C++ Variable Template

A variable template (introduced in C++14) is a declaration that defines a family of variables or static data members parameterized by one or more template arguments. It extends the C++ template mechanism—previously restricted to classes, functions, and type aliases—directly to variable declarations, allowing the compiler to generate distinct, type-safe variables from a single parameterized definition.

Syntax

The base syntax for declaring a variable template at namespace or class scope requires the template keyword followed by the template parameter list, the variable type, its identifier, and an optional initializer.

template <template-parameter-list>
type variable_name = initializer;

Instantiation and Memory Semantics

When a variable template is instantiated with a specific set of template arguments, the compiler generates a unique variable. Each instantiation occupies its own distinct memory address.

template <typename T>
constexpr T pi = T(3.1415926535897932385L);

// Implicit instantiations
const float pi_float = pi<float>;   // Generates a float variable
const double pi_double = pi<double>; // Generates a double variable

// pi<float> and pi<double> have different memory addresses

Specialization

Variable templates support both explicit (full) specialization and partial specialization, utilizing the same resolution rules as class templates.

Explicit Specialization

You can define a completely different type and initializer for a specific template argument.

template <typename T>
constexpr T default_value = T();

// Explicit specialization for const char*
template <>
constexpr const char* default_value<const char*> = "null-string";

Partial Specialization

You can specialize the variable template for a subset of types, such as pointers or arrays.

template <typename T>
constexpr bool is_pointer = false;

// Partial specialization for any pointer type
template <typename T>
constexpr bool is_pointer<T*> = true;

Scope and Linkage

Variable templates can be declared at namespace scope or as static data members within a class. They cannot be declared at block scope (inside a function). Namespace Scope:

namespace math {
    template <typename T>
    constexpr T e = T(2.71828182845904523536L);
}

Class Scope (Static Data Members): A non-templated class can contain a templated static data member.

struct Limits {
    template <typename T>
    static const T minimum;
};

// Out-of-line definition
template <typename T>
const T Limits::minimum = /* implementation */;

Type Deduction and Constraints

Variable templates do not support template argument deduction from the initializer; the template arguments must always be explicitly specified (e.g., var<int>, not var). However, starting in C++20, variable templates can be constrained using Concepts to restrict the types that can be used for instantiation.

#include <concepts>

// Constrained variable template: T must satisfy std::integral
template <std::integral T>
constexpr T zero_value = 0;

// zero_value<int> is valid.
// zero_value<double> results in a compilation error.

Type Traits Equivalence

Variable templates are the underlying mechanism for the _v suffix conventions found in the <type_traits> library. They act as syntactic sugar over accessing the ::value static member of a class template.

// Traditional class template approach (C++11)
bool b1 = std::is_same<int, int>::value;

// Variable template equivalent (C++17)
bool b2 = std::is_same_v<int, int>;

// Internal implementation of the variable template
template <typename T, typename U>
inline constexpr bool is_same_v = is_same<T, U>::value;