C++14中的variable template(变量模版)用于定义一系列变量或静态数据成员，语法如下：
template < parameter-list > variable-declaration(1). variable-declaration: 变量的声明,声明的变量名成为模版名;
(2).parameter-list: 模版参数的非空逗号分隔列表，每个参数都是non-type parameter, a type parameter, a template parameter, or a parameter pack of any of those.
从变量模版实例化的变量称为实例化变量。从静态数据成员模版实例化的静态数据成员称为实例化静态数据成员。
以下为测试代码：
namespace {
// reference: https://en.cppreference.com/w/cpp/language/variable_template
template<class T>
constexpr T pi = T(3.1415926535897932385); // variable template
template<class T>
T circular_area(T r) // function template
{
return pi<T> * r * r; // pi<T> is a variable template instantiation
}
struct limits {
template<typename T>
static const T min; // declaration of a static data member template
};
template<typename T>
const T limits::min = { T(1.2) }; // definition of a static data member template
template<class T>
class X {
public:
static T s; // declaration of a non-template static data member of a class template
};
template<class T>
T X<T>::s = 1.2; // definition of a non-template data member of a class template
// reference: https://www.anycodings.com/1questions/4931124/c14-variable-templates-what-is-their-purpose-any-usage-example
template<int N>
const int square = N * N;
template<typename T>
T n = T(5);
} // namespace
int test_variable_template()
{
// 1. function template
std::cout << "int value:" << circular_area(1) << "\n"; // int value:3
std::cout << "float value:" << circular_area(1.f) << "\n"; // float value:3.14159
// 2. static data member template
std::cout << "int limits::min:" << limits::min<int> << "\n"; // int limits::min:1
std::cout << "float limits::min:" << limits::min<float> << "\n"; // float limits::min:1.2
std::cout << "int X::s:" << X<int>::s << "\n"; // int X::s:1
std::cout << "float X::s:" << X<float>::s << "\n"; // float X::s:1.2
// 3. compile time code
std::cout << "square:" << square<5> << "\n"; // square:25
// 4. it seems to instantiate the variables separately for the type
n<int> = 10;
std::cout << "n<int>:" << n<int> << "\n"; // n<int>:10
std::cout << "n<float>:" << n<float> << "\n"; // n<float>:5
return 0;
}windows和linux执行结果如下图所示：
