用 const char*const 解释 constexpr

Question

我有以下代码：

static constexpr const char*const myString = "myString";

能否请您解释一下与以下内容的区别：

static const char*const myString = "myString";

在这种情况下，constexpr 有什么新功能？

Answer 1

C++ 标准（9.4.2 静态数据成员）中的以下引用描述了差异

3 If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignmentexpression is a constant expression (5.19). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [ Note: In both these cases, the member may appear in constant expressions. —end note ] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.

例如考虑两个程序

struct A
{
    const static double x = 1.0;
};

int main() 
{
    return 0;
}

struct A
{
    constexpr static double x = 1.0;
};

int main() 
{
    return 0;
}

第一个不会编译，而第二个会编译。

同样适用于指针

这个节目

struct A
{
    static constexpr const char * myString = "myString";
};

int main() 
{
    return 0;
}

将编译此程序

struct A
{
    static const char * const myString = "myString";
};

int main() 
{
    return 0;
}

不会编译。