C++ 类的实例化对象大小

发表于

实习生笔试面试时碰到过多次的问题，通过运行代码记录一下结论。测试平台为64位。

空类

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class A {};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    return 0;
}
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sizeof(a) is 1

空类的大小为1

只含有构造函数、析构函数、成员函数的类

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class A {
public:
    A() {};
    ~A() {};
    void fun() {};
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    return 0;
}
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sizeof(a) is 1

大小与构造函数、析构函数、成员函数无关

含有成员变量的类

需要考虑对齐

C++中数据类型占据字节长度
| 数据类型 | 大小 |
| —— | —- |
| char | 1 |
| bool | 1 |
| short | 2 |
| int | 4 |
| float | 4 |
| double | 8 |

简单类

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class A {
public:
    char ch;
    int num;
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    return 0;
}
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sizeof(a) is 8

size = 1(char) + 3(padding) + 4(int) = 8

成员变量不同的顺序

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class A {
public:
    double d;
    char ch;
    int num;
};
class B {
public:
    char ch;
    double d;
    int num;
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    B b;
    cout << "sizeof(b) is " << sizeof(b) << endl;
    return 0;
}
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sizeof(a) is 16
sizeof(b) is 24

大小为占用最大空间的类型所占用的字节数的倍数

sizeof(a) = 8(double) + 1(char) + 3(padding) + 4(int) = 16

sizeof(b) = 1(char) + 7(padding) + 8(double) + 4(int) + 4(padding) = 24

指针

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class A {
public:
    int* p;
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    return 0;
}
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sizeof(a) is 8

指针变量的大小与机器平台有关，64位机器为8个字节

静态成员变量

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class A {
public:
    char ch;
    int num;
    static int count;
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    return 0;
}
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sizeof(a) is 8

静态成员变量不影响

虚函数

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class A {
public:
    virtual void fun1() {}
    virtual void fun2() {}
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    return 0;
}
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sizeof(a) is 8

有虚函数时，占用个内存会包含一个虚表指针(64位大小为8字节)，且不论虚函数的个数多少只有一个

虚表指针放在对象内存的起始位置

继承

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class A {
public:
    char ch1;
};
class B:public A{
public:
    char ch2;
    int num;
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    B b;
    cout << "sizeof(b) is " << sizeof(b) << endl;
    return 0;
}
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sizeof(a) is 1
sizeof(b) is 8

大小叠加并考虑对齐，基类成员在前

同名变量

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class A {
public:
    int num;
};
class B:public A{
public:
    int num;
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    B b;
    cout << "sizeof(b) is " << sizeof(b) << endl;
    return 0;
}
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2
sizeof(a) is 4
sizeof(b) is 8

含有虚函数

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class A {
public:
    virtual void fun1() {}
};
class B:public A{
public:
    virtual void fun2() {}
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    B b;
    cout << "sizeof(b) is " << sizeof(b) << endl;
    return 0;
}
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sizeof(a) is 8
sizeof(b) is 8

基类有虚函数，派生类也有虚函数，在虚表指针指向的虚表中添加一个函数指针，所以派生类仍仅有一个虚表指针

虚继承

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class A {
public:
    int num;
    virtual void fun1() {}
};
class B:virtual public A{
public:
    virtual void fun2() {}
};
int main() {
    A a;
    cout << "sizeof(a) is " << sizeof(a) << endl;
    B b;
    cout << "sizeof(b) is " << sizeof(b) << endl;
    return 0;
}
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2
sizeof(a) is 16
sizeof(b) is 24

与编译器有关，测试使用的是gcc

sizeof(b) = vptr_b(8) + sizeof(a) = 24

其他编译器还会有子类指向父类的指针