C++ how to return a const object from a class variable

Question

So I have a class called MusicComposer that has an AudioSignal object as a data member called music. Music is non-const. However, I need to return music as a const AudioSignal in a method called getMusic.

In the source file:

const AudioSignal& MusicComposer::getMusic(){

}

music is declared as:

AudioSignal music;

^ this is in the header file.

I declare music as non-const because music needs to be changed before it is returned by the getMusic method. However, I can't figure out for the life of me how to return a const version of it. How can I return a const version of music?

No, the reference returned will be const. The object inside MusicComposer will still stay as is. — agrum
– agrum, Commented May 16, 2014 at 23:40
@user1799156 const is a property of the expression accessing the object, not the object itself. — Potatoswatter
– Potatoswatter, Commented May 16, 2014 at 23:40
@user1799156 Well, only when references are involved. An object (not reference) declared const will resist non-const access, and forcing the issue (with const_cast or similarly) produces undefined behavior (i.e., may crash). — Potatoswatter
– Potatoswatter, Commented May 16, 2014 at 23:56

Mooing Duck · Accepted Answer · 2014-05-16 23:42:25Z

2

C++ can automatically change mutable to const for you whenever. No biggie. It's going the other way that's hard.

return music;

If you really want to be explicit you could do this, but it's wierd code, and wierd is bad.

return static_cast<const AudioSignal&>(music);

Also, Cyber observed that getter methods themselves are usually const, which lets them be called even when MusicComposer is const.

const AudioSignal& MusicComposer::getMusic() const {
                                             ^^^^^

answered May 16, 2014 at 23:42

Mooing Duck

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Comments

gsamaras · Accepted Answer · 2014-05-16 23:44:55Z

1

Take a look in this example:

#include <iostream>

class A { // declare class A
  int integer;
};

class B { // declare class B
 public:
  const A& get() { // get() will return a const ref
    return a;      // here it is returning the ref of data member a
  }
 private:
  A a;
};

int main() {
  A a;
  B b;
  a = b.get();

  return 0;
}

As mentioned by potatoswatter,

const is a property of the expression accessing the object, not the object itself

answered May 16, 2014 at 23:44

gsamaras

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