类实例化和构造函数

2024-09-01 03:12
文章标签 实例 构造函数

本文主要是介绍类实例化和构造函数，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

类实例化和构造函数

  • 类如何创立，如何调用构造函数
    • 源码
    • rv汇编
    • 行为分析
  • 一般成员函数
  • 虚函数
    • 源码
    • 汇编
    • 行为分析
  • 纯虚函数
    • 汇编
    • 行为分析
  • 多态
    • 源码
    • 汇编
    • 行为分析
  • 为什么构造函数不能是虚函数

类如何创立，如何调用构造函数

源码

#include <iostream>
using namespace std;// 抽象父类
class Base {int a,b;
public:Base(){a=1;b=2;}};// 子类
class Derived : public Base {
public:int c;Derived(){c=3;}
};int main() {Derived b ; return 0;
}

rv汇编

__cxx_global_var_init:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16lui     a0, %hi(std::__ioinit)addi    a0, a0, %lo(std::__ioinit)sw      a0, -12(s0)call    std::ios_base::Init::Init() [complete object constructor]lw      a1, -12(s0)lui     a0, %hi(_ZNSt8ios_base4InitD1Ev)addi    a0, a0, %lo(_ZNSt8ios_base4InitD1Ev)lui     a2, %hi(__dso_handle)addi    a2, a2, %lo(__dso_handle)call    __cxa_atexitlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retmain:addi    sp, sp, -32sw      ra, 28(sp)sw      s0, 24(sp)addi    s0, sp, 32mv      a0, zerosw      a0, -28(s0)sw      a0, -12(s0)addi    a0, s0, -24call    Derived::Derived() [base object constructor]lw      a0, -28(s0)lw      s0, 24(sp)lw      ra, 28(sp)addi    sp, sp, 32retDerived::Derived() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)sw      a0, -16(s0)call    Base::Base() [base object constructor]lw      a1, -16(s0)addi    a0, zero, 3sw      a0, 8(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::Base() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a1, -12(s0)addi    a0, zero, 1sw      a0, 0(a1)addi    a0, zero, 2sw      a0, 4(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16ret_GLOBAL__sub_I_example.cpp:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16call    __cxx_global_var_initlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retstd::__ioinit:.zero   1

行为分析

  • 在main会去调用Derived的构造函数，并且在调用之前，会给a0寄存器一个地址，这个地址其实是this指针，也就是Derived实例化对象b的地址，main在传入之前已经为b实例开辟了栈空间
  • Derived的构造函数的也会传入b的地址给Base的构造函数
  • 进入Base构造就不难看出，这里拿着最开始b的地址在做实际的输出化了
        lw      a1, -12(s0)addi    a0, zero, 1sw      a0, 0(a1)addi    a0, zero, 2sw      a0, 4(a1)lw      s0, 8(sp)

这里是先把地址放在栈上，然后a1拿着地址，之后就往偏移位置放值，这里对应的就是初始化父类的成员变量

  • 调用完回到Derived的构造函数，再初始化子类成员变量
        lw      a1, -16(s0)addi    a0, zero, 3sw      a0, 8(a1)

一般成员函数

#include <iostream>
using namespace std;// 抽象父类
class Base {int a,b;
public:Base(){a=1;b=2;}};// 子类
class Derived : public Base {
public:int c;Derived(){c=3;}void show(){cout<<c<<endl;}
};int main() {Derived b ; b.show();return 0;
}

这里其实和构造函数一样，调用之气会传入变量的地址，也就是this指针，以便函数使用成员变量

虚函数

源码

#include <iostream>
using namespace std;class Base {
public:int a,b;Base(){a=1;b=2;}virtual void show1(){cout<<a<<endl;}virtual void show2(){cout<<a<<endl;}};// 子类
class Derived : public Base {
public:int c;Derived(){c=3;}virtual void show2(){cout<<a<<endl;}
};int main() {Derived b ; b.show1();return 0;
}

汇编

__cxx_global_var_init:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16lui     a0, %hi(std::__ioinit)addi    a0, a0, %lo(std::__ioinit)sw      a0, -12(s0)call    std::ios_base::Init::Init() [complete object constructor]lw      a1, -12(s0)lui     a0, %hi(_ZNSt8ios_base4InitD1Ev)addi    a0, a0, %lo(_ZNSt8ios_base4InitD1Ev)lui     a2, %hi(__dso_handle)addi    a2, a2, %lo(__dso_handle)call    __cxa_atexitlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retmain:addi    sp, sp, -48sw      ra, 44(sp)sw      s0, 40(sp)addi    s0, sp, 48mv      a0, zerosw      a0, -36(s0)sw      a0, -12(s0)addi    a0, s0, -32sw      a0, -40(s0)call    Derived::Derived() [base object constructor]lw      a0, -40(s0)call    Base::show1()lw      a0, -36(s0)lw      s0, 40(sp)lw      ra, 44(sp)addi    sp, sp, 48retDerived::Derived() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)sw      a0, -16(s0)call    Base::Base() [base object constructor]lw      a1, -16(s0)lui     a0, %hi(vtable for Derived)addi    a0, a0, %lo(vtable for Derived)addi    a0, a0, 8sw      a0, 0(a1)addi    a0, zero, 3sw      a0, 12(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::show1():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::Base() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a1, -12(s0)lui     a0, %hi(vtable for Base)addi    a0, a0, %lo(vtable for Base)addi    a0, a0, 8sw      a0, 0(a1)addi    a0, zero, 1sw      a0, 4(a1)addi    a0, zero, 2sw      a0, 8(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retDerived::show2():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::show2():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16ret_GLOBAL__sub_I_example.cpp:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16call    __cxx_global_var_initlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retstd::__ioinit:.zero   1vtable for Derived:.word   0.word   typeinfo for Derived.word   Base::show1().word   Derived::show2()typeinfo name for Derived:.asciz  "7Derived"typeinfo name for Base:.asciz  "4Base"typeinfo for Base:.word   _ZTVN10__cxxabiv117__class_type_infoE+8.word   typeinfo name for Basetypeinfo for Derived:.word   _ZTVN10__cxxabiv120__si_class_type_infoE+8.word   typeinfo name for Derived.word   typeinfo for Basevtable for Base:.word   0.word   typeinfo for Base.word   Base::show1().word   Base::show2()

行为分析

有虚函数的话，对象的首地址就会设置为虚函数表的位置，然后父类设置之后子类再设置覆盖掉。

纯虚函数

源码

#include <iostream>
using namespace std;class Base {
public:int a,b;Base(){a=1;b=2;}virtual void show1(){cout<<a<<endl;}virtual void show2()=0;};// 子类
class Derived : public Base {
public:int c;Derived(){c=3;}virtual void show2(){cout<<a<<endl;}
};int main() {Derived b ; b.show1();return 0;
}

汇编

__cxx_global_var_init:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16lui     a0, %hi(std::__ioinit)addi    a0, a0, %lo(std::__ioinit)sw      a0, -12(s0)call    std::ios_base::Init::Init() [complete object constructor]lw      a1, -12(s0)lui     a0, %hi(_ZNSt8ios_base4InitD1Ev)addi    a0, a0, %lo(_ZNSt8ios_base4InitD1Ev)lui     a2, %hi(__dso_handle)addi    a2, a2, %lo(__dso_handle)call    __cxa_atexitlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retmain:addi    sp, sp, -48sw      ra, 44(sp)sw      s0, 40(sp)addi    s0, sp, 48mv      a0, zerosw      a0, -36(s0)sw      a0, -12(s0)addi    a0, s0, -32sw      a0, -40(s0)call    Derived::Derived() [base object constructor]lw      a0, -40(s0)call    Base::show1()lw      a0, -36(s0)lw      s0, 40(sp)lw      ra, 44(sp)addi    sp, sp, 48retDerived::Derived() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)sw      a0, -16(s0)call    Base::Base() [base object constructor]lw      a1, -16(s0)lui     a0, %hi(vtable for Derived)addi    a0, a0, %lo(vtable for Derived)addi    a0, a0, 8sw      a0, 0(a1)addi    a0, zero, 3sw      a0, 12(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::show1():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::Base() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a1, -12(s0)lui     a0, %hi(vtable for Base)addi    a0, a0, %lo(vtable for Base)addi    a0, a0, 8sw      a0, 0(a1)addi    a0, zero, 1sw      a0, 4(a1)addi    a0, zero, 2sw      a0, 8(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retDerived::show2():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16ret_GLOBAL__sub_I_example.cpp:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16call    __cxx_global_var_initlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retstd::__ioinit:.zero   1vtable for Derived:.word   0.word   typeinfo for Derived.word   Base::show1().word   Derived::show2()typeinfo name for Derived:.asciz  "7Derived"typeinfo name for Base:.asciz  "4Base"typeinfo for Base:.word   _ZTVN10__cxxabiv117__class_type_infoE+8.word   typeinfo name for Basetypeinfo for Derived:.word   _ZTVN10__cxxabiv120__si_class_type_infoE+8.word   typeinfo name for Derived.word   typeinfo for Basevtable for Base:.word   0.word   typeinfo for Base.word   Base::show1().word   __cxa_pure_virtual

行为分析

抽象类的虚函数表里标注了纯虚函数，这里是找不到base的show2实现的

vtable for Base:.word   0.word   typeinfo for Base.word   Base::show1().word   __cxa_pure_virtual

行为和没有纯虚函数一样，也是替换表

多态

源码

#include <iostream>
using namespace std;class Base {
public:int a,b;Base(){a=1;b=2;}virtual void show2(){cout<<a<<endl;}virtual void show1(){cout<<a<<endl;}};// 子类
class Derived : public Base {
public:int c;Derived(){c=3;}virtual void show2(){cout<<a<<endl;}virtual void show3(){cout<<a<<endl;}
};// 修正后的函数
void callShow2(Base* x){x->show2();
}int main() {Derived b ; b.show1();callShow2(&b); // 调用修正后的函数来展示 show2 的效果return 0;
}

汇编

__cxx_global_var_init:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16lui     a0, %hi(std::__ioinit)addi    a0, a0, %lo(std::__ioinit)sw      a0, -12(s0)call    std::ios_base::Init::Init() [complete object constructor]lw      a1, -12(s0)lui     a0, %hi(_ZNSt8ios_base4InitD1Ev)addi    a0, a0, %lo(_ZNSt8ios_base4InitD1Ev)lui     a2, %hi(__dso_handle)addi    a2, a2, %lo(__dso_handle)call    __cxa_atexitlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retcallShow2(Base*):addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 0(a0)lw      a1, 0(a1)jalr    a1lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retmain:addi    sp, sp, -48sw      ra, 44(sp)sw      s0, 40(sp)addi    s0, sp, 48mv      a0, zerosw      a0, -36(s0)sw      a0, -12(s0)addi    a0, s0, -32sw      a0, -40(s0)call    Derived::Derived() [base object constructor]lw      a0, -40(s0)call    Base::show1()lw      a0, -40(s0)call    callShow2(Base*)lw      a0, -36(s0)lw      s0, 40(sp)lw      ra, 44(sp)addi    sp, sp, 48retDerived::Derived() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)sw      a0, -16(s0)call    Base::Base() [base object constructor]lw      a1, -16(s0)lui     a0, %hi(vtable for Derived)addi    a0, a0, %lo(vtable for Derived)addi    a0, a0, 8sw      a0, 0(a1)addi    a0, zero, 3sw      a0, 12(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::show1():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::Base() [base object constructor]:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a1, -12(s0)lui     a0, %hi(vtable for Base)addi    a0, a0, %lo(vtable for Base)addi    a0, a0, 8sw      a0, 0(a1)addi    a0, zero, 1sw      a0, 4(a1)addi    a0, zero, 2sw      a0, 8(a1)lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retDerived::show2():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retDerived::show3():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retBase::show2():addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16sw      a0, -12(s0)lw      a0, -12(s0)lw      a1, 4(a0)lui     a0, %hi(_ZSt4cout)addi    a0, a0, %lo(_ZSt4cout)call    std::ostream::operator<<(int)lui     a1, %hi(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)addi    a1, a1, %lo(_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_)call    std::ostream::operator<<(std::ostream& (*)(std::ostream&))lw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16ret_GLOBAL__sub_I_example.cpp:addi    sp, sp, -16sw      ra, 12(sp)sw      s0, 8(sp)addi    s0, sp, 16call    __cxx_global_var_initlw      s0, 8(sp)lw      ra, 12(sp)addi    sp, sp, 16retstd::__ioinit:.zero   1vtable for Derived:.word   0.word   typeinfo for Derived.word   Derived::show2().word   Base::show1().word   Derived::show3()typeinfo name for Derived:.asciz  "7Derived"typeinfo name for Base:.asciz  "4Base"typeinfo for Base:.word   _ZTVN10__cxxabiv117__class_type_infoE+8.word   typeinfo name for Basetypeinfo for Derived:.word   _ZTVN10__cxxabiv120__si_class_type_infoE+8.word   typeinfo name for Derived.word   typeinfo for Basevtable for Base:.word   0.word   typeinfo for Base.word   Base::show2().word   Base::show1()

行为分析

首先观察表，子类是肯定包含了父类的所有虚函数的，只是会把重新实现的，替换为自己的，所以体现多态就是这样，我们传入参数是基类指针，因为默认的虚函数表是在首地址的，大家都一样，然后我们都想调用show2(),因为穿进来的指针指向的地址拿到的虚函数表是不一样的，虽然偏移一样，但是最后调用的就是不同的虚函数实现。

为什么构造函数不能是虚函数

由之前的行为分析知道，子类实例化过程中，会去调用父类的构造函数，将对象的首地址设置为父类的虚函数表，再去设置父类的成员变量，再退回到子类构造函数重复一样的操作。
对于虚函数，是为了覆盖父类的行为实现多态，那么假设构造是虚函数，怎么去覆盖父类的构造行为，虚函数的；
进一步将，子类的行为是，调用父类构造，然后覆盖他的虚函数表，然后父类构造变成了子类构造，这完全乱套。
所以来说，构造根本就和多态不是一个东西，混在一起完全没有意义，他只是帮助设置初值和虚函数表的，与多态行为无关。

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