將常見sizeof()考核點匯總如下，細則可參考文末鏈接，可先瀏覽以下示例。
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
using namespace std;
class A{};
class A1{
void test1();
};
class A2{
virtual void test();
};
class B
{
public:
int a;
double b;
private:
int setValue();
};
class C:public B
{
public:
int s;
};
class D:public B,public A
{
public:
int s;
int ch;
};
struct stTestA
{
int a;
char b;
};
struct stTestB
{
char a;
double b;
};
struct stTestC
{
int a;
double b;
short c;
};
struct stTestD
{
char a;
int b;
double c;
};
void main()
{
cout<<sizeof(char)<<endl; //1
cout<<sizeof(short)<<endl; //2
cout<<sizeof(int)<<endl; //4
cout<<sizeof(float)<<endl; //4
cout<<sizeof(double)<<endl; //8
cout<<sizeof(A)<<endl; //1 空類為占用字節為1
cout<<sizeof(A1)<<endl; //1 普通函數不計
cout<<sizeof(A2)<<endl; //4 虛函數表指針 4字節
cout<<sizeof(B)<<endl; //16 字節對齊
cout<<"C:"<<sizeof(C)<<endl; //24
cout<<"D:"<<sizeof(D)<<endl; //24
cout<<sizeof(stTestA)<<endl; //8
cout<<sizeof(stTestB)<<endl; //16
cout<<sizeof(stTestC)<<endl; //24
cout<<sizeof(stTestD)<<endl; //16 字節對齊細則 體會例子間差別
cout<<sizeof(std::string)<<endl; //32 與編譯器及環境有關
string *p;
cout<<sizeof(p)<<endl; //4 任何指針都是4（32bit環境）或8（64bit環境）
}
參考文檔
https://www.cnblogs.com/bigbigtree/p/3580585.html