C ++-对const感到困惑

Question

I am reading this tutorial about const objects http://www.learncpp.com/cpp-tutorial/810-const-class-objects-and-member-functions/ .

I understand const objects can only access const functions. What confuses me is when we pass a const object to a copy constructor, how can it access non-const functions?

Example:

class Dog {
     protected:
         string m_name;
         int m_age; 
     public:
         Dog() : m_name("Fido"), m_breed("Corgi"), m_color("Black"), m_age(1) {}
         Dog(const Dog& d) { m_name = d.getName(); m_age = d.getAge(); }
         String getName() { return m_name; }
         int getAge() { return m_age; }
};

So when the copy constructor is called with a const dog object, how can this object access the getName() and getAge() functions?

Answer 1

A const member function is a member function that can't modify the class instance it's being called on.

So if you have a const object, you can only call the functions that are declared const , by appending const after the function prototype:

void const_function(int i) const { /* function body */ }
                           ^^^^^

That's the only place the const can appear in, in order to make the function const . Other places just modify the return type or the arguments.

---

A copy constructor is a just a constructor that takes as an argument a const reference to an object of the same type. The reference is const because the constructor does not modify the object passed as the argument, it just copies from it and constructs the new object with the copied data:

Foo(const Foo& copy_from_this_object) { /* copy member data from argument */ }

Also, the copy constructor doesn't actually copy the const ness from its argument, it just copies all the data from it. Ie it doesn't matter whether the argument was const or not, it's just copied , not modified . The newly constructed may or may not be const . It has nothing to do with the const ness of the copied-from object.

---

Edit (after OP's edit):

So when the copy constructor is called with a const dog object, how can this object access the getName() and getAge() functions?

It cannot .

This is because the member functions are not properly marked const where they should have:

getName and getAge don't modify their class instance . Therefore they should be const ! After making them const they can be called on the const object in the copy constructor, and the code will compile fine.

Note: In general, you should always make getters const . That's because their task should be to only retrieve information about the object, not modify it.

Answer 2

You just have to try compiling your code to see it doesn't work - from ideone.com here :

prog.cpp: In copy constructor 'Dog::Dog(const Dog&)':
prog.cpp:13:46: error: passing 'const Dog' as 'this' argument of 'std::string Dog::getName()' discards qualifiers [-fpermissive]
       Dog(const Dog& d) { m_name = d.getName(); m_age = d.getAge(); }
                                              ^
prog.cpp:13:66: error: passing 'const Dog' as 'this' argument of 'int Dog::getAge()' discards qualifiers [-fpermissive]
       Dog(const Dog& d) { m_name = d.getName(); m_age = d.getAge(); }
                                                                  ^

You can fix it by making the "get" functions const :

string getName() const { return m_name; }
int getAge() const { return m_age; }

You can see that version compile successfully here

Answer 3

It can't .

Try it before asking.