C++中继承 声明基类析构函数为虚函数作用，单继承和多继承关系的内存分布

1，基类析构函数不为虚函数

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

class CBase
{
public:
    CBase() {
        m_one = 0;
        printf("this is CBase construct
");
    }
    ~CBase() {
        printf("this is ~CBase deconstruct
");
    }

    void setNumOne(int n)
    {
        m_one = n;
    }
    int getNumOne()
    {
        return m_one;
    }
private:
    int m_one;
};

class CDrived:public CBase
{
public:
    CDrived() {
        m_two = 0;
        printf("this is CDrived construct
");
    }
    ~CDrived() {
        printf("this is ~CDrived deconstruct
");
    }

    void setNumTwo(int n)
    {
        m_two = n;
    }
    int getNumTwo()
    {
        return m_two;
    }
private:
    int m_two;
};

int main()
{
    CBase *p = new CDrived;
    delete p;
    std::cout << "Hello World!
"; 
}

输出：

this is CBase construct
this is CDrived construct
this is ~CBase deconstruct
Hello World!

可以发现继承类析构函数没有调用，若继承类中有一些资源需要释放，则不能释放，故需要将基类析构函数声明为虚函数。

 

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

class CBase
{
public:
    CBase() {
        m_one = 0;
        printf("this is CBase construct
");
    }
    virtual ~CBase() {
        printf("this is ~CBase deconstruct
");
    }

    void setNumOne(int n)
    {
        m_one = n;
    }
    int getNumOne()
    {
        return m_one;
    }
private:
    int m_one;
};

class CDrived:public CBase
{
public:
    CDrived() {
        m_two = 0;
        printf("this is CDrived construct
");
    }
    ~CDrived() {
        printf("this is ~CDrived deconstruct
");
    }

    void setNumTwo(int n)
    {
        m_two = n;
    }
    int getNumTwo()
    {
        return m_two;
    }
private:
    int m_two;
};

int main()
{
    CBase *p = new CDrived;
    delete p;
    std::cout << "Hello World!
"; 
}

输出：

this is CBase construct
this is CDrived construct
this is ~CDrived deconstruct
this is ~CBase deconstruct
Hello World!

2，

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

class CBase
{
public:
    CBase() {
        m_one = 0;
        printf("this is CBase construct
");
    }
    virtual ~CBase() {
        printf("this is ~CBase deconstruct
");
    }

    virtual void setNumOne(int n)
    {
        m_one = n;
    }
    virtual int getNumOne()
    {
        return m_one;
    }
private:
    int m_one;
};

class CDrived:public CBase
{
public:
    CDrived() {
        m_two = 0;
        printf("this is CDrived construct
");
    }
    ~CDrived() {
        printf("this is ~CDrived deconstruct
");
    }

    void setNumTwo(int n)
    {
        m_two = n;
    }
    int getNumTwo()
    {
        return m_two;
    }
private:
    int m_two;
};

int main()
{
    CDrived *p = new CDrived;
    printf("sizeof(CDrived) = %d
", sizeof(CDrived)); // 12
    delete p;
    std::cout << "Hello World!
"; 
}

3，多继承

单继承只有一个虚表指针，而多继承往往有多个

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

class CFather
{
public:
    CFather() {
    
    }
    ~CFather() {

    }

    virtual void setTall(int tall)
    {
        m_tall = tall;
    }

private:
    int m_tall;
};

class CMother
{
public:
    CMother() {

    }
    ~CMother() {

    }

    virtual void setWeight(int weight)
    {
        m_weight = weight;
    }

private:
    int m_weight;
};

class CSon:public CFather,public CMother
{
public:
    CSon() {

    }
    ~CSon() {

    }

    virtual void setAge(int age) // 地址存放在第一个虚表指针后面
    {
        m_age = age;
    }

private:
    int m_age;
};

int main()
{
    CSon cSon;
    cSon.setAge(8);
    printf("sizeof(CSon) = %d
", sizeof(CSon)); // 20
    std::cout << "Hello World!
"; 
}