sizeof操作符关于类相关使用

最新推荐文章于 2024-01-29 17:49:31 发布

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本文通过几个具体的C++代码示例，详细解析了类的大小如何受到类成员、虚函数等因素的影响，并给出了不同情况下类的实际大小。

首先，看一个题目：以下代码使用VC编译成32位可执行程序，请问：nLenA、nLenAObject、nLenB、nLenAObject、nLenC、nLenCObject的值分别为

#include<iostream>
#include<stdlib.h>

using namespace std;

class t_classA
{
	public:
		t_classA(){};
		~t_classA(){};
};

class t_classB
{
	public:
		t_classB(){};
		virtual ~t_classB(){};
};

class t_classC:public t_classA,public t_classB
{
	public:
		t_classC(){};
		virtual ~t_classC(){};
};

int main()
{
	int nLenA = sizeof(t_classA);
	cout<<"nLenA: "<<nLenA<<endl;

	t_classA oA;
	int nLenAObject = sizeof(oA);
	cout<<"nLenAObject: "<<nLenAObject<<endl;

	int nLenB = sizeof(t_classB);
	cout<<"nLenB: "<<nLenB<<endl;

	t_classB oB;
	int nLenBObject = sizeof(oB);
	cout<<"nLenBObject: "<<nLenBObject<<endl;

	int nLenC = sizeof(t_classC);
	cout<<"nLenC: "<<nLenC<<endl;

	t_classC oC;
	int nLenCObject = sizeof(oC);
	cout<<"nLenCObject: "<<nLenCObject<<endl;
}

下面进行全面的分析：

1、空类

#include<iostream>
#include<stdlib.h>

using namespace std;

class A//1
{
};

int main()
{
	A a;
	cout<<sizeof(A)<<endl;//1
	cout<<sizeof(a)<<endl;//1
}

2、一般非空

#include<iostream>
#include<stdlib.h>

using namespace std;

class A
{
	int a;//4
	int *p;//4
	char c;//1
};

int main()
{
	A a;
	cout<<sizeof(A)<<endl;//12
	cout<<sizeof(a)<<endl;//12
}

3.有虚函数

#include<iostream>
#include<stdlib.h>

using namespace std;

class A
{
	public:
	    A(){};
		virtual ~A(){};//4
	private:
	    int b;//4
	    int *p;//4
	    char c;//1
};

int main()
{
	A a;
	cout<<sizeof(A)<<endl;//16
	cout<<sizeof(a)<<endl;//16
}

4.虚函数继承

#include<iostream>
#include<stdlib.h>

using namespace std;

class A//16
{
	public:
	    A(){};
		virtual ~A(){};
	private:
	    int b;
	    int *p;
	    char c;
};
class B:public A
{
	public:
		B(){};
		~B(){};
	private:
	     int b;//4	
};
int main()
{
	B b;
	cout<<sizeof(B)<<endl;//20
	cout<<sizeof(b)<<endl;//20
}

5.静态成员函数

#include<iostream>
#include<stdlib.h>

using namespace std;

class A//1
{
	static int n;//0
};

int main()
{
	A a;
	cout<<sizeof(A)<<endl;//1
	cout<<sizeof(a)<<endl;//1
}

6.普通成员函数

#include<iostream>
#include<stdlib.h>

using namespace std;

class A//1
{
	func();//0
};

int main()
{
	A a;
	cout<<sizeof(A)<<endl;//1
	cout<<sizeof(a)<<endl;//1
}

所以结果如下：

nLenA=1 
nLenAObject=1 
nLenB=4 
nLenAObject=4
nLenC=4
nLenCObject=4

参考：http://blog.youkuaiyun.com/zzhongcy/article/details/38361755