如何在 C++ 中定义字符串常量？
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How do I define string constants in C++?
提问by Thom
Possible Duplicate:
C++ static constant string (class member)
static const C++ class member initialized gives a duplicate symbol error when linking
My experience with C++ pre-dated the addition of the string class, so I'm starting over in some ways.
我在 C++ 方面的经验早于字符串类的添加，所以我在某些方面重新开始。
I'm defining my header file for my class and want to create a static constant for a url. I'm attempting this by doing as follows:
我正在为我的班级定义头文件，并希望为 url 创建一个静态常量。我正在尝试这样做：
#include <string>
class MainController{
private:
static const std::string SOME_URL;
}
const std::string MainController::SOME_URL = "www.google.com";
But this give me a duplicate definition during link.
但这在链接期间给了我一个重复的定义。
How can I accomplish this?
我怎样才能做到这一点？
采纳答案by David Nehme
Move the
移动
const std::string MainController::SOME_URL = "www.google.com";
to a cpp file. If you have it in a header, then every .cpp that includes it will have a copy and you will get the duplicate symbol error during the link.
到一个cpp文件。如果您将它放在标题中，那么包含它的每个 .cpp 都会有一个副本，并且您将在链接期间收到重复符号错误。
回答by JRL
You need to put the line
你需要把线
const std::string MainController::SOME_URL = "www.google.com";
in the cpp file, not the header, because of the one-definition rule. And the fact that you cannot directly initialize it in the class is because std::stringis not an integral type (like int).
在 cpp 文件中，而不是标题中，因为一个定义规则。并且您不能在类中直接初始化它的事实是因为std::string它不是整数类型（如 int）。
Alternatively, depending on your use case, you might consider not making a static member but using an anonymous namespace instead. See this post for pro/cons.
或者，根据您的用例，您可能会考虑不创建静态成员，而是使用匿名命名空间。有关利弊，请参阅此帖子。
回答by Nawaz
Define the class in the header file:
在头文件中定义类：
//file.h
class MainController{
private:
static const std::string SOME_URL;
}
And then, in source file:
然后，在源文件中：
//file.cpp
#include "file.h"
const std::string MainController::SOME_URL = "www.google.com";
回答by Mark B
You should put the const std::string MainController::SOME_URL = "www.google.com";definition into a single source file, not in the header.
您应该将const std::string MainController::SOME_URL = "www.google.com";定义放在一个源文件中，而不是放在头文件中。
