有一個constexpr靜態字符串給出一個鏈接錯誤

Question

下面的程序給我一個鏈接時錯誤：

#include <iostream> 

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

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

的錯誤消息是

/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) 

確定。讓我們嘗試解決這個問題：我添加了struct體外的定義：

#include <iostream> 

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

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

鏘給我下面的錯誤消息。

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，現在我知道你想要什麼：

#include <iostream> 

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

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

而且又是一個錯誤：

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

還有狗咬自己尾巴。 :(

有沒有辦法使用在類中聲明的編譯時常量字符數組？原因是，我想要一個類內的數據，是我需要一個類型traits類，可以幫助我做模板的東西。

Answer 1

應該工作：

#include <iostream> 

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

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

在標準（n4140§9.4.2/ 3），你可以找到：

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.

Answer 2

正如評論所說，這個版本工作正常：

struct Test { static constexpr auto text = "Text"; }; 

但text將是一個const char*，而不是char[]。