使用静态成员函数访问静态数据成员

Question

我正在尝试使用静态成员函数访问静态数据成员，以便我可以调用此函数并检索数据成员。目标是increase/decrease这个静态数据成员来统计程序中存在多少个对象。

作业逐字记录：

A static member function called getNumObjects should be provided that takes no parameters and returns an int indicating the number of objects of type Complex currently in existence.

到目前为止我的代码：

Complex.hpp

class Complex{
public:
...
// Get the number of complex objects in the current program
        static int& getNumObjects();
...

private:
...
        static int counter; // static counter
...
}

Complex.cpp

// Initialize object counter for getNumObjects()
int Complex::counter = 0;
// Get number of objects existing in the current program
static int& Complex::getNumObjects(){
        return counter;
}

测试Complex.cpp

// checks to see how many complex objects are open currently
        std::cout << "How many complex objecst are in existence? ";
        int num = Complex::getNumObjects();
        std::cout << num << '\n';
        std::cout << "successful\n";

我不明白为什么编译器总是向我抛出这个错误：

 error: cannot declare member function ‘static int& Complex::getNumObjects()’ to have static linkage [-fpermissive]
 static int& Complex::getNumObjects(){

或此错误：

In function ‘int getNumObjects()’:
/../../../ error: ‘counter’ was not declared in this scope
  return counter;
         ^~~~~~~
/../../../ note: suggested alternative: ‘toupper’
  return counter;
         ^~~~~~~
         toupper

我进行了广泛的搜索，我似乎已经很好地初始化了我的私有数据成员以及函数。 getNumObjects() 表示为 class，那么为什么说函数的范围不正确？

Answer 1

static 关键字只能在 class 声明中使用。方法实现应省略 static 声明：

int& Complex::getNumObjects(){
        return counter;
}

奇怪的是，您已经为您的静态成员变量遵循了这条规则；-)

Answer 2

I don't understand why the compiler keeps throwing me this error:

因为你在成员函数定义中重复了static。该函数已声明 static； C++ 语法要求您在 cpp 文件中省略 static：

int Complex::counter = 0;
// Get number of objects existing in the current program
int& Complex::getNumObjects(){
    return counter;
}

注意： return int& 可能不是一个好主意，因为调用者可以修改 counter 而不是你的知识：

Complex::getNumObjects() = -123; // <<== returning reference makes this legal

这非常糟糕，因为它完全破坏了封装。本质上，您的 counter 最终被公开，就好像它是一个 public 成员变量一样。

您应该将函数更改为 return int：

int Complex::getNumObjects(){
    return counter;
}

Answer 3

您绝对必须跟踪析构函数和构造函数中的对象。此外，您真的希望您的计数器达到 atomic，以便在多个线程上运行时不会出现竞争条件。

struct Complex
{
  Complex()
  { ++counter; }

  Complex(Complex const&other)
  : _real(other._real), _imag(other._imag)
  { ++counter; }

  Complex(double r, double i=0)
  : _real(r), _imag(i)
  { ++counter; }

  // and so on for all other constructors

  Complex&operator=(Complex const&) = default;

  Complex&operator=(double x)
  {
    _real = x;
    _imag = 0;
    return *this;
  }

 ~Complex() { --counter; }

  static int getNumObjects() { return counter; }

private:
  double _real,_imag;
  static std::atomic<int> counter;
};

实际上并不需要移动构造函数（因为堆上的数据不由 Complex 管理）。

使用静态成员函数访问静态数据成员

Using Static Member Function to Access Static Data Member

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