什么是C ++中的C＃静态实例的等价物？ [英] What is the equivalent of a C# Static instance in C++?

问题描述

我有很多C＃代码，我必须写在C + +。我在C ++中没有太多的经验。

I have a lot of C# Code that I have to write in C++. I don't have much experience in C++.

我使用 Visual Studio 2012 来构建。该项目是 C ++（而不是C ++ / CLI）

I am using Visual Studio 2012 to build. The project is an Static Library in C++ (not in C++/CLI).

在C＃代码中有一个类，创建实例

In the C# code there is a class for which some static instances are created

C＃

namespace MyNamespace
{
      public class MyClass
      {
          //Class Code

         static public MyClass Instance1 = new MyClass(/*parameters*/);
         static public MyClass Instance2 = new MyClass(/*other parameters*/);

我需要在C ++中做类似的操作，到目前为止：

I need to do something similar in C++, so far I got:

C ++

namespace MyNamespace
{
    class MyClass
    {
        //Class Code

     }//end of class

    static MyClass& Instance1 = MyClass(/*parameters*/);
    static MyClass& Instance2 = MyClass(/*other parameters*/);

}//end of Namespace

是不一样的，通过添加单词静态我做的是，我的Instance1和Instance2只在当前文件的范围内可见。这是正确的吗？

However, from what I read this is not quite the same, by adding the word "static" what I am doing is that my Instance1 and Instance2 are only visible within the scope of the current file. Is this correct?

所以我不想要一堆Instance1，我只想在整个程序中一个。
我读到，而不是使用 static ，我可以使用 extern ，告诉编译器变量的定义在另一个文件，因此我会结束只有一个实例Instance1和Instance2，而不是多个实例。

So I don't want a bunch of Instance1, i just want one in the entire program. I read that instead of using static, I could use extern, which tells the compiler the definition of the variable is in another file, thus I would wind up with only one instance of Instance1 and Instance2 and not multiple ones.

所以我试过：

C ++ MyClass.h

extern MyClass& Instance1;
extern MyClass& Instance2;

C ++ MyClass.cpp

MyClass& Instance1 = MyClass(/*parameters*/);
MyClass& Instance2 = MyClass(/*other parameters*/);

这个构建正常，但是当我尝试运行我的测试时， ：

 Failed to set up the execution context to run the test 

在使用 extern 之前，我的测试运行正常，但是当我添加它时，它们将无法工作。这使我相信我不能正确地声明，或者Visual Studio 2012不能正确支持这些功能的一些功能？

Before using extern my tests ran just fine, but when I add it they won't work. Which leads me to believe I am not declaring things properly, or maybe Visual Studio 2012 doesn't support properly some of this functionality?

推荐答案

p> static 在C ++中有不同的含义。当与变量声明一起使用时，它意味着你正在想的：没有链接的变量，它只是可用的文件中，它被声明或甚至，如果它是一个局部变量，只有一个副本（好方法来声明局部变量，它们应该在它们被使用的地方附近是静态的）。

static has different meanings in C++ according to where it is used. When used with a variable declaration it means what you were thinking: no linkage for the variable, it is available just to the file in which it is declared or even, if it's a local variable, that there is just one copy of it (good way to declare local variables which are supposed to be static near where they are used).

在类声明中，虽然static意味着与C＃中相同的东西，你必须有一个源文件实际声明静态变量（或在方法中声明为 static ），这是因为 static 变量必须在某处保留空间。这可以通过多种方式实现，例如：

In a class declaration though static means the same thing as in C# with the difference that you must have a source file which actually declares the static variable (or declare it as static inside a method), this because this static variable must be have the space reserved somewhere. This can be done in multiple ways, for example:

static MyClass& instance() {
  static MyClass realInstance;
  return realInstance;
}

这样你就可以看到使用 static ：一个是告诉实例（）方法是 static 类的命名空间，而不是单个实例），并且一个声明一个静态变量，即使它是一个局部变量，它只有一个副本。

In this way you can see both ways of using static: one is to tell that the instance() method is static (attached to the class namespace, not to a single instance) in the contained class, and one to declare a static variable that will have just one copy of itself even if it's a local variable.

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