C++ 内存布局 - Part5: 继承关系中 构造析构与vptr的调整

这里以单继承为例，汇编采用AT&T格式，先看示例代码：

#include <iostream>class Base {
public:Base() {std::cout << "Base Constructor, this ptr: " << this << std::endl;printVptr();}virtual ~Base() {std::cout << "Base Destructor, this ptr: " << this << std::endl;printVptr();}virtual void printVptr() { std::cout << "Base::printVptr, vptr = " << *(void **)this << std::endl; }int a;int b;
};class Derived : public Base {
public:Derived() {std::cout << "Derived Constructor, this ptr: " << this << std::endl;printVptr();}~Derived() {std::cout << "Derived Destructor, this ptr: " << this << std::endl;printVptr();}void printVptr() override { std::cout << "Derived::printVptr, vptr = " << *(void **)this << std::endl; }int c;
};int main() {std::cout << "Base size: " << sizeof(Base) << std::endl;std::cout << "Derived size: " << sizeof(Derived) << std::endl;Base *obj = new Derived();std::cout << "before delete, this ptr is: " << obj << std::endl;delete obj;return 0;
}

编译运行结果：

Base size: 16
Derived size: 24
Base Constructor, this ptr: 0xa1a2c0
Base::printVptr, vptr = 0x401090
Derived Constructor, this ptr: 0xa1a2c0
Derived::printVptr, vptr = 0x401068
before delete, this ptr is: 0xa1a2c0
Derived Destructor, this ptr: 0xa1a2c0
Derived::printVptr, vptr = 0x401068
Base Destructor, this ptr: 0xa1a2c0
Base::printVptr, vptr = 0x401090

从结果中可以看到， 在执行Base部分的构造和析构时，vptr指向的是基类的虚表，执行Derived部分的构造和析构时，vptr指向的是派生类的虚表。

查看汇编代码，以Base的析构为例，其他的构造析构都是类似：

Base::~Base() [base object destructor]:pushq   %rbpmovq    %rsp, %rbpsubq    $16, %rspmovq    %rdi, -8(%rbp)             将this指针入栈movl    $vtable for Base+16, %edx  将虚表指针存入edxmovq    -8(%rbp), %rax             将this指针存入raxmovq    %rdx, (%rax)  将虚表指针存入rax指向的内存，也就是对象内存的开始部分，vptrmovl    $.LC1, %esimovl    $_ZSt4cout, %edicall    std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)movq    %rax, %rdxmovq    -8(%rbp), %raxmovq    %rax, %rsimovq    %rdx, %rdicall    std::basic_ostream<char, std::char_traits<char> >::operator<<(void const*)movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, %esimovq    %rax, %rdicall    std::basic_ostream<char, std::char_traits<char> >::operator<<(std::basic_ostream<char, std::char_traits<char> >& (*)(std::basic_ostream<char, std::char_traits<char> >&))movq    -8(%rbp), %raxmovq    %rax, %rdicall    Base::printVptr()nopleaveret

因此，整个构造析构的顺序为：

当new一个派生类对象时，首先会执行基类的构造函数，这时这个构造中的对象，其vptr指向基类的虚表，当基类部分构造完毕，继续执行派生类的构造函数时，此时对象的vptr指向派生类的虚表。

当delete这个派生类对象时，首先执行派生类的析构函数，此时对象的vptr仍然指向派生类的虚表，派生类的析构函数会继续执行基类的析构函数，此时对象的vptr会指向基类的虚表。最后派生类的析构函数会负责释放整个对象内存： call    operator delete(void*, unsigned long)