const member functions and const return values

Why there are two function's versions in one class(const and non-const)? Let's have a test now.

#include <iostream>
using namespace std;

class A
{
public:
A() {}
void fun() { cout<<"I am not const."<<endl; }
void fun() const { cout<<"I am const."<<endl; }
};

int main()
{
A test1;
test1.fun();

const A test2;
test2.fun();

return 0;
}

It is clear to see that the const object will match the const version. However, what happens when we don't have the const member function?

#include <iostream>
using namespace std;

class A
{
public:
A() {}
void fun() { cout<<"I am not const."<<endl; }
//	void fun() const { cout<<"I am const."<<endl; }
};

int main()
{
A test1;
test1.fun();

const A test2;
test2.fun();

return 0;
}

An error occurs here. How about lacking of the non-const member function version?

#include <iostream>
using namespace std;

class A
{
public:
A() {}
//	void fun() { cout<<"I am not const."<<endl; }
void fun() const { cout<<"I am const."<<endl; }
};

int main()
{
A test1;
test1.fun();

const A test2;
test2.fun();

return 0;
}

It runs well. However, there is a difference--both const!

The conclusion is that const function is more flexible, while the non-const version is relatively strict.

What about the const return values? Let's see.

#include <iostream>
using namespace std;

class A
{
public:
A() { a=0; }
int& fun() { cout<<"I am not const."<<endl; return a; }
const int& fun() const { cout<<"I am const."<<endl; return a; }
void get() { cout<<"Its value is "<<a<<endl; }
private:
int a;
};

int main()
{
A test1;
test1.get();
test1.fun() = 10;
test1.get();

return 0;
}

The value has been changed. Notice that it is a private member value. Now try the const version.

#include <iostream>
using namespace std;

class A
{
public:
A() { a=0; }
int& fun() { cout<<"I am not const."<<endl; return a; }
const int& fun() const { cout<<"I am const."<<endl; return a; }
void get() { cout<<"Its value is "<<a<<endl; }
private:
int a;
};

int main()
{
A test1;
test1.get();
test1.fun() = 10;
test1.get();

const A test2;
test2.fun() = 10;

return 0;
}

In conclusion, through "&" we can change the private value in class, while it doesn't work provided it is a const return value.