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.

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 or defaulted. For function templates, the arguments are explicitly provided, defaulted, or deduced from context.

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
}

[edit] 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 and cv-qualification 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)

[edit] 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
}

[edit] 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. 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

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

If the default is specified for a template parameter of a primary class 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

[edit] Examples

[edit] 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:

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)
Technical Specifications

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


type name(optional)	(1)

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

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


`typename` name(optional)	(1)

`class` name(optional)	(2)

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

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


`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)

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Template parameters and template arguments

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[edit] Non-type template parameter

[edit] Type template parameter

[edit] Template template parameter

[edit] Template arguments