cpp文件中名称空间内常量的定义 [英] Definition of constant within namespace in cpp file
问题描述
在VS 2010中，我已经达到了这种奇怪的效果(对我来说).任何人都可以给它一些启发.
I've approached this strange ( for me ) effect in VS 2010. Can anyone smart shed some light on it please.
//Header.h
#include <string>
namespace MySpace {
extern const std::string SOME_CONST_STRING;
}
//Implementation.cpp
#include "Header.h"
using namespace MySpace;
const std::string SOME_CONST_STRING = "CONST_STRING_VALUE";
这会导致链接器输出错误LNK2001:无法解析的外部符号const MySpace :: SOME_CONST_STRING.
This causes linker to output error LNK2001: unresolved external symbol const MySpace::SOME_CONST_STRING.
但是当我这样更改Implementation.cpp时:
However when I change Implementation.cpp like this:
//Implementation.cpp
#include "Header.h"
namespace MySpace {
const std::string SOME_CONST_STRING = "CONST_STRING_VALUE";
}
代码建立正常.
是否更喜欢在cpp文件中定义名称空间而不是使用它呢?
Is it good example for prefering defining of namespace in cpp file rather than using it ?
推荐答案
名称空间的全部要点是，特定名称空间中的名称独立于该名称空间之外的名称.程序可能同时具有 MySpace :: SOME_CONST_STRING 和全局 :: SOME_CONST_STRING ，这是两个完全不相关的符号.链接器使用一个定义来满足对另一个的引用是错误的.
The whole point of namespaces is that names in a particular namespace are independent of names outside that namespace. A program could have both a MySpace::SOME_CONST_STRING and a global ::SOME_CONST_STRING, and these are two completely unrelated symbols. It would be wrong for the linker to use a definition of one to satisfy a reference to the other.
定义变量时，必须在所需的名称空间中对其进行定义.
When you define a variable, you have to define it in the namespace where you want it to be.
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