Are the following const function equivalent?

Question

My question is simple. I was programing a getter when I noticed that I forgot the last const in my expression. But while I was writing it, I doubted: I know that in const function you can't change the returned element, so in the second one the is const std::vector implicit?

const std::vector<int> foo() const{}
std::vector<int> foo() const{}

you can return whatever you like from a const function, you just cant change the object its invoked on — pm100, Mar 25 '22 at 22:45
I seem to remember returning a `const` value prevents Return Value Optimization, but I'm not sure if that's still true. — François Andrieux, Mar 25 '22 at 22:52
no the const is not implicit, you are returning a writable vector — pm100, Mar 25 '22 at 22:52
the compiler will tell you if you try to do something invalid when you declare a function const — pm100, Mar 25 '22 at 22:54
so if I'm returning a writable vector, what guarentee the const at the finish of the function? @pm100 — renton01, Mar 25 '22 at 22:55
Since the returned `vector` is a value, a copy, you can fold it, spindle it, and mutilate it without affecting the instance you got the copy from. The trailing `const` is satisfied. — user4581301, Mar 25 '22 at 22:56
the fact that you did not change this inside foo. https://stackoverflow.com/questions/751681/meaning-of-const-last-in-a-function-declaration-of-a-class — pm100, Mar 25 '22 at 23:00
It will be returning a copy by the way and that's missing from the body also there shouldn't be semicolons at the ends there. — QuentinUK, Mar 26 '22 at 00:01

score 1 · Answer 1 · answered Mar 25 '22 at 22:49

1

You can return non-const values. You cannot return a non-const reference. Both of these functions return a copy, thus the const for the return type is not required.

If you were to return

std::vector<int> &

Then you would need the const.

answered Mar 25 '22 at 22:49

ChrisMM

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score 1 · Answer 2 · edited Mar 26 '22 at 00:39

Your concern is that if you are rturning a reference to data from within the object does that have to be const. Yes. This

class Test {
    int m_num;
public:
    int& getNum()   const { return m_num; }
};

does not compile. You need

class Test {
    int m_num;
public:
    const int& getNum()   const { return m_num; }
};

your case with a vector

class Test {
    int m_num;
    std::vector<int> m_vec;
public:
    const int& getNum()   const { return m_num; }
    std::vector<int> getVec()   const { return m_vec; }
};

works fine because you are returning a copy

but this is not ok

class Test {
    int m_num;
    std::vector<int> m_vec;
public:
    const int& getNum()   const { return m_num; }
    std::vector<int> & getVec()   const { return m_vec; }
};

has to be

class Test {
    int m_num;
    std::vector<int> m_vec;
public:
    const int& getNum()   const { return m_num; }
    const std::vector<int> & getVec()   const { return m_vec; }
};

Are the following const function equivalent?

2 Answers2