C++ 在类定义中定义静态常量整数成员
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Defining static const integer members in class definition
提问by HighCommander4
My understanding is that C++ allows static const members to be defined inside a class so long as it's an integer type.
我的理解是 C++ 允许在类中定义静态常量成员，只要它是整数类型。
Why, then, does the following code give me a linker error?
那么，为什么下面的代码会给我一个链接器错误？
#include <algorithm>
#include <iostream>
class test
{
public:
static const int N = 10;
};
int main()
{
std::cout << test::N << "\n";
std::min(9, test::N);
}
The error I get is:
我得到的错误是：
test.cpp:(.text+0x130): undefined reference to `test::N'
collect2: ld returned 1 exit status
Interestingly, if I comment out the call to std::min, the code compiles and links just fine (even though test::N is also referenced on the previous line).
有趣的是，如果我注释掉对 std::min 的调用，代码编译和链接就好了（即使 test::N 在前一行也被引用）。
Any idea as to what's going on?
知道发生了什么吗？
My compiler is gcc 4.4 on Linux.
我的编译器是 Linux 上的 gcc 4.4。
采纳答案by Edward Strange
My understanding is that C++ allows static const members to be defined inside a class so long as it's an integer type.
我的理解是 C++ 允许在类中定义静态常量成员，只要它是整数类型。
You are sort of correct. You are allowed to initialize static const integrals in the class declaration but that is not a definition.
你说得对。您可以在类声明中初始化静态常量积分，但这不是定义。
Interestingly, if I comment out the call to std::min, the code compiles and links just fine (even though test::N is also referenced on the previous line).
有趣的是，如果我注释掉对 std::min 的调用，代码编译和链接就好了（即使 test::N 在前一行也被引用）。
Any idea as to what's going on?
知道发生了什么吗？
std::min takes its parameters by const reference. If it took them by value you'd not have this problem but since you need a reference you also need a definition.
std::min 通过常量引用获取其参数。如果按价值计算它们，您就不会遇到这个问题，但由于您需要参考，因此您还需要一个定义。
Here's chapter/verse:
这是章节/诗句：
9.4.2/4- If a staticdata member is of constintegral or constenumeration type, its declaration in the class definition can specify a constant-initializerwhich shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.
9.4.2/4- 如果一个static数据成员是const整型或const枚举类型，它在类定义中的声明可以指定一个常量初始化器，它应该是一个整型常量表达式（5.19）。在这种情况下，成员可以出现在整数常量表达式中。如果在程序中使用该成员，则该成员仍应在名称空间范围内定义，并且名称空间范围定义不应包含初始化程序。
See Chu's answer for a possible workaround.
有关可能的解决方法，请参阅 Chu 的回答。
回答by HostileFork says dont trust SE
Bjarne Stroustrup's example in his C++ FAQsuggests you are correct, and only need a definition if you take the address.
Bjarne Stroustrup在他的 C++ FAQ 中的例子表明你是正确的，如果你接受地址，只需要一个定义。
class AE {
// ...
public:
static const int c6 = 7;
static const int c7 = 31;
};
const int AE::c7; // definition
int f()
{
const int* p1 = &AE::c6; // error: c6 not an lvalue
const int* p2 = &AE::c7; // ok
// ...
}
He says "You can take the address of a static member if (and only if) it has an out-of-class definition". Which suggests it would work otherwise. Maybe your min function invokes addresses somehow behind the scenes.
他说：“当（且仅当）静态成员具有类外定义时，您才能获取它的地址”。这表明它会以其他方式工作。也许您的 min 函数在幕后以某种方式调用了地址。
回答by Stephen Chu
Another way to do this, for integer types anyway, is to define constants as enums in the class:
无论如何，对于整数类型，另一种方法是将常量定义为类中的枚举：
class test
{
public:
enum { N = 10 };
};
回答by Amardeep AC9MF
Not just int's. But you can't define the value in the class declaration. If you have:
不仅仅是int。但是您不能在类声明中定义该值。如果你有：
class classname
{
public:
static int const N;
}
in the .h file then you must have:
在 .h 文件中，您必须具有：
int const classname::N = 10;
in the .cpp file.
在 .cpp 文件中。
回答by karadoc
Here's another way to work around the problem:
这是解决该问题的另一种方法：
std::min(9, int(test::N));
(I think Crazy Eddie's answer correctly describes why the problem exists.)
（我认为 Crazy Eddie 的回答正确地描述了问题存在的原因。）
回答by Carlo Wood
As of C++11 you can use:
从 C++11 开始，您可以使用：
static constexpr int N = 10;
static constexpr int N = 10;
This theoretically still requires you to define the constant in a .cpp file, but as long as you don't take the address of Nit is very unlikely that any compiler implementation will produce an error ;).
这理论上仍然需要您在 .cpp 文件中定义常量，但只要您不获取N它的地址，任何编译器实现都不太可能产生错误；)。
回答by fredoverflow
C++ allows static const members to be defined inside a class
C++ 允许在类中定义静态常量成员
Nope, 3.1 §2 says:
不，3.1 §2 说：
A declaration is a definition unlessit declares a function without specifying the function's body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification (7.5) and neither an initializer nor a functionbody, it declares a static data member in a class definition(9.4), it is a class name declaration (9.1), it is an opaque-enum-declaration (7.2), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4).
声明是一个定义，除非它声明一个函数而没有指定函数体 (8.4)，它包含 extern 说明符 (7.1.1) 或链接规范 (7.5) 并且既没有初始化程序也没有函数体，它声明了一个静态数据类定义(9.4) 中的成员，它是一个类名声明 (9.1)，它是一个不透明枚举声明 (7.2)，或者它是一个 typedef 声明 (7.1.3)，一个 using 声明 (7.3. 3) 或 using 指令 (7.3.4)。
