Hello sword4869/learn_cpp

# const functino in general class

class Aclass{
    bool operator<(const Aclass &b) const;
    void operator=(const Aclass &b);
};

什么时候是const函数，什么不是呢？

不需要修改时，就设计为const函数，比如小于运算符重载；需要修改时就不，比如赋值运算符重载。

# 1. const object

const MyClass obj;

# 1.1. constructor

When you've declare a const object, you can't change its data members during the object's lifetime. But all const member variables must be initialized.

So the const object(include member varialbes)'s initialization is done via a parameterized and public constructor. If no public default constructor is provided, a compiler error will occur.

# 1.2. member initializer list

C++ provides a handy syntax for initializing members of the class called the member initializer list (also called a constructor initializer).

Why need this special syntax? Because the normal constructor is wrong.

class MyClass {
public:
    MyClass(int a, int b) {
        regVar = a;
        constVar = b;
    }
private:
    int regVar;
    const int constVar;
};
//error

The initialization list eliminates the need to place explicit assignments in the constructor body.

class MyClass {
public:
    MyClass(int a, int b): regVar(a), constVar(b){}
private:
    int regVar;
    const int constVar;
};

In the declaration of constructor, it is as same as normal class.

class MyClass {
public:
    MyClass(int a, int b);
private:
    int regVar;
    const int constVar;
};
MyClass::MyClass(int a, int b): regVar(a), constVar(b){
    cout << regVar << end;
    cout << constVar << end;
}

PS: Even in cases in which member variables are not constant, it makes good sense to use the member initializer syntax.

# 1.3. forbid changes

Once a const class object has been initialized via the constructor, you cannot modify the object's member variables. This includes both directly making changes to public member variables and calling member functions that set the value of member variables.

# 1.4. const object call const function

A constant object can't call regular functions. Hence, for a constant object to work you need a constant function.

For const member functions that are defined outside of the class definition, the const keyword must be used on both the function prototype and definition.

MyClass.h

class MyClass{ 
public: 
    void myPrint() const; 
};

MyClass.cpp

#include "MyClass.h" 
#include  <iostream>
using namespace std;
void MyClas:myPrint() const {
    cout <<"Hello" <<endl;
}

main.h

#include "MyClass.h" 
#include  <iostream>
using namespace std;
int main(){
    const MyClass obj;
    obj.myPrint();
    return 0;
}

← constructor.md →