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# Template parameters and template arguments

From cppreference.com
< cpp‎ | language

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 one of

- non-type template parameter
- type template parameter
- template template parameter

#### [edit] Non-type template parameter

type name(optional) | (1) |
type name(optional) `=` default | (2) |
type `...` name(optional) | (3) | (since C++11)

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

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

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

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

#### [edit] 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.

#### [edit] Template template parameter

`template` `<` parameter-list `>` `class` name(optional) | (1) |
`template` `<` parameter-list `>` `class` name(optional) `=` default | (2) |
`template` `<` parameter-list `>` `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`.

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

    template<class T> class myarray {};

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

### [edit] 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 or defaulted. For function templates, the arguments are explicitly provided, defaulted, or deduced from context.

#### [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.
- 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.
- 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 data.

#### [edit] Template type arguments

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

    template <class 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<class T> class A { // primary template
        int x;
    };
    template<class T> class A<T*> { // partial specialization
        long x;
    };

    // 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<class T> struct eval; // primary template

    template< template<class, class...> class TT, class T1, class... Rest>
    struct eval<TT<T1, Rest...>> {}; // partial specialization of eval

    template <class T1> struct A;
    template <class T1, class T2> struct B;
    template <int N> struct C;
    template <class T1, int N> struct D;
    template <class T1, class 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

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.

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

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

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

    template<class T = int> class X;
    template<class 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 <class T = float> struct B {};

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

### [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:

    42

| This section is incomplete Reason: more examples

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