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我們先了解derived class在調(diào)用constructor的時(shí)候,首先先調(diào)用的是其父類的構(gòu)造函數(shù)
why?
首先我們的derived class obj中有所有的父類class的成員,在derived clas obj中其父類成員的可訪問(wèn)程度根據(jù)父類成員所在位置所定(子類對(duì)象可以訪問(wèn)父類的public和protected成員),所以在我們的子類構(gòu)造的時(shí)候要先構(gòu)造父類class(主要將其父類成員初始化)
沒有構(gòu)造函數(shù)編譯器會(huì)默認(rèn)分配一個(gè)編譯器給你,其功能僅僅是初始化成員而已
如下圖、
#includeclass Base{public:
Base(){std::cout<< "constructor of Base "<<'\n'; }
};
class Derived: public Base{public:
Derived(){std::cout<< "constructor of derived "<< '\n'; }
};
int main(){Derived* d1 = new Derived();
return 0;
} 打印如下
constructor of Base
constructor of derived當(dāng)然我們構(gòu)造子類的時(shí)候也可以選擇性的調(diào)用父類構(gòu)造函數(shù)
#includeclass Base{public:
Base(){std::cout<< "constructor of Base "<<'\n'; }
Base(int a) : Base_value(a) {std::cout<< "constructor of base and init base_value and base_value is "<< Base_value<< '\n';}
int Base_value;
};
class Derived: public Base{public:
Derived(): Base(){std::cout<< "constructor of derived "<< '\n'; }
Derived(int a): Base(a){std::cout<< "call second constructor of Base_Value"<< '\n';}
};
int main(){Derived* d1 = new Derived(100);
// Derived obj;
return 0;
} 我們上面講到了子類中其實(shí)是有父類的成員,所以在構(gòu)造子類的時(shí)候先調(diào)用父類的構(gòu)造函數(shù)構(gòu)造父類,有構(gòu)造那么就有析構(gòu)函數(shù),析構(gòu)的順序正好相反,先析構(gòu)子類,再析構(gòu)父類如下
#includeclass Base{public:
Base(){std::cout<< "constructor of Base "<<'\n'; }
Base(int a) : Base_value(a) {std::cout<< "constructor of base and init base_value and base_value is "<< Base_value<< '\n';}
~Base(){std::cout<< "destructor of Base"<<'\n';}
int Base_value;
};
class Derived: public Base{public:
Derived(): Base(){std::cout<< "constructor of derived "<< '\n'; }
Derived(int a): Base(a){std::cout<< "call second constructor of Base_Value"<< '\n';}
~Derived(){std::cout<< "destructor of Derived"<< '\n';}
};
int main(){Derived* d1 = new Derived(100);
// Derived obj;
std::cout<< "---------------"<< '\n';
delete d1;
return 0;
} 打印如下
constructor of base and init base_value and base_value is 100
call second constructor of Base_Value
---------------
destructor of Derived
destructor of Base我們?cè)倏聪旅娲a在deletederived class的時(shí)候只有base的destructor被執(zhí)行,而derived class的destructor并沒有被執(zhí)行
#includeclass Base{public:
virtual void fun() {std::cout<< "base fun"<< std::endl; }
~Base() {std::cout<< "base destructor"<< std::endl;}
};
class Derive: public Base{public:
virtual void fun() {std::cout<< "derived fun"<< '\n';}
~Derive() {std::cout<< "Derive destructor"<<'\n'; }
};
int main(){Base *b1 = new Base();
Base *b2 = new Derive();
b1->fun();
b2->fun();
delete b1;
delete b2;
} 打印如下
base fun
derived fun
base destructor
base destructor為什么會(huì)這樣?我們注意我們new完Derived class后將其賦值給Base class指針,此時(shí)我們new完Derived class生成的obj1雖然有derived class自有的成員變量,但是obj1歸根結(jié)底是Base class的obj,所以不可訪問(wèn)Derived Class的額外成員這叫做object slicing
此時(shí)又有一個(gè)問(wèn)題誕生,因?yàn)槲覀儗⒆宇惖膶?duì)象指針obj1賦值給父類,那么意味著obj1只能使用父類部分的成員,我們直接實(shí)例化父類這樣實(shí)例化后的對(duì)象又比obj1占用的空間小,又簡(jiǎn)單不是挺好嗎?為什么還要將子類對(duì)象obj1賦值給父類?這里有一個(gè)用處,就是父類中的一些成員是virtual,那么子類對(duì)象中父類vptr指向的函數(shù)為子類override后的地址，如下圖
class X {int x;
string str;
public:
X() {}
virtual ~X() {}
virtual void printAll() {}
};
class Y : public X {int y;
public:
Y() {}
~Y() {}
void printAll() {}
};Y memory layout
| |
|------------------------------|<------ Y class object memory layout
| int X::x |
stack |------------------------------|
| | int string::len |
| |string X::str ----------------|
| | char* string::str |
\|/ |------------------------------| |-------|--------------------------|
| X::_vptr |------| | type_info Y |
|------------------------------| |--------------------------|
| int Y::y | | address of Y::~Y() |
|------------------------------| |--------------------------|
| o | | address of Y::printAll() |
| o | |--------------------------|
| o |
------|------------------------------|--------
| X::X() |
|------------------------------| |
| X::~X() | |
|------------------------------| |
| X::printAll() | \|/
|------------------------------| text segment
| Y::Y() |
|------------------------------|
| Y::~Y() |
|------------------------------|
| Y::printAll() |
|------------------------------|
| string::string() |
|------------------------------|
| string::~string() |
|------------------------------|
| string::length() |
|------------------------------|
| o |
| o |
| o |
| |上圖中Y繼承了X,那么Y對(duì)象中應(yīng)該含有X的vptr,上圖中Y的確含有X的vptr,但是X的vptr不再指向X的函數(shù)而是指向class Y override后的函數(shù)
所以我們?cè)谖鰳?gòu)函數(shù)中加上virtual關(guān)鍵字后子類對(duì)象memory layout中對(duì)應(yīng)的父類vptr指針(子類沒有定義virtual函數(shù),所以子類memory layout中沒有關(guān)于子類的vptr指針，如果子類對(duì)象obj1賦值給父類,那么obj1就會(huì)使用其內(nèi)存中x的vptr指向的虛函數(shù))指向的析構(gòu)函數(shù)變成了子類override的那一個(gè)(父類對(duì)象memory layout中vptr還是父類virtual function的地址)，相反如果我們的子類中沒有對(duì)父類的虛函數(shù)重載定義,那么子類中的vptr指向的是父類函數(shù)的地址,然后編譯器執(zhí)行的時(shí)候會(huì)先在vptr中找對(duì)應(yīng)的函數(shù)地址,因?yàn)槲覀冏宇悰]有重新定義父類虛函數(shù),那么vptr中指向的還是父類對(duì)應(yīng)的虛函數(shù),則最后編譯器執(zhí)行的是父類的函數(shù)而不是子類的函數(shù).
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