c++ - 拥有 constexpr 静态字符串会导致链接器错误

Question

下面的程序给我一个链接时错误:

#include <iostream>

struct Test { static constexpr char text[] = "Text"; };

int main()
{
    std::cout << Test::text << std::endl; // error: undefined reference to `Test::text'
}

错误信息是

/tmp/main-35f287.o: In function `main':
main.cpp:(.text+0x4): undefined reference to `Test::text'
main.cpp:(.text+0x13): undefined reference to `Test::text'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

好的。让我们尝试解决这个问题:我在 struct 主体之外添加了一个定义:

#include <iostream>

struct Test { static constexpr char text[] = "Text"; };
constexpr char Test::text[] = "Text";

int main()
{
    std::cout << Test::text << std::endl;
}

Clang 给我以下错误信息。

main.cpp:4:35: error: static data member 'text' already has an initializer
    constexpr char Test::text[] = "Text";
                                  ^
main.cpp:3:50: note: previous initialization is here
    struct Test { static constexpr char text[] = "Text"; };

哦，好吧，我想，现在我知道你想要什么了:

#include <iostream>

struct Test { static constexpr char text[]; };
constexpr char Test::text[] = "Text";

int main()
{
    std::cout << Test::text << std::endl;
}

又是一个错误:

main.cpp:3:41: error: declaration of constexpr static data member 'text' requires an initializer
    struct Test { static constexpr char text[]; };

然后狗咬了自己的尾部。 :(

有没有办法使用在类中声明的编译时常量字符数组？我想要一个类中的数据的原因是我需要一个类型特征类来帮助我做模板的事情。

Answer 1

应该工作:

#include <iostream>

struct Test { static constexpr char text[] = "Text"; };
constexpr char Test::text[];

int main()
{
    std::cout << Test::text << std::endl;
}

在标准 (n4140 §9.4.2/3) 中，您可以找到:

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.