How can I make my vector update when passed in a function?

Question

When I have this piece of code:

int push666 (vector<int> vect){

    vect.push_back(666);
}

int main()
{

    vector<int> a;
    a.reserve(1);
    push666(a);

    cout << a[0];

    return 0;
}

The cout will simply print out some garbage value. It seems like functions don't have a lasting effect on the vector. What can I do about it?

`push666` needs to take a `vector&`, not a `vector`. And you need to read a [book](http://stackoverflow.com/q/388242/241631). — Praetorian, Nov 20 '15 at 20:29
I am going to read one. Your list made it much easier for me, thank you! — , Nov 22 '15 at 10:56

score 5 · Accepted Answer · answered Nov 20 '15 at 20:30

5

You pass the vector by reference instead of by value.

int push666 (vector<int>& vect){  
                    // ^^^  
    vect.push_back(666);
}

answered Nov 20 '15 at 20:30

R Sahu

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cadaniluk · Answer 2 · 2015-11-21T08:24:45.363

C++ supports passing by value and by reference.

In your code, the std::vector<int> is passed to the function by value - it is copied and this very copy is modified and std::vector<int>::push_backed.

In order to prevent the copy¹, so the function directly operates on a, you need to pass it by reference:

int push666 (vector<int>& vect){
    vect.push_back(666);
}

The other way, the C way (or bad C++ way) is to pass a pointer:

int push666 (vector<int>* vect){
    vect->push_back(666);
}

Note that the pointer is passed by value here.

¹ The standard doesn't specify that but a reference is probably implemented with a pointer, so technically some copying is made.

It's "call by XXX", not "copy by XXX". – Barmar Nov 20 '15 at 20:53 — Barmar, Nov 20 '15 at 20:53

score 1 · Answer 3 · answered Nov 20 '15 at 20:34

You should pass it by reference:

int push666 (vector<int>& vect){

    vect.push_back(666);
}

or what is less common by pointer:

int push666 (vector<int>* vect){

    vect->push_back(666);
}
push666(&a); // use & to take address of a

both ways allows you to modify arguments passed to functions

How can I make my vector update when passed in a function?

3 Answers3