3-C++中类大小影响因素

• 对齐规则
• 虚继承
• 虚函数（多个合并）
• 静态变量
• 成员变量的大小
  

#include <iostream>
#include <cstddef>using namespace std;// 空类 
class EmptyClass {
};// char
class charClass {char c;	
};
// int
class intClass {int a;	
};// char+int
class charintClass {char c;	int a;	
};// int+double
class intdoubleClass {	int a;	double d;
};// 只有静态成员和函数
class StaticMemberClass {
private:static int static_var;    // 不影响类大小int normal_var;           // 影响
public:static void static_func() {}  // 不影响void normal_func() {}         // 不影响
};
// 静态成员变量必须在类外定义
int StaticMemberClass::static_var = 0;// 有虚函数的类
class VirtualFunctionClass {
private:char c;public:virtual void virtual_func() {}    // 引入虚函数表指针void normal_func() {}
};// 4. 单继承
class Base {
private:int base_data;
};class Derived : public Base {
private:int derived_data;
};// 5. 多继承
class Base1 {
private:int base1_data;
};class Base2 {
private:int base2_data;
};class MultipleDerived : public Base1, public Base2 {
private:int derived_data;
};// 虚继承
class VirtualBase {
private:int virtual_base_data;
};class VirtualDerived : virtual public VirtualBase {
private://	vbtptr 虚基类表指针int virtual_derived_data;char c;// virtual_base_data
};// 包含虚函数的多继承
class BaseWithVirtual {
private:int base_data;
public:virtual void base_virtual() {}
};class AnotherBase {
private:int another_data;
};class ComplexDerived : public BaseWithVirtual, public AnotherBase {
private:int complex_data;
public:virtual void derived_virtual() {}
};// 测试内存对齐
class AlignmentTest {
private:char c;      // 1字节int i;       // 4字节double d;    // 8字节short s;     // 2字节
};class OptimizedAlignment {
private:double d;    // 8字节int i;       // 4字节short s;     // 2字节char c1;      // 1字节char c2;      // 1字节
};int main() {// 输出各平台相关信息cout << "=== 平台信息 ===" << endl;cout << "指针大小: " << sizeof(void*) << " 字节" << endl;cout << "int大小: " << sizeof(int) << " 字节" << endl;cout << "double大小: " << sizeof(double) << " 字节" << endl;cout << "=== C++类大小影响因素分析 ===" << endl;cout << "sizeof(EmptyClass): " << sizeof(EmptyClass) << " 字节" << endl;cout << "sizeof(charClass): " << sizeof(charClass) << " 字节" << endl;cout << "sizeof(intClass): " << sizeof(intClass) << " 字节" << endl;cout << "sizeof(charintClass): " << sizeof(charintClass) << " 字节" << endl;cout << "sizeof(intdoubleClass): " << sizeof(intdoubleClass) << " 字节" << endl;cout << "sizeof(StaticMemberClass): " << sizeof(StaticMemberClass) << " 字节" << endl;cout << "sizeof(VirtualFunctionClass): " << sizeof(VirtualFunctionClass) << " 字节" << endl;cout << endl;cout << "=== 继承对类大小的影响 ===" << endl;cout << "sizeof(Base): " << sizeof(Base) << " 字节" << endl;cout << "sizeof(Derived): " << sizeof(Derived) << " 字节" << endl;cout << "sizeof(Base1): " << sizeof(Base1) << " 字节" << endl;cout << "sizeof(Base2): " << sizeof(Base2) << " 字节" << endl;cout << "sizeof(MultipleDerived): " << sizeof(MultipleDerived) << " 字节" << endl;cout << endl;cout << "=== 虚继承对类大小的影响 ===" << endl;cout << "sizeof(VirtualBase): " << sizeof(VirtualBase) << " 字节" << endl;cout << "sizeof(VirtualDerived): " << sizeof(VirtualDerived) << " 字节" << endl;cout << endl;cout << "=== 虚函数在多继承中的影响 ===" << endl;cout << "sizeof(BaseWithVirtual): " << sizeof(BaseWithVirtual) << " 字节" << endl;cout << "sizeof(AnotherBase): " << sizeof(AnotherBase) << " 字节" << endl;cout << "sizeof(ComplexDerived): " << sizeof(ComplexDerived) << " 字节" << endl;cout << endl;cout << "=== 内存对齐的影响 ===" << endl;cout << "sizeof(AlignmentTest): " << sizeof(AlignmentTest) << " 字节" << endl;cout << "sizeof(OptimizedAlignment): " << sizeof(OptimizedAlignment) << " 字节" << endl;cout << endl;return 0;
}