Return an array pointer from a function with non-constant array size in C++

Question

I built a function to generate a random 2-d array in C++. I was hoping that I could set the size of the array at compile time, so I included variables for the number of rows and columns in the array. However, when I try to compile the function, I get an error about the storage size of the array is not constant. This seems to do with the the static keyword that I have to add to the array definition, so that I can return the pointer from the function. I was not sure if there is a way around this error? Any suggestions.

double * generate_random_array(int rows, int cols, double lower_, double upper_){

static double test_array[rows][cols];

for (int i = 0; i < sizeof test_array / sizeof test_array[0]; i++) {
    for (int j = 0; j < sizeof test_array[0] / sizeof(double); j++) {
        test_array[i][j] = generate_random_numbers(lower_, upper_);
    }
}
return(test_array);

}

rows and cols are not known at compile time, you can call generate_random_array with whatever arguments you want. — François Andrieux
– François Andrieux, Commented Mar 10, 2017 at 19:00
Yeah, I was just generating the random array as part of some test functions. So I can change the row, column length in the test case, and then compile the test case. Sorry if I was unclear about that. Yeah, I am going to test with vectors as well, but thanks for the tip. — krishnab
– krishnab, Commented Mar 10, 2017 at 19:04

Vittorio Romeo · Accepted Answer · 2017-03-10 19:07:44Z

6

You can make generate_random_array a template, enforcing rows and cols to be known at compile-time:

template <int rows, int cols>
double* generate_random_array(double lower_, double upper_)
{
    /* ... as before ... */
}

Example usage:

generate_random_array<5, 10>(51.4, 66.0);

Nevertheless, you should use std::array instead of C-style arrays. If you want resizable arrays, then you should use std::vector instead.

std::array example:

template <int rows, int cols>
auto generate_random_array(double lower_, double upper_)
{
    const auto idx = [](int x, int y){ return y * rows + x; };
    std::array<double, rows * cols> result;

    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            result[idx(i, j)] = generate_random_numbers(lower_, upper_);
        }
    }

    return result;
}

Example usage:

auto test_array = generate_random_array<5, 10>(11.0, 66.33);

live wandbox example

edited Mar 10, 2017 at 19:07

answered Mar 10, 2017 at 19:03

Vittorio Romeo

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4 Comments

krishnab Over a year ago

That is great. This should work just fine. I would not have thought of using a template, but I am still new to C++. Thanks for the tip about using std::array and std::vector in the future.

krishnab Over a year ago

Say Vittorio, just had a question. I noticed in the answer to this post you actually create and return a std::array from the function by value. I thought that returning a C++ array by value was not possible--you could only return a pointer to the first element. Does std::array act like std::vector in that you can return those containers by value in functions?

Vittorio Romeo Over a year ago

@krishnab: yes, it does.

krishnab Over a year ago

Oh great. Thanks for the tip. I did not know that.

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