Returning an array from a function [duplicate] - c++

Question

This question already has answers here:
Return array in a function
(20 answers)
Closed 10 years ago.
Here is my code
double hour_payload_add(int entries , double array[])
{
int index=0 ,k=0;
int totalpayload=0;
double returnarray[entries/120];
while(k< entries)
{
totalpayload = totalpayload + array[k];
if(k%120==0)
{
returnarray[index] = totalpayload;
index++;
totalpayload=0;
}
}
return returnarray;
}
here I have called it
double hourgraph[hrentries];
hourgraph= hour_payload_add(entries , graph);
as I want to return an array what should I do to return without using pointers?

Answer 1

Pass the array by reference to the function and don't return anything. Small example:
void hour_payload_add(int entries , double array[], double (&returnArray)[SIZE])
{
// returnArray will be updated as it's external to function.
}
or
void hour_payload_add(int entries , double array[], double *returnArray) // <-- pointer
{
// returnArray will be updated as it's external to function.
}
usage:
double returnArray[SIZE];
hour_payload_add(entries, array, returnArray);

Answer 2

Two lines changed:
std::vector<double> hour_payload_add(int entries , std::vector<double> array) // HERE
{
int index=0 ,k=0;
int totalpayload=0;
std::vector<double> returnarray(entries/120); // HERE
/// ....
return returnarray;
}
Okay, maybe add one at the top
#include <vector>
For full bonus, I suggest changing it a bit more:
std::vector<double> hourgraph(hrentries);
hourgraph = hour_payload_add(graph);
// ...
std::vector<double> hour_payload_add(std::vector<double> array)
{
const size_t entries = array.size();

Answer 3

For that you should be using a pointer and create the array before you fill it (not in the function that is filling it).
double* hour_payload_add(int entries , double array[])
However I prefer to pass arrays as a reference.
void hour_payload_add(int entries , double array[], double& ret_array[])
Or even better (as others will say) would be to use vectors.
void hour_payload_add(std::vector<double>array, std::vector<double>& ret_array)

Answer 4

You have to create your array using new. So your function should be like this:
double* hour_payload_add(int entries , double array[])
{
int index=0 ,k=0;
int totalpayload=0;
double* returnarray = new double[entries/120];
while(k< entries)
{
totalpayload = totalpayload + array[k];
if(k%120==0)
{
returnarray[index] = totalpayload;
index++;
totalpayload=0;
}
}
return returnarray;
}
After using the array you must free the space by calling delete[] on the array to avoid memory leaks:
int entries = 5;
double* array = hour_payload_add(entries);
for (int i=0; i < entries; i++)
{
std::cout << array[i] << std::endl;
}
delete[] array;

Answer 5

You cannot return arrays in C++ - you would rather return a pointer to a piece of memory.
Moreover, you cannot return a pointer to a locally declared array that is on the stack because the stack is restored right after the function terminates. You have to allocate it dynamically with new or malloc, and return a pointer to its beginning.

Answer 6

In general you should not use arrays in C++, instead preferring the use of the STL Container Classes (std::vector is the most array-like of these). Of course there are exceptions, and in some cases you should use an array (never say never), but in your example I would suggest the use of vector instead.
Sehe's answer provides an example of using the std::vector, and initialising it with the size of the array:
std::vector<double> returnarray(entries/120);
This uses a constructor that sets the default size of the array. More usefully a vector can change size dynamically, and you can, as suggested, return it from a function. Depending on the optimisation done by the compiler, it may not even create a copy of the vector - but this is compiler-dependent.
My suggestion is to look at documentation on vector and other containers in preference to an array.

Answer 7

You cannot return an c style Array from a function in C++. But Yes you can return a std::array container from a function
You can return starting address of the c style array which is created locally on inside the function but that array gets cleared off when function returns and you are left with a pointer pointing to not what you wanted.
You will have to dynamically allocate a array using new and then return pointer to it.