# Variable template (since C++14)

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A variable template defines a family of variables or static data members.

### Syntax

`template` `<` parameter-list `>` variable-declaration | (1) |
`template` `<` parameter-list `>` `requires` constraint variable-declaration | (2) | (since C++20)

variable-declaration | - | a declaration of a variable. The declared variable name becomes a template name.
parameter-list | - | a non-empty comma-separated list of the template parameters, each of which is either non-type parameter, a type parameter, a template parameter, or a parameter pack of any of those.
constraint | - | a constraint expression which restricts the template parameters accepted by this variable template

### Explanation

A variable instantiated from a variable template is called an instantiated variable. A static data member instantiated from a static data member template is called an instantiated static data member.

A variable template may be introduced by a template declaration at namespace scope, where variable-declaration declares a variable.

    template<class T>
    constexpr T pi = T(3.1415926535897932385L); // variable template

    template<class T>
    T circular_area(T r) // function template
    {
        return pi<T> * r * r; // pi<T> is a variable template instantiation
    }

When used at class scope, variable template declares a static data member template.

    using namespace std::literals;
    struct matrix_constants
    {
        template<class T>
        using pauli = hermitian_matrix<T, 2>; // alias template

        template<class T> // static data member template
        static constexpr pauli<T> sigmaX = {{0, 1}, {1, 0}};

        template<class T>
        static constexpr pauli<T> sigmaY = {{0, -1i}, {1i, 0}};

        template<class T>
        static constexpr pauli<T> sigmaZ = {{1, 0}, {0, -1}};
    };

As with other static members, a definition of a static data member template may be required. Such definition is provided outside the class definition. A template declaration of a static data member at namespace scope may also be a definition of a non-template data member of a class template:

    struct limits
    {
        template<typename T>
        static const T min; // declaration of a static data member template
    };

    template<typename T>
    const T limits::min = { }; // definition of a static data member template

    template<class T>
    class X
    {
        static T s; // declaration of a non-template static data member of a class template
    };

    template<class T>
    T X<T>::s = 0; // definition of a non-template data member of a class template

Unless a variable template was explicitly specialized or explicitly instantiated, it is implicitly instantiated when a specialization of the variable template is referenced in a context that requires a variable definition to exist or if the existence of the definition affects the semantics of the program, i.e. if the variable is needed for constant evaluation by an expression (the definition may be not used).

The existence of a definition of a variable is considered to affect the semantics of the program if the variable is needed for constant evaluation by an expression, even if constant evaluation of the expression is not required or if constant expression evaluation does not use the definition.

### Notes

Until variable templates were introduced in C++14, parametrized variables were typically implemented as either static data members of class templates or as constexpr function templates returning the desired values.

Variable templates cannot be used as template template arguments.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_variable_templates` | `201304L` | (C++14) | Variable templates

### 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 2255 | C++14 | it was unclear whether a specialization of a static data member template is a static data member | it is

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