Cascading Function Calls

Question

Using a function that looks like:

IntegerSet &insert(int m);

I'm trying to create a function that supports cascading calls that insert values into an object's array.

My function:

IntegerSet &IntegerSet::insert(int m) {
    IntegerSet newObj;
    for (int i = 0; i < 256; i++) {
        newObj.set1[i] = set1[i]; //copy the array into the new object
    }
    newObj.set1[m] = true;
    return newObj;
}

The object being returned is empty, which I suspect has to do with the reference.

Attempting to change it by altering the & to look like

IntegerSet IntegerSet::&insert(int m)

makes it refuse to compile because it 'expects an identifier'.

You're returning a reference to a local variable. Perhaps you might expand your example to include the definition for IntegerSet and how you're actually using it. You should read this: https://stackoverflow.com/questions/4643713/c-returning-reference-to-local-variable — Retired Ninja, May 07 '17 at 05:11

score 3 · Accepted Answer · answered May 07 '17 at 05:27

This is probably what you intended in order to make fluent syntax work:

IntegerSet& IntegerSet::insert(int m)
{
    this->set1[m] = true;
    return *this;
}

For fluent syntax, you typically want a lot of member functions called on the same object, not proliferation of temporary objects.

score 1 · Answer 2 · answered May 07 '17 at 05:31

You should not return a reference to a local, as it will have been destroyed by the time the function returns. However, you probably want to modify the IntegerSet on which you call insert instead and return a reference to *this. This is much more efficient than making a copy each time.

IntegerSet &IntegerSet::insert(int m) {
    set1[m] = true;
    return *this;
}

This way you can chain function calls on an IntegerSet object like so:

iset.insert(1).insert(2).insert(3);

Cascading Function Calls

2 Answers2