有一个constexpr静态字符串给出链接器错误 [英] Having a constexpr static string gives a linker error

问题描述

以下程序给我一个链接时间错误：

  #include< iostream& 
 
 struct Test {static constexpr char text [] =Text; }; 
 
 int main（）
 {
 std :: cout< Test :: text<< std :: endl; //错误：未定义引用`Test :: text'
} 
  

错误消息

  /tmp/main-35f287.o：在main函数中：
 main.cpp :(。文本+ 0x4）：未定义的引用`Test :: text'
 main.cpp :(。text + 0x13）：未定义引用`Test :: text'
 clang：error：退出代码1（使用-v查看调用）
  

让我们尝试解决：我在 struct 正文之外添加一个定义：

  #include< iostream> 
 
 struct Test {static constexpr char text [] =Text; }; 
 constexpr char Test :: text [] =Text; 
 
 int main（）
 {
 std :: cout< Test :: text<< std :: endl; 
} 
  

Clang给了我以下错误信息。

  main.cpp：4：35：error：static data member'text'已经有一个初始化程序
 constexpr char Test :: text [] =Text; 
 ^ 
 main.cpp：3：50：注意：以前的初始化在这里
 struct Test {static constexpr char text [] =Text; }; 
  

哦，好吧，我想，现在我知道你想要什么：

  #include< iostream> 
 
 struct Test {static constexpr char text []; }; 
 constexpr char Test :: text [] =Text; 
 
 int main（）
 {
 std :: cout< Test :: text<< std :: endl; 
} 
  

又一次错误：

  main.cpp：3：41：error：声明constexpr静态数据成员'text'需要一个初始化器
 struct Test {static constexpr char text []; }; 
  

那里的狗咬了自己的尾巴。 （

有没有办法使用在类中声明的编译时常数字符数组？原因，我想在类中的数据，需要一个类型traits类，帮助我做模板的东西。

解决方案

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：
 
 
 
 
 
 
 
 
 
 
 
 可以在类
定义中使用constexpr说明符声明文字类型的静态数据成员 ;如果是这样，它的声明
指定一个括号或初始化器，其中作为赋值表达式的每个初始化子句
是一个常量表达式[注意：在
这两种情况下，成员可能出现在常量表达式中。 -end 
 note]  如果在程序中为
 odr-used（3.2），那么成员仍然在命名空间范围内定义，并且命名空间范围定义应为
不包含初始值 。
 
 
 
The following program gives me a link-time error:
#include <iostream>

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

int main()
{
    std::cout << Test::text << std::endl; // error: undefined reference to `Test::text'
}
The error message is
/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)
Ok. Let's try to fix that: I add a definition outside the struct body:
#include <iostream>

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

int main()
{
    std::cout << Test::text << std::endl;
}
Clang gives me the following error message. 
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"; };
Oh, well, I thought, now I know what you want:
#include <iostream>

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

int main()
{
    std::cout << Test::text << std::endl;
}
And again an error: 
main.cpp:3:41: error: declaration of constexpr static data member 'text' requires an initializer
    struct Test { static constexpr char text[]; };
And there the dog bites its own tail. :( 


Is there a way to use compile-time constant character arrays that are declared inside a class? The reason, I want the data inside a class, is that I need a type traits class that helps me do template stuff. 
解决方案
Should work:
#include <iostream>

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

int main()
{
    std::cout << Test::text << std::endl;
}
In standard (n4140 §9.4.2/3) you can find:

  
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.


                        
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