Returning multiple data items from a function in C or C++

Question

I am confused on a couple homework questions I have... Can you return multiple data items from a function by using return()? Can a function only return one value, unless it is a pointer to an array?

I believe that the answer is that a function can return multiple data items by returning a structure. Then, returning a pointer to an array is not the only way - if that is a way?

But there seems to be a lot of discussion on this topic and so I want to make sure I have at least the basic idea correct: You can return multiple data items using a structure but using pointer (I don't understand this) will use memory more efficiently. Is this correct?

Answer 1

With C++0x/C++11 you can use this:

#include <string>
#include <iostream>
#include <tuple>

using namespace std;

tuple<int, unsigned, string, int> getValues()
{
    return tuple<int, unsigned, string, int>(1, 2, "3", 4);
}

int main()
{
    int a;
    unsigned b;
    string c;
    int d;

    tie(a, b, c, d) = getValues();

    cout << a << ", " << b << ", " << c << ", " << d << endl;
    return 0;
}

Compile it with

g++ tuple_test.cpp -std=c++0x -o tuple_test

And and if you run the programm it will output this:

1, 2, 3, 4

But it's better to use references like this (i would say):

void getValues(int& a, unsigned& b, string& c, int& d)
{
    a = 1;
    b = 2;
    c = "3";
    d = 4;
}

int main()
{
    ...
    getValues(a, b, c, d)
    ...
}

Uch thats what I get for not reading the question carefully...

Can a function only return one value, unless it is a pointer to an array?

Yeah you only can return 1 single value, but this single value can include multiply values (struct, class, array).

You can return multiple data items using a structure but using pointer (I don't understand this) will use memory more efficiently. Is this correct?

True. But when you use pointers it depends on how you use it. When you dynamic allocate it each function call it wont be very efficient and you would need to deallocate the memory manually after usage. When you use a global-array/struct it will be efficient. But can give you problems when you call the function multiply times.

Answer 2

In addition to what is already said in this thread, in C++11 you can return structures initialized using uniform initialization:

struct XYZ {
    int x;
    int y;
    int z;
};

XYZ foo() {
    return {1, 2, 3};
}

Personally I prefer returning structures with named members rather than tuples because the latter doesn't provide names for its members.

Answer 3

That is correct.

You can however "return" multiple items, by adding parameters that are passed by reference, then writing the multiple results to them.

Answer 4

A function can indeed only return one 'thing' with its return statement. That thing can, however, be a pointer (C & C++ arrays are simply pointers in disguise), a reference (a pointer which can't be reseated or array-indexed) or an object, which may encapsulate multiple things.

If you return a structure, you're passing back the entire structure. If you return a pointer or reference, you are returning only the address of the structure - so you had better not return a reference or pointer to a structure that goes out of scope when the function returns! Doing so invokes undefined behavior, which most likely (but not always) is a segmentation fault.