Sizeof的計算看內存分配，sizeof內存分配

Sizeof的計算看內存分配，sizeof內存分配

本文記錄了有關sizeof的一些計算，主要有下面的四種情況：(如有錯誤,敬請留言)

#include <iostream>
#include <stdlib.h>
using namespace std;

//如果數組變量被傳入函數中做sizeof()運算，則和指針的運算沒有區別。
void Func( char str[100] )
{
    cout<<"Fun: == "<<sizeof(str)<<endl;//4字節的指針
}

int main()
{
    char str[] = "Hello";
    char *p = str;
    int n = 10;

    cout<<"str == "<<sizeof(str)<<endl;
    //結果=6=strlen("Hello")+1;數值中要留一個元素保存字符串結束符
    Func(str);

    //2個結果都是4，win32下，指針和int都是4字節
    cout << "p ==" << sizeof(p) << endl;
    cout << "n ==" << sizeof(n) << endl;//結果4

    void *q = malloc(100);//100字節的堆內存
    cout << "void * == " << sizeof(q) << endl;
    //還是4(指針類型的大小)

    char *ch = (char *)malloc(sizeof(char)*10);
    cout << "char * == " <<sizeof(ch) << endl;
    //還是4(指針類型的大小)
    
    return 0;
}
//數組和指針的sizeof()是有區別的。
//對於指針，不論何種類型，其大小都是4字節的。

通過上面的示例,我們可以看到:sizeof計算的都是數據類型占用空間的大小,除了一種情況,就是初始化char str[]="Hello"的時候，sizeof計算的是字符串的長度+1.

sizeof計算類對象所占用空間的大小-用到了字節對齊

#include <iostream>
using namespace std;

class A
{
public:
    int i;//4
};
class AA
{
public:
    double d;
    int i;//8+4+4（補全）
};
class B
{
public:
    char ch;//1
};
class C
{
public:
    int i;
    short j;//4+2+2(補全)
};
class D
{
public:
    int i;
    short j;
    char ch;//4+2+1+1(補全)
};
class E
{
public:
    int i;
    int ii;
    short l;
    char ch;
    char chr;//4+4+2+1+1
};
class F
{
public:
    int i;
    int ii;
    int iii;
    short j;
    char ch;
    char chr;//4+4+4+2+1+1
};
class FF
{
public:
    short j;
    int i;
    char ch;
    int ii;
    char chr;
    int iii;//2+2（補全）+4+1+3（補全）+4+1+3（補全）+4=24
};
int main()
{
    cout<<"sizeof(double) == "<<sizeof(double)<<endl;//4字節
    cout<<"sizeof(float) == "<<sizeof(float)<<endl;//4字節
    cout<<"sizeof(int) == "<<sizeof(int)<<endl;//4字節
    cout<<"sizeof(short) == "<<sizeof(short)<<endl;//2
    cout<<"sizeof(char) == "<<sizeof(char)<<endl<<endl;//1

    cout<<"sizeof(A) == "<<sizeof(A)<<endl;
    cout<<"sizeof(AA) == "<<sizeof(AA)<<endl;//最寬的類型是8字節的。
    cout<<"sizeof(B) == "<<sizeof(B)<<endl;
    cout<<"sizeof(C) == "<<sizeof(C)<<endl;
    cout<<"sizeof(D) == "<<sizeof(D)<<endl;
    cout<<"sizeof(E) == "<<sizeof(E)<<endl;
    cout<<"sizeof(F) == "<<sizeof(F)<<endl;
    cout<<"sizeof(FF) == "<<sizeof(FF)<<endl;//同樣的類型個數，只是順序變了，就不一樣了。
    return 0;
}

sixeof計算含有虛函數的類對象的空間大小

#include <iostream>
using namespace std;
class Base
{
public:
    Base(int x):a(x){}
    void print()
    {
        cout<<"base"<<endl;
    }
private:
    int a;
};
class Dervied:public Base
{
public:
    Dervied(int x):Base(x-1),b(x){}
    void print()
    {
        cout<<"Dervied"<<endl;
    }
private:
    int b;
};
class A
{
public:
    A(int x):a(x){}
    virtual void print()
    {
        cout<<"A"<<endl;
    }
private:
    int a;
};
class B:public A
{
public:
    B(int x):A(x-1),b(x){}
    virtual void print()
    {
        cout<<"B"<<endl;
    }
private:
    int b;
};

int main()
{
    Base obj1(1);
    cout<<"size of base obj1 is "<<sizeof(obj1)<<endl;
    Dervied obj2(2);
    cout<<"size of Dervied obj2 is "<<sizeof(obj2)<<endl;

    A a(1);
    cout<<"size of A obj is "<<sizeof(a)<<endl;
    B b(2);
    cout<<"size of B obj is "<<sizeof(b)<<endl;
    return 0;
}

普通函數不占用內存，只要有虛函數，就會占用一個指針大小的內存，原因，系統多用了一個指針維護這個類的虛函數表， 並注意到，這個虛函數無論含有多少項（類中含有多少個虛函數）都不會再影響類的大小了。

sizeof計算虛擬繼承的類對象的空間大小

#include <iostream>
using namespace std;
class A{};
class B{};
class C:public A,public B{};
class D:virtual public A{};
class E:virtual public A,virtual public B{};
class F
{
public:
    int a;
    static int b;
};
int F::b = 10;
int main()
{
    cout<<"sizeof(A) =="<<sizeof(A)<<endl;
    cout<<"sizeof(B) =="<<sizeof(B)<<endl;
    cout<<"sizeof(C) =="<<sizeof(C)<<endl;
    cout<<"sizeof(D) =="<<sizeof(D)<<endl;
    cout<<"sizeof(E) =="<<sizeof(E)<<endl;
    cout<<"sizeof(F) =="<<sizeof(F)<<endl;
    return 0;
}