【C++】sizeof各种类型的大小

#include 
using namespace std;

////////// 测试基础类型 //////////
int GetArraySize(char str[])
{
    return sizeof(str);
}

void TestSizeofBase()
{
    cout << "////////// 测试基础类型 //////////" << endl;
    char* pStr = "hello world";
    char arrStr[] = "hello world";
    cout << "char: " << sizeof(char) << endl;
    cout << "short: " << sizeof(short) << endl;
    cout << "int: " << sizeof(int) << endl;
    cout << "float: " << sizeof(float) << endl;
    cout << "double: " << sizeof(double) << endl;
    cout << "char*: " << sizeof(pStr) << endl;
    cout << "string: " << sizeof("hello world") << endl;
    cout << "char[]: " << sizeof(arrStr) << endl;
    cout << "char[] as param: " << GetArraySize(arrStr) << endl;
    cout << endl;
}

//////////// 测试空类 ////////////
class EmptyA
{
};

class EmptyB : public EmptyA
{
};

struct EmptyStruct
{
};

void TestSizeofEmpty()
{
    cout << "//////////// 测试空类 ////////////" << endl;
    cout << "EmptyA: " << sizeof(EmptyA) << endl;
    cout << "EmptyB: " << sizeof(EmptyB) << endl;
    cout << "EmptyStruct: " << sizeof(EmptyStruct) << endl;
    cout << endl;
}


////////// 测试内存对齐 //////////
class AlignA
{
    char a;
};

class AlignB
{
    char a;
    short b;
};

class AlignC
{
    char a;
    short b;
    double c;
};

#pragma pack(1)
class AlignD
{
    char a;
    short b;
    double c;
};
#pragma pack()

void TestSizeofAlign()
{
    cout << "////////// 测试内存对齐 //////////" << endl;
    cout << "AlignA: " << sizeof(AlignA) << endl;
    cout << "AlignB: " << sizeof(AlignB) << endl;
    cout << "AlignC: " << sizeof(AlignC) << endl;
    cout << "AlignD: " << sizeof(AlignD) << endl;
    cout << endl;
}

//////////// 测试多态 ////////////

class VirtualA
{
    virtual void test();
};
class VirtualB
{
    virtual void test();
    virtual void test2();
};
void VirtualB::test() {}
void VirtualB::test2() {}

class VirtualC
{
    char a;
    virtual void test();
    virtual void test2();
};

void TestSizeofVirtual()
{
    cout << "//////////// 测试多态 ////////////" << endl;
    VirtualB objB;
    cout << "VirtualA: " << sizeof(VirtualA) << endl;
    cout << "VirtualB: " << sizeof(VirtualB) << endl;
    cout << "VirtualB Object: " << sizeof(objB) << endl;
    cout << "VirtualC: " << sizeof(VirtualC) << endl;
    cout << endl;
}

void TestSizeof()
{
    TestSizeofBase();
    TestSizeofEmpty();
    TestSizeofAlign();
    TestSizeofVirtual();
}

int main(int argc, char **argv)
{
    TestSizeof();
    getchar();
    return 0;
}

输出结果

 

简要说明

sizeof(GetArraySize(arrStr)) 4，是因为参数传递为形参时，把数组转为了指针。 
sizeof(空类) 1，是因为实例化类需要编译器给它分配内存空间，不能分配为size为0的内存，所以编译器默认分配了一个字节，以便标记可能初始化的类实例，同时使空类占用的空间也最少（即1字节）。 
内存对齐，对象的首地址能够被其最宽基本类型成员的大小所整除；每个成员相对于结构体首地址的偏移量（offset）都是成员大小的整数倍， 如有需要编译器会在成员之间加上填充字节（internal adding）；总大小为最宽基本类型成员大小的整数倍。 
虚函数，虚函数表指针，大小相当于sizeof(void *)，C++中为4个字节。