c++虚函数表

概述

首先,相较于C语言，C++语言并没有额外增加内存消耗(确切说，在没有虚函数情况下)。对于一个C++类对象,每个对象有独立的数据成员(非static),但是内存中成员函数只有一份,该类的所有对象共享成员函数。

编译器在编译阶段,进行函数的重构，即将成员函数进行非成员化。通过将this指针作为函数的第一个参数,通过this指针即可以找到对象的数据成员

使用GDB调试 C++ 虚函数

class Base
{
public:
        int a;
        int b;
        virtual void function(){}
        virtual void function001(){}

        Base(){}
        void NotVirtFunc(){}
};

class Derive : public Base
{
public:
    Derive(){}
    int c;
    virtual void function(){}
    virtual void function006(){}
};

class Test
{
    public:
    int vv;
    virtual void bar(){}
    virtual void foo(){}

    Test(){}
};

class Derive2 : public Derive, public Test
{
public:
    Derive2(){}
    int d2;
    virtual void function(){}
    virtual void function006(){}
    virtual void bar(){}
};

int main()
{
    Base b;                         // 没有继承
    b.function();

    Derive d;                       // 继承
    d.function();

    Derive2* d2 = new Derive2();    //多继承
    d2->function();
    delete d2;
    return 0;
}

(gdb) p b
$1 = {_vptr.Base = 0x400a90 <vtable for Base+16>, a = 0, b = 0}
(gdb) info vtbl b
vtable for 'Base' @ 0x400a90 (subobject @ 0x7fffffffe2e0):
[0]: 0x40081e <Base::function()>
[1]: 0x400828 <Base::function001()>

(gdb) p d
$2 = {<Base> = {_vptr.Base = 0x400a50 <vtable for Derive+16>, a = 0, b = 0}, c = 1}
(gdb) info vtbl d
vtable for 'Derive' @ 0x400a50 (subobject @ 0x7fffffffe2c0):
[0]: 0x40086e <Derive::function()>
[1]: 0x400828 <Base::function001()>
[2]: 0x400878 <Derive::function006()>

(gdb) p *d2        
$5 = {<Derive> = {<Base> = {_vptr.Base = 0x4009d0 <vtable for Derive2+16>, a = 0, b = 0}, c = 0}, 
        <Test> = {_vptr.Test = 0x400a00 <vtable for Derive2+64>, vv = 0}, d2 = 0}
(gdb) info vtbl d2
vtable for 'Derive2' @ 0x4009d0 (subobject @ 0x603010):
[0]: 0x4008ee <Derive2::function()>
[1]: 0x400828 <Base::function001()>
[2]: 0x4008f8 <Derive2::function006()>
[3]: 0x400902 <Derive2::bar()>

vtable for 'Test' @ 0x400a00 (subobject @ 0x603028):
[0]: 0x40090c <non-virtual thunk to Derive2::bar()>
[1]: 0x40088c <Test::foo()>