Why return string& from a const method won't compile?

Question

Given the following code:

#include <iostream>
#include <string>
using namespace std;

class A
{
private:
    string m_name;
    string m_first;
public:
    A(): m_first("string") {}
    virtual void print() const {}
    string& getName() const {return m_first;}  // won't compile 
    const string& getLastName() const {return m_name;}  // compile
};


int main()
{
    A a;
    return 0;
}

Compiler presents : "invalid initialization of reference of type 'std::string&' from expression of type 'const std::string'" Why can't I return "m_first" from getName() ? I thought that the const on the tail of the function states that the function will not change 'this'... but I'm not trying to change this , just return a data member.

Answer 1

Because inside a const method, all non- mutable members are implicitly const . So, you're trying to bind a reference to non-const std::string (your return value) to an object of type const std::string , which is illegal(because it would allow modification of const data), hence the error.

Answer 2

By returning a reference, you are saying you can modify the class data-member that the reference-variable is implicitly pointing to, and therefore modify the class ... but you have dedicated the class method as a constant method, meaning it is not allowed to change any class member variables that have not been specifically declared mutable. So by returning a non-constant reference, you are breaking the encapsulation "contract" that the class interface has established. Your options are to either return a temporary object (ie, that creates a copy of the object), or a constant-reference. So you could either do

const string& getName() const {return m_first;}

or

string getName() const { return m_first; } //copies m_first and returns the copy

Answer 3

Your code promises that the reference won't change the m_name member, but you return a reference that can change it. What you want is a string const& return type.

This returns a "read-only" reference to m_name.

See also: the C++ FAQ on const correctness

Answer 4

When you return string & , it allows modifying class member... but the function is const , so it is not allowed to allow such a situation. But when you return const string & , it doesn't allow modifying class instance.

Answer 5

如果你调用A.getName().ModifyTheString() ==>这意味着你修改了this 。