constexpr静态数据成员给出未定义的引用错误 [英] constexpr static data member giving undefined reference error

问题描述

我正在处理一个内核，我想让我的静态数据成员 constexpr 所以我可以有一个枚举类中的值。但是，如果我这样做，我得到一个未定义的引用错误。

工作：

pre> // terminal.hpp
class Terminal
{
static uint32_t col_map [16];
};

// terminal.cpp
uint32_t Terminal :: col_map [16] = {
0x000000，0x0000AA，0x00AA00，0x00AAAA，
0xAA0000，0xAA00AA，0xAA5500，0xAAAAAA ，
0x555555，0x55​​55FF，0x55​​FF55，0x55​​FFFF，
0xFF5555，0xFF55FF，0xFFFF55,0xFFFFFF
};


不工作：

  // terminal.hpp 
 class Terminal 
 {
 constexpr static uint32_t col_map [16] = {
 0x000000，0x0000AA，0x00AA00，0x00AAAA，
 0xAA0000，0xAA00AA，0xAA5500，0xAAAAAA，
 0x555555，0x55​​55FF，0x55​​FF55，0x55​​FFFF，
 0xFF5555,0xFF55FF，0xFFFF55,0xFFFFFF 
}; 
 
枚举类颜色：uint32_t 
 {
 Black = col_map [0]，
 White = col_map [15] 
} 
}; 
  

注意，我试图在常规编译器上重现这个失败，所以我认为它有关内核或交叉编译器。

链接器错误：

  o：在函数`Terminal :: drawcolormap（）'中：
 terminal.cpp :( .text + 0x6f）：未定义引用`Terminal :: col_map'
  pre> 
 
 
我总是运行 make clean; 
 $ p 
 
解决方案
静态数据成员应在类外部定义，如果它是odr使用的---  period 。此规则没有例外。
 
 
 
如果数据成员 constexpr ，它还必须已在 内定义类定义。 
 
 
 
正确的代码：
 
 $ b $ 
  class Terminal 
 {
 constexpr static uint32_t col_map [16] = {/ * ... * /}; 
 // ... 
}; 
 constexpr uint32_t Terminal :: col_map [16]; //定义
  
这是违反直觉的，但这只是它的方式。
 
I'm working on a kernel and I want to make my static data member constexpr so I can have its values in an enum class. However, if I do so I get an undefined reference error. It only seems to work if I make it non-constexpr and initialize it outside of the class.

Working:
// terminal.hpp
class Terminal
{
    static uint32_t col_map[16];
};

// terminal.cpp
uint32_t Terminal::col_map[16] = {
    0x000000, 0x0000AA, 0x00AA00, 0x00AAAA,
    0xAA0000, 0xAA00AA, 0xAA5500, 0xAAAAAA,
    0x555555, 0x5555FF, 0x55FF55, 0x55FFFF,
    0xFF5555, 0xFF55FF, 0xFFFF55, 0xFFFFFF
};
Not working:
// terminal.hpp
class Terminal
{
    constexpr static uint32_t col_map[16] = {
        0x000000, 0x0000AA, 0x00AA00, 0x00AAAA,
        0xAA0000, 0xAA00AA, 0xAA5500, 0xAAAAAA,
        0x555555, 0x5555FF, 0x55FF55, 0x55FFFF,
        0xFF5555, 0xFF55FF, 0xFFFF55, 0xFFFFFF
    };

    enum class Color : uint32_t
    {
        Black = col_map[0],
        White = col_map[15]
    };  
};
Note that I tried to reproduce this on a regular compiler unsuccessfully so I think it has something to do with the kernel or cross compiler.

Linker error:
terminal.o: In function `Terminal::drawcolormap()':
terminal.cpp:(.text+0x6f): undefined reference to `Terminal::col_map'
I always run make clean; make.
解决方案
A static data member shall be defined outside the class if it is odr-used---period. There are no exceptions to this rule.

In the case that the data member is constexpr, it must also be initialized inside the class definition. However this does not abrogate the requirement to define it outside the class.

Correct code:
class Terminal
{
    constexpr static uint32_t col_map[16] = { /* ... */ };
    // ...
};
constexpr uint32_t Terminal::col_map[16]; // definition
It's counterintuitive, but that's just the way it is.

                        
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