C++ returning an array

Question

I'm new to C++ programming and i want to make a function that gives you all the common dividers into an array, and afterwards use it in int main()

Is returning an array from a function possible :D?

Thank you!

Code: http://pastebin.com/K8195wzF

Answer 1

Returning a dynamically-sized container like std::vector has the best use case here:

#include <vector>
...

std::vector<int> dividers(int x, int y) {
    std::vector<int> divi;

    ...
    return divi;
}

Returning by value creates a copy, ensuring no dangling pointers. Such is the case when returning a local array through a pointer: the memory will be lost when the function exits leaving the pointer to point to garbage.

Answer 2

Yes it is possible to return an array in form of a pointer, but this is not recommended as you won't know its size in advance. But take care not to return the address of a local variable (allocated on the stack!!). Another problem with your code is that you didn't initialize d. The preferred C++ way would be to return a vector of the type (you do not need d anymore)

   #include <vector>
   ....
   vector<int> dividers(int y,int x){
    int yx;
    vector<int> divi;
    divi.reserve(100);
    if(y > x){
      yx = y;
    }
    if(x > y){
      yx = x;
    }

    for (int n = 1; n <= yx; n++){
      if ((n%y==0) && (n%x==0)){
        divi.push_back(n);
      }
    }

    return divi;
  }

You should read about iterators too to modify the loop that is in your main function.

Big mistake in my code removed... Thanks for pointing it out. I did the same mistake with vector that what you had with the array. Fixed now... Stupid me...

Answer 3

No it's not possible. You could return a vector, or you could pass a pointer to the first element of an array to your function.

Looking at your code, the pointer option would look something like this

int dividers(int y,int x, int* divi){
    ...
}

int main()
{
    int divi[100];
    ...
    dividers(x, y, divi);
}

The vector solution might look like this

std::vector<int> dividers(int y,int x){
    std::vector<int> divi;
    ...
        divi.push_back(n); // when you want to add a divider to your vector
    ...
    return divi;
}

int main()
{
    std::vector<int> divi;
    ...
    divi = dividers(x, y);
}

Answer 4

You could do the following:

typedef int arrT[10]

arrT *func() {}

whereby one is defining arrT to be an alias of an array of 10 ints or

int (*func())[10]{}

whereby one is defining func when dereferenced to return an array of 10 ints.

A using declaration such that using arrT = int[10] is also possible.

As others have noted, returning a std::vector from a function containing the type desired is possible with a function declared as std::vector<int> func(){}.

Also, you could use another container std::array<> and forget built-in arrays altogether.

Reference:

C++ Primer; Lippman, Lajoie, Moo