C++ 类的大小
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Size of C++ classes
提问by nurabha
Here is the code which prints size of different classes
这是打印不同类大小的代码
#include <iostream>
using namespace std;
class EmptyClass
{
};
class AbstractClass
{
public:
virtual void funcOne() = 0;
virtual void funcTwo() = 0;
};
class NotAbstrClass
{
public: int virtFunc( int );
};
class MixClass
{
public:
virtual void clFunc( int );
static int i;
int j;
};
int main()
{
// Print size of class or class objects
cout<<"Size of empty class: "<< sizeof(EmptyClass)<<endl;
cout<<"Size of Abstract class :"<< sizeof(AbstractClass)<<endl;
cout<<"Size of Non Abstract class: "<< sizeof(NotAbstrClass)<<endl;
cout<<"Size of Mix class: "<< sizeof(MixClass)<<endl;
return 0;
}
The output of the program on C++11 compiler is
在 C++11 编译器上程序的输出是
Size of empty class: 1
Size of Abstract class :4
Size of Non Abstract class: 1
Size of Mix class: 8
I understand why Empty class has size 1 Size of empty class object. For abstract class, the object stores a pointer for implementing virtual function call mechanisms. But what about the sizes of other class objects (NotAbstrClass and MixClass) ?
我明白为什么 Empty class 有 size 1 Size of empty class object。对于抽象类，对象存储了一个指针，用于实现虚函数调用机制。但是其他类对象（NotAbstrClass 和 MixClass）的大小呢？
回答by HappyTran
According to Girish Shetty:
根据 Girish Shetty 的说法：
There are many factors that decide the size of an object of a class in C++.
These factors are:
- Size of all non-static data members
- Order of data members
- Byte alignment or byte padding
- Size of its immediate base class
- The existence of virtual function(s) (Dynamic polymorphism using virtual functions).
- Compiler being used
- Mode of inheritance (virtual inheritance)
在 C++ 中，决定类的对象大小的因素有很多。
这些因素是：
- 所有非静态数据成员的大小
- 数据成员的顺序
- 字节对齐或字节填充
- 其直接基类的大小
- 虚函数的存在（使用虚函数的动态多态）。
- 正在使用的编译器
- 继承方式（虚拟继承）
Here there are some related website, I think it can be helpful to you.
这里有一些相关的网站，我想它可以对你有所帮助。
Determine the size of class object: http://www.cprogramming.com/tutorial/size_of_class_object.html
确定类对象的大小：http: //www.cprogramming.com/tutorial/size_of_class_object.html
Memory layout: http://www.phpcompiler.org/articles/virtualinheritance.html
内存布局：http: //www.phpcompiler.org/articles/virtualinheritance.html
And, if you use MVSC, you can dump all memory layout of all class in your solution with -d1reportAllClassLayoutlike that:
而且，如果您使用 MVSC，您可以-d1reportAllClassLayout像这样转储解决方案中所有类的所有内存布局：
cl -d1reportAllClassLayout main.cpp
回答by Praetorian
NotAbstrClasshas no data members, so it too is an empty class. Since classes cannot be zero-sized, you get the same treatment as EmptyClass.
NotAbstrClass没有数据成员，所以它也是一个空类。由于类不能为零大小，因此您将获得与EmptyClass.
MixClasshas a virtual function, and 1 non-static data member. It seems each of these (vptrand int) occupy 4 bytes on your platform, so the size is 8 bytes.
MixClass有一个虚函数和 1 个非静态数据成员。似乎这些 (vptr和int) 中的每一个都在您的平台上占用 4 个字节，因此大小为 8 个字节。
回答by Marius
NotAbstrClass is like an empty class when we talk about bit sizes since it has no data. MixClass has the virtual function pointer (4 bytes on a 32-bit machine) and an int (also 4 bytes).
当我们谈论位大小时，NotAbstrClass 就像一个空类，因为它没有数据。MixClass 具有虚拟函数指针（在 32 位机器上为 4 个字节）和一个 int（也是 4 个字节）。
