c++ - 默认情况下，静态变量是否在 C++17 的模板中内联？

Question

在 C++17 的模板中是否默认内联静态变量？这是一个例子:

template<typename T>
struct SomeClass {
    static T test;
};

struct SomeClass2 {
    static constexpr int test = 9;
};

这些变量是内联的还是仍然需要一个外线定义才能使用 ODR？

Answer 1

static constexpr 也将隐含地为 inline，否则您需要将其标记为 inline

template<typename T>
struct SomeClass {
    inline static T test; // Now inline
};

struct SomeClass2 {
    static constexpr int test = 9; // inline
};

参照。来自 n4606 [depr.static_constexpr]

For compatibility with prior C++ International Standards, a constexpr static data member may be redundantly redeclared outside the class with no initializer. This usage is deprecated.

Example:

struct A {
  static constexpr int n = 5; // definition (declaration in C++ 2014)
};
const int A::n; // redundant declaration (definition in C++ 2014)

和 [dcl.constexpr](巴里打败了我)

A function or static data member declared with the constexpr specifier is implicitly an inline function or variable (7.1.6).