why copy constructor use private property directly in C++

问题

Look at following:

class node
{
    int freq;

public:
    node(const node &other)
    {
        freq = other.freq;
    }

    int getFreq()
    {
        return freq;
    }
};

It works well. However, when I replace freq = obj.freq with freq = obj.getFreq(), it gives me this error:

'int node::getFreq(void)': cannot convert 'this' pointer from 'const node' to 'node &'

Why? freq is a private member, it makes more sense that we should use the interface getFreq to access it.

回答1:

It won't compile, because your function is not declared const:

int getFreq() const; // accessor function that does not modify the object

Thus, you can not call it with const instance: const node &obj.

Accessing obj.freq works, because it adapts to the const instance, making obj.freq not modifiable - to do this with a member function would be a nonsense (code inside member function lacking const specifier might (and should) require modifiable entities).

来源：https://stackoverflow.com/questions/33844477/why-copy-constructor-use-private-property-directly-in-c