CODEWITHSUNDEEP
c++functionreference
jazzybazz
jazzybazz

Reputation: 1887

Need help understanding functions that returns a reference

What is the difference between

What is written in the textbook:

MyClass& Myclass::operator++() {
  do something
  return *this;
}

and

MyClass Myclass::operator++() {
  do something
  return *this;
}

* means "value pointed by" ....

is it that the second example will return a copy of the object pointed by this (a copy of *this) while the first example will return *this itself?

if this is the case then what difference does it make? to improve execution time?

Upvotes: 0

Views: 63

Answers (3)

David Rodríguez - dribeas
David Rodríguez - dribeas

Reputation: 208323

As you already mention in the answer, the first one returns a reference and the second one is making a copy. The main difference is that the semantics are completely different, consider:

const auto & r = ++a;
++a;
assert(a == r);    // true/false depending on what is returned

In the particular case of prefix operator++, the correct semantics (the one that most users will expect) is yielding a reference. Doing any other thing will cause confusion and probably end up being the source of bugs in the source code.

In a more general question, if the function had no predefined expected semantics, the difference would still be the same regarding usage. The will be a higher cost if you copy (for those types for which the copy might be expensive), but if you need reference semantics you will need to yield a reference, and if you need copy semantics you will need to return by value no matter how cheap/expensive that is.

Note that regarding the cost, it might be less than you expect, depending on what you are actually doing. If the caller is going to use the return to initialize a new object, the cost is probably going to be the same as RVO will kick in and remove the unnecessary extra copy.

Upvotes: 0

Pradheep
Pradheep

Reputation: 3819

the difference between the two is that

in case 1

you are returning a reference ie you are returning a constant pointer to the object.

In case 2

you are creating a new object and returning the object

Lets dig a little deeper to understand things better

In case 1

since you are returning the pointer to the existing object any changes you make with affect the original object

In case 2

since its a copy there is no impact on the original object

In terms of speed and memory

In case of 1

since you returning the address of an existing object there is no or little overhead

In case 2

you are creating a new copy ...so constructor of the object would be invoked and hence there is overhead in terms of memory and time taken

Upvotes: 1

Timothy Shields
Timothy Shields

Reputation: 79441

The second version will copy the MyClass object (i.e. *this). The first will return only a reference to it.

It's not that either one is "correct" - it's that they have different meanings. Sometimes you want to clone an object so that you have two (possibly) independent copies. Sometimes you want to share a single object.

Upvotes: 0

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