C++ 前置声明详解及实例

#include <iostream>
using namespace std;

void fun(char ch, int *pValue, double dValue);

void main()
{
  int nValue = 100;
  double dValue = 111.22;
  fun('a', &nValue, dValue);

  system("pause");
}

void fun(char ch, int *pValue, double dValue)
{
  return;
}

/*
 * 自定义类型前置声明
 */
#include <iostream>
using namespace std;

class B;

class A 
{
private:
  // 内置类型
  int m_nInt;
  int& m_nRInt;
  int* m_pInt;

  // 自定义类型
//  B b; // error！
  B* m_pB;
  B& m_b;

public: 
  A (B *pBPara = NULL) : m_nInt(100)
    , m_nRInt(m_nInt)
    , m_pInt(NULL)
    , m_pB(NULL)
    , m_b((NULL == pBPara) ? (*m_pB) : (*pBPara))
  {
    cout << "A()" << endl;
  }
  ~A()
  {
    cout << "~A()" << endl;
  }

  void funA()
  {
//    m_pB->doAnything(); // build error C2027: use of undefined type 'B'
  }
};

class B
{
private:
  int m_n;

public: 
  B (int n = 100) : m_n(n)
  {
    cout << "B()" << endl;
  }
  ~B()
  {
    cout << "~B()" << endl;
  }
  void doAnything()
  {
    cout << "B::anythig()" << endl;
  }
};

void main()
{
  A objA;
  system("pause");
}

/*
 * TestForwardDeclar.h
 */
#ifndef D_TESTFORWARDDECLAR_H_
#define D_TESTFORWARDDECLAR_H_

#include <iostream>

class B; // 前置声明自定义类型

class A 
{
private:
  // 内置类型
  int m_nInt;
  int& m_nRInt;
  int* m_pInt;

  // 自定义类型
//  B b; // error！
  B* m_pB;
  B& m_b;

public: 
  A (B *pBPara = NULL);
  ~A ();
  void funA();
};

class B
{
private:
  int m_n;

public: 
  B (int n = 100);
  ~B ();
  void doAnything();
};

#endif

/*
 * TestForwardDeclar.cpp
 */

#include "TestForwardDeclar.h"
#include <iostream>

A::A (B *pBPara) 
  : m_nInt(100)
  , m_nRInt(m_nInt)
  , m_pInt(NULL)
  , m_pB(NULL)
  , m_b((NULL == pBPara) ? (*m_pB) : (*pBPara))
{
  std::cout << "A()" << std::endl;
}

A::~A()
{
  std::cout << "~A()" << std::endl;
}

void A::funA()
{
  m_pB->doAnything(); // 分开头文件和实现文件即可
}


B::B (int n) : m_n(n)
{
  std::cout << "B()" << std::endl;
}

B::~B()
{
  std::cout << "~B()" << std::endl;
}

void B::doAnything()
{
  std::cout << "B::anythig()" << std::endl;
}

#include "TestForwardDeclar.h"
 
 void main()
 {
  A objA;
}