> x1; // error: string literal as template-argument
 
template<int* p> class X {};
int a[10];
struct S
{
    int m;
    static int s;
} s;
X<&a[2]> x3;  // error: address of array element
X<&s.m> x4;   // error: address of non-static member
X<&s.s> x5;   // ok: address of static member
X<&S::s> x6;  // ok: address of static member
 
template<const int& CRI> struct B {};
B<1> b2;     // error: temporary would be required for template argument
int c = 1;
B<c> b1;     // ok

(since C++17)

Template type arguments

A template argument for a type template parameter must be a type-id, which may name an incomplete type:

template<typename T> class X {}; // class template
 
struct A; // incomplete type
typedef struct {} B; // type alias to an unnamed type
 
int main()
{
    X<A> x1; // ok: 'A' names a type
    X<A*> x2; // ok: 'A*' names a type
    X<B> x3; // ok: 'B' names a type
}

Template template arguments

A template argument for a template template parameter must be an id-expression which names a class template or a template alias.

When the argument is a class template, only the primary template is considered when matching the parameter. The partial specializations, if any, are only considered when a specialization based on this template template parameter happens to be instantiated.

template<typename T> class A { int x; }; // primary template
template<class T> class A<T*> { long x; }; // partial specialization
 
// class template with a template template parameter V
template<template<typename> class V> class C
{
    V<int> y; // uses the primary template
    V<int*> z; // uses the partial specialization
};
 
C<A> c; // c.y.x has type int, c.z.x has type long

To match a template template argument A to a template template parameter P, each of the template parameters of A must match corresponding template parameters of P exactly (until C++14)P must be at least as specialized as A (since C++14). If P's parameter list includes a parameter pack, zero or more template parameters (or parameter packs) from A's template parameter list are matched by it.

template<typename T> struct eval; // primary template 
 
template<template<typename, typename...> class TT, typename T1, typename... Rest>
struct eval<TT<T1, Rest...>> {}; // partial specialization of eval
 
template<typename T1> struct A;
template<typename T1, typename T2> struct B;
template<int N> struct C;
template<typename T1, int N> struct D;
template<typename T1, typename T2, int N = 17> struct E;
 
eval<A<int>> eA; // ok: matches partial specialization of eval
eval<B<int, float>> eB; // ok: matches partial specialization of eval
eval<C<17>> eC; // error: C does not match TT in partial specialization because
                // TT's first parameter is a type template parameter,
                // while 17 does not name a type
eval<D<int, 17>> eD; // error: D does not match TT in partial specialization
                     // because TT's second parameter is a type parameter pack,
                     // while 17 does not name a type
eval<E<int, float>> eE; // error: E does not match TT in partial specialization
                        // because E's third (default) parameter is a non-type

template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };
template <class ...Types> class C { /* ... */ };
 
template<template<class> class P> class X { /* ... */ };
X<A> xa; // OK
X<B> xb; // OK in C++14 after CWG 150
         // Error earlier: not an exact match
X<C> xc; // OK in C++14 after CWG 150
         // Error earlier: not an exact match
 
template<template<class ...> class Q> class Y { /* ... */ };
Y<A> ya; // OK
Y<B> yb; // OK
Y<C> yc; // OK
 
template<auto n> class D { /* ... */ }; // note: C++17
template<template<int> class R> class Z { /* ... */ };
Z<D> zd; // OK
 
template <int> struct SI { /* ... */ };
template <template <auto> class> void FA();  // note: C++17
FA<SI>();  // Error

Formally, a template template-parameter P is at least as specialized as a template template argument A if, given the following rewrite to two function templates, the function template corresponding to P is at least as specialized as the function template corresponding to A according to the partial ordering rules for function templates. Given an invented class template X with the template parameter list of A (including default arguments):

Each of the two function templates has the same template parameters, respectively, as P or A.
Each function template has a single function parameter whose type is a specialization of X with template arguments corresponding to the template parameters from the respective function template where, for each template parameter PP in the template parameter list of the function template, a corresponding template argument AA is formed. If PP declares a parameter pack, then AA is the pack expansion PP...; otherwise, AA is the id-expression PP.

If the rewrite produces an invalid type, then P is not at least as specialized as A.

(since C++14)

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, non-type, or template), but not to parameter packs.

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. In a function template, a parameter pack may be followed by more type parameters only if they have defaults or can be deduced from the function arguments.

Default parameters are not allowed

in the out-of-class definition of a member template (they have to be provided in the declaration inside the class body)
in friend class template declarations

in any function template declaration or definition

(until C++11)

On a friend function template declaration, default template arguments are allowed only if the declaration is a definition, and no other declarations of this function appear in this translation unit.

(since C++11)

Default template arguments that appear in the declarations and the definition 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

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

The default template argument is implicitly instantiated when the value of that default argument is needed, except if the template is used to name a function:

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>*

(since C++14)

Examples

Non-type template parameters

Run this code

#include <iostream>
 
// simple non-type template parameter
template<int N>
struct S { int a[N]; };
 
template<const char*>
struct S2 {};
 
// complicated non-type 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] > s2; // error: array element has no linkage
S2<okay> s2; // works
 
int a[5];
int f(int n) { return n; }
 
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';
}

Output:

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
CWG 150	C++14	template-template arguments had to match parameter lists of template-template parameters exactly	more specialized also allowed

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

parameters and arguments
class templates
function templates
class member templates
variable templates(C++14)
template argument deduction
class template deduction(C++17)
explicit (full) specialization
partial specialization
dependent names
parameter packs(C++11)
sizeof... (C++11)
fold-expressions(C++17)
SFINAE
Constraints and concepts(concepts TS)