How to Return an Array from a function in c++?

Question

I'm new to C++ and am more used to Python. I have found posts answering this question, but I haven't been able to modify what I've found to work in my code. The goal of my function is simple, I want it to give me an array with x, y, and z coordinates where I give it polar coordinates. Here's what I have that isn't working:

double coordinate_finder(double coordinates[], double radius)
{
    double x = radius * sin(radians(90 - coordinates[0])) * cos(radians(coordinates[1]));
    double y = radius * sin(radians(90 - coordinates[0])) * sin(radians(coordinates[1]));
    double z = radius * cos(radians(90 - coordinates[0]));

    double XYZ[] = { x,y,z };
    return XYZ;

}

I feel like what I want it to do reads pretty easily but I'd like it to return XYZ when the function is called later in the code. By the way radians() is a function I made which converts degrees to radians.

you should look into the concept of pointer, if you want to do in the c way — Indiano
– Indiano, Commented Dec 12, 2022 at 19:38
You can just overwrite the three values in coordinates if you want. But make sure to make internal copies if necessary. — user1196549
– user1196549, Commented Dec 12, 2022 at 19:49

Remy Lebeau · Accepted Answer · 2022-12-12 19:56:17Z

7

You can't return a C-style array as-is, but you can return a pointer to a dynamically allocated array, eg:

double* coordinate_finder(double coordinates[], double radius)
{
    ...

    double *XYZ = new double[3];
    XYZ[0] = x;
    XYZ[1] = y;
    XYZ[2] = z;

    return XYZ;
}

And then the caller will have to free it:

double* xyz = coordinate_finder(...);
...
delete[] xyz;

This can be made safer for the caller using std::unique_ptr:

#include <memory>

std::unique_ptr<double[]> coordinate_finder(double coordinates[], double radius)
{
    ...

    auto XYZ = std::make_unique<double[]>(3);
    XYZ[0] = x;
    XYZ[1] = y;
    XYZ[2] = z;

    return XYZ;
}

Or std::vector:

#include <vector>

std::vector<double> coordinate_finder(double coordinates[], double radius)
{
    ...

    std::vector<double> XYZ(3);
    XYZ[0] = x;
    XYZ[1] = y;
    XYZ[2] = z;

    return XYZ;
}

Or a static variable (or thread_local, if multi-threading is a factor):

double* coordinate_finder(double coordinates[], double radius)
{
    static double XYZ[3];
    // thread_local double XYZ[3];

    ...

    XYZ[0] = x;
    XYZ[1] = y;
    XYZ[2] = z;

    return XYZ;
}

However, it would be better to simply return a std::array instead:

#include <array>

std::array<double,3> coordinate_finder(double coordinates[], double radius)
{
    ...
    std::array<double,3> XYZ = { x,y,z };
    return XYZ;
}

Or, a std::tuple:

#include <tuple>

std::tuple<double,double,double> coordinate_finder(double coordinates[], double radius)
{
    ...
    std::tuple<double,double,double> XYZ{ x,y,z };
    return XYZ;
}

Or, a custom struct/class:

struct xyz
{
    double x;
    double y;
    double z;
};

xyz coordinate_finder(double coordinates[], double radius)
{
    ...
    xyz XYZ{ x,y,z };
    return XYZ;
}

edited Dec 12, 2022 at 19:56

answered Dec 12, 2022 at 19:40

Remy Lebeau

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2 Comments

user1196549 Over a year ago

You can return a C-style array that is malloc'ed by the function. You need to free it later. (But passing it as an argument is even simpler.)

Kevin Anderson Over a year ago

May also want to include a vector<double> example, mentioning copy elision, as that'll be "cheap" creation as well. And/or std::move and how that would work.

NobodyImportant · Accepted Answer · 2025-08-21 03:18:24Z

1

The only way to return a C-style array from a function is to wrap it in a struct. Which is basically what std::array does.

auto coordinate_finder(double coordinates[], double radius)
{
    double x = radius * sin(radians(90 - coordinates[0])) * cos(radians(coordinates[1]));
    double y = radius * sin(radians(90 - coordinates[0])) * sin(radians(coordinates[1]));
    double z = radius * cos(radians(90 - coordinates[0]));

    struct {
        double XYZ[3];
    } ret = { {x, y, z} };

    return ret;
}

answered Aug 21 at 3:18

NobodyImportant

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