C++类的构造、拷贝构造、析构函数等

1:

一个空的class在C++编译器处理过后就不再为空,编译器会自动地为我们声明一些member function,如果你写

class A{}; 
编译器处理后,就相当于:

class A

public: 
A();  //默认构造函数
A(const A&);  //拷贝构造函数
~A();  //析构函数
A& operator=(const A& rhs); 
A* operator&();  //取地址运算符

const A* operator&() const;
};

基本上符合我上文的回答,只是多了取地址运算符的两个函数。

这种回答对否?

其实对于这样的一个空类来说,是完全没有必要的,而编译器也不是这样做的。编译器的做法是:

只有你需要用到这些函数并且你又没有显示的声明这些函数的时候,编译器才会贴心的自动声明相应的函数。

比如   
A   a;   
编译器就会根据上面的实例,给类A生成构造函数和析构函数。   
当使用   
A   b(b);   
编译器就会生成类A的拷贝构造函数。   
A   c;   
c   =   a;   
编译器生成赋值运算符函数   
A   &d   =   a;   
编译器生成取地址运算符函数。   
    
经过我们的分析可以这样理解:对于一个没有实例化的空类,编译器是不会给它生成任何函数的,当实例化一个空类后,编译器会根据需要生成相应的函数。这条理论同样适合非空类(只声明变量,而不声明函数)。

类的const成员变量、static const、static成员变量的初始化 

结论:

  • 静态常量数据成员可以在类内初始化(即类内声明的同时初始化),也可以在类外,即类的实现文件中初始化,不能在构造函数中初始化,也不能在构造函数的初始化列表中初始化;
  • 静态非常量数据成员只能在类外,即类的实现文件中初始化,也不能在构造函数中初始化,不能在构造函数的初始化列表中初始化;
  • 非静态的常量数据成员不能在类内初始化,也不能在构造函数中初始化,而只能且必须在构造函数的初始化列表中初始化;
  • 非静态的非常量数据成员不能在类内初始化,可以在构造函数中初始化,也可以在构造函数的初始化列表中初始化;

总结如下表:

类型 初始化方式

类内(声明)

类外(类实现文件)

构造函数中

构造函数的初始化列表

非静态非常量数据成员

N

N

Y

Y

非静态常量数据成员

N

N

N

Y (must)

静态非常量数据成员

N

Y (must)

N

N

静态常量数据成员

Y

Y

N

N

/**
* <Effective C++>, page 14
* const data of class
* platform: visual studio 2005, win32
* filename: item2.1.cpp
*/
#include <iostream>
using namespace std;
&nbsp;
class MyTest
{
//(1) error C2864: 'MyTest::MaxNumber1' : only static const integral data members can be initialized within a class
//int MaxNumber1 = 5;
&nbsp;
//(2) error C2864: 'MyTest::MaxNumber2' : only static const integral data members can be initialized within a class
//const int MaxNumber2 = 5;
&nbsp;
//(3) error C2864: 'MyTest::MaxNumber3' : only static const integral data members can be initialized within a class
//static int MaxNumber3 = 5;
&nbsp;
//(4) ok
static const int MaxNumber4 = ;
static const char cconst4 = 'B';
&nbsp;
//(5) error C2864: 'MyTest::dconst4' : only static const integral data members can be initialized within a class
//static const double dconst4 = 200.00;
&nbsp;
public:
//(6) error C2758: 'MyTest::MaxNumber2' : must be initialized in constructor base/member initializer list
MyTest()
{
cout<<"MyTest constructor! "<<endl;
cout<<"MaxNumber4 = "<<MaxNumber4<<endl;
cout<<"cconst4 = "<<cconst4<<endl;
}
};
&nbsp;
int main()
{
MyTest obj; return ;
}

代码注释中的(1),(2),(3)表示step编号。

从(1),(2),(3)中,我们可以看出,只有static const integral data member(静态整型常量数据成员)才能在类内初始化。从(4),(5)中也可以得到证明。其中,char型相当于整型。

运行结果如下。

MyTest constructor!

MaxNumber = 5

cconst1 = A

cconst2 = B

dconst1 = 100

/**
* <Effective C++>, page 14
* const data of class
* platform: visual studio 2005, win32
* filename: item2.2.cpp
*/
#include <iostream>
using namespace std;
&nbsp;
class MyTest
{
int MaxNumber1;
const int MaxNumber2;
static int MaxNumber3;
&nbsp;
static const int MaxNumber4 = ;
static const char cconst4 = 'B';
&nbsp;
static const int MaxNumber5;
&nbsp;
public:
//(1) error C2758: 'MyTest::MaxNumber2' : must be initialized in constructor base/member initializer list
//(4) error C2438: 'MaxNumber3' : cannot initialize static class data via constructor
//(7) error C2438: 'MaxNumber5' : cannot initialize static class data via constructor
MyTest():MaxNumber1(), MaxNumber2()//, MaxNumber5(5)//, MaxNumber3(5)
{
//(2) error C2166: l-value specifies const object
//MaxNumber2 = 5;
&nbsp;
//(3) error LNK2001: unresolved external symbol "private: static int MyTest::MaxNumber3" (?MaxNumber3@MyTest@@0HA)
//MaxNumber3 = 5;
&nbsp;
//(6) error C3892: 'MaxNumber5' : you cannot assign to a variable that is const
//MaxNumber5 = 5;
&nbsp;
cout<<"MyTest constructor! "<<endl;
cout<<"MaxNumber1 = "<<MaxNumber1<<endl;
cout<<"MaxNumber2 = "<<MaxNumber2<<endl;
cout<<"MaxNumber3 = "<<MaxNumber3<<endl;
cout<<"MaxNumber4 = "<<MaxNumber4<<endl;
cout<<"MaxNumber5 = "<<MaxNumber5<<endl;
cout<<"cconst4 = "<<cconst4<<endl;
}
};
&nbsp;
//(5) ok
int MyTest::MaxNumber3 = ;
&nbsp;
//(8) ok
const int MyTest::MaxNumber5 = ;
&nbsp;
//(9) error C2761: 'int MyTest::MaxNumber1' : member function redeclaration not allowed
//int MyTest::MaxNumber1 = 5;
&nbsp;
int main()
{
MyTest obj;
&nbsp;
return ;
}

运行结果如下。

MyTest constructor!

MaxNumber1 = 5

MaxNumber2 = 5

MaxNumber3 = 5

MaxNumber4 = 5

MaxNumber5 = 5

cconst4 = B

代码注释中的(1),(2),(3)表示step编号。

从(1),(2)可以看出,非静态的常量数据成员必须在构造函数的初始化列表中初始化;如果在构造函数中初始化,会出现error c2166的错误,即常量对象是只读(read only)的,不能对其赋值。

从(3),(4),(5)可知,静态非常量数据成员只能在类外(类的实现文件)初始化。

从(6),(7),(8)可知,静态常量数据成员也可以在类外(类的实现文件)初始化。

3 初始化顺序

c++ 中类变量的初始化顺序,大体如下:

  1. 基类的静态成员初始化;
  2. 派生类的静态成员初始化;
  3. 基类的对象成员初始化;
  4. 基类的构造函数;
  5. 派生类的对象成员初始化;
  6. 派生类的构造函数;
  7. #include <iostream>
    using namespace std; class Inner{
    public:
    Inner(int i=): in(i) { cout << "Inner.constructor. " << in<<"\n";}
    Inner(Inner& inner): in(inner.in) { cout << "Inner.copy consturctor" << "\n";};
    int getValue(){ return in; } private:
    int in;
    }; class Inner2{
    public:
    Inner2(double i=0.0): in2(i) { cout << "Inner2.constructor. " << in2<<"\n";}
    Inner2(Inner2& inner2): in2(inner2.in2) { cout << "Inner2.copy consturctor" << "\n";};
    int getValue(){ return in2; }
    private:
    double in2;
    }; class Base{
    public:
    Base(int v=): value(v){ value = ; cout << "Base.constructor. " << value<<"\n";}
    Base(Base& b): value(b.value) { cout << "Base.copy consturctor" << "\n";}
    int getValue(){ return value; }
    protected:
    int value;
    Inner2 inner2;
    static Inner inner;
    static int baseInt;
    }; Inner Base::inner();
    int Base::baseInt = ; class Derive: public Base{
    public:
    Derive(string s = "hello", int base=, int i=, double i2=): derive(s), in2(i2), in(i) { cout << "derive.derive == " << derive << ", Derive.consturctor" << "\n";};
    //Derive(string s = "hello", int base=0, int i=0, double i2=0){ cout << "Derive.consturctor" << "\n";};
    Derive(Derive& d): derive(d.derive) { cout << "Derive.copy consturctor" << "\n";}
    void printValue(){
    cout << "derive.base.value == " << value << ", derive.derive == " << derive <<
    ", derive.in == " << in.getValue() << ", derive.in2 == " << in2.getValue()
    <<"base int: "<<baseInt<< "\n"; } private:
    string derive;
    Inner in;
    Inner2 in2;
    static Inner inner;
    //static int baseInt;
    }; Inner Derive::inner();
    int main()
    {
    Derive d("world", , , 30.0);
    d.printValue();
    // Derive d2 = d;
    cout << endl;
    }

    Inner.constructor. 1000

    Inner.constructor. 100

    Inner2.constructor. 0

    Base.constructor. 1

    Inner.constructor. 20

    Inner2.constructor. 30

    derive.derive == world, Derive.consturctor

    derive.base.value == 1, derive.derive == world, derive.in == 20, derive.in2 == 30base int: 100

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