面试中你不可回避的C、C++的问题(三)

本节继续上一次的关于sizeof的讲解：

这次主要是探讨一下，关于sizeof对于类以及对象之间的内存的大小的关系

二维指针域数组的关系

#include <stdio.h>

int main()
{
//存储的是指针所以是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;
}空类的大小，以及带有虚函数的空类的大小
#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;
}再来讨论一下，带有类的成员函数以及继承类对象的大小
#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;
}