面試中你不可迴避的C、C++的問題(三)

本節繼續上一次的關於sizeof的講解：

這次主要是探討一下，關於sizeof對於類以及對象之間的內存的大小的關係

二維指針域數組的關係

#include <stdio.h>

int main()
{
    //存儲的是指針所以是3*4*4=48
    int ** a[3][4];
    printf("%d\n",sizeof(a));//48
    char** b[3][4];
    printf("%d\n",sizeof(b));//48
    return 0;
}

空類的大小，以及帶有虛函數的空類的大小

#include <stdio.h>
#include <iostream>

using namespace std;

class A
{
};

class B
{
public:
    virtual void f() {}
};

int main()
{
    A a;
    cout << sizeof(a) << endl;//1
    B b;
    cout << sizeof(b) << endl;//4
    return 0;
}

再來討論一下，帶有類的成員函數以及繼承類對象的大小

#include <stdio.h>
#include <iostream>
#include <complex>

using namespace std;

class Base
{
public:
    Base() { cout << "Base Constructor !" << endl; }
    ~Base() { cout << "Base Destructor !" << endl; }
    virtual void f(int ) { cout << "Base::f(int)" << endl; }
    virtual void f(double) { cout << "Base::f(double)" << endl; }
    virtual void g(int i=10) { cout << "Base::g()" << i << endl; }
    void g2(int i=10) { cout << "Base::g2()" << i << endl; }
};

class Derived:public Base
{
public:
    Derived() { cout << "Derived Constructor !" << endl; }
    ~Derived() { cout << "Derived Destructor !" << endl; }
    void f(complex<double>) { cout << "Derived::f(complex)" << endl; }
    virtual void g(int i=20) { cout << "Derived::g()" << i << endl; }
};

int main()
{
    Base c;
    Derived d;
    Base* pb = new Derived;
    cout << sizeof(Base) << endl;//4
    cout << sizeof(Derived) << endl;//4
    cout << sizeof(pb) << endl;//4
    cout << sizeof(c) << endl;//4
    cout << sizeof(d) << endl;//4
    return 0;
}