如何访问类的静态成员？ [英] How to access static members of a class?

问题描述

我开始学习C ++和Qt，但有时我从一本书粘贴的最简单的代码导致错误。

我在使用QtCreator IDE的Ubuntu 10.04上使用 g ++ 4.4.2 g ++编译器语法和其他编译器之间有区别吗？例如，当我尝试访问静态成员时，总是会出错。

  #include< iostream> 
 using namespace std; 
 class A 
 {
 public：
 static int x; 
 static int getX（）{return x;} 
}; 
 int main（）
 {
 int A :: x = 100; // error：invalid use of qualified-name'A :: x'
 cout<< A :: getX（）; //错误：：未定义引用'A :: x'
 return 0; 
} 
  

我认为它与声明在这里和在这里（isn' t it？）。

解决方案

你声明静态成员很好，但不是在任何地方定义。

基本上've说有一些静态成员，但从来没有为它留出一些内存，你需要：

  int A： ：x = 100; 
  

在类别之外和 b $ b

I am starting to learn C++ and Qt, but sometimes the simplest code that I paste from a book results in errors.

I'm using g++4.4.2 on Ubuntu 10.04 with QtCreator IDE. Is there a difference between the g++ compiler syntax and other compilers? For example when I try to access static members something always goes wrong.

#include <iostream>
using namespace std;
class A
{
   public:
      static int x;
      static int getX() {return x;}
};
int main()
{
   int A::x = 100; // error: invalid use of qualified-name 'A::x'
   cout<<A::getX(); // error: : undefined reference to 'A::x'
   return 0;
}

I think it's exactly the same as declared here and here (isn't it?). So what's wrong with the above code?

解决方案

You've declared the static members fine, but not defined them anywhere.

Basically what you've said "there exists some static member", but never set aside some memory for it, you need:

int A::x = 100;

Somewhere outside the class and not inside main.

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