Returning a matrix array in C++ function

As the comment states, your return type of the function input_x is wrong.

Your matrix mat is an array of arrays of int. This is a c-style array and you can only return it as an int** which would directly lead to a dangling reference.

Instead of using c-style arrays it's better to use standard containers, like std::vector as Fred Larson pointed out.

See the documentation here about std::vector In general https://en.cppreference.com is a good start to see if any standard library type suits your needs.

You can initialize your matrix like this:

std::vector<std::vector<int>> mat(num_rows)
for (auto & row : mat)
    row.resize(num_cols);

Don't forget to include the respective header

#include <vector>

One of the strengths of C++ is that many complex data types already exist in the standard library. And even more exist in common extension libraries like Boost. You would be well-advised to use those instead of "making do" with C style arrays or structs, if for no other reason than to avoid error-prone manual memory management.

Svenja showcased the "vector of vector" approach using the standard type std::vector. But there is an even better match for your use case in Boost.uBLAS' Matrix type:

#include <iostream>
#include <boost/numeric/ublas/matrix.hpp>
#include <boost/numeric/ublas/io.hpp>

// Avoid "using namespace". Keep the global namespace clean,
// and use explicit namespacing for minimal ambiguity.
namespace bu = boost::numeric::ublas;

// Use (unsigned) size_t for sizes. Use padding spaces for
// readability.
bu::matrix< int > input_x( size_t num_rows, size_t num_cols )
{
    bu::matrix< int > matrix( num_rows, num_cols );

    // Use "speaking" variable names instead of "i" and "j".
    // Use prefix "++"; while it makes no difference with POD
    // types, it does make a difference with complex ones.
    for ( size_t r = 0; r < num_rows; ++r )
    {
        for ( int c = 0; c < num_cols; ++c )
        {
            // Give all the useful information you can.
            // Don't forget to std::endl your output if
            // the next thing you do is expecting input.
            std::cout << "value " << r << ", " << c << std::endl;

            // Avoid the use of "named temporary" variables.
            // Name only what you actually need later on, and
            // if you can (like here), just work without any
            // intermediary like "user_input".
            std::cin >> matrix( r, c );
        }
    }

    return matrix;
}

int main()
{
    size_t rows;
    size_t cols;

    std::cout << "How many rows?" << std::endl;
    std::cin >> rows;
    std::cout << "How many columns?" << std::endl;
    std::cin >> cols;

    // In C++ you do not need to declare all variables at the
    // beginning of the block. Declare at the point you actually
    // need the variable.
    bu::matrix< int > matrix = input_x( rows, cols );

    return 0;
}

The value returned by the function is not stored.

input_x(rows,cols); // should be stored in a variable

Also, you have to change the return type of function to a matrix.

int input_x(int num_rows, int num_cols){

To solve the problem:

1. Create a matrix dynamically
2. Return the matrix

Here is the code for the above approach.

/******************************************************************************
                                  Online C++ Compiler.
    *******************************************************************************/

    #include <iostream>

    using namespace std;


    int** input_x(int num_rows, int num_cols){

       int **mat = (int **)malloc(num_rows * sizeof(int *));
       int user_input;

        for (int i = 0; i < num_rows; i++) {
            mat[i] = (int *)malloc(num_cols * sizeof(int));
        }
        for (int i=0; i<num_rows; i++){
            for (int j=0; j<num_cols; j++){
                    cout<<"Enter the value X:";
                    cin>>user_input;
                    mat[i][j]=user_input;

            }
        }

       return mat;
    }



    int main()
    {
        int** mat;
        int input;
        int rows;
        int cols;

        cout<<"How many row?";
        cin>> rows;
        cout<<"How many columns?";
        cin>> cols;


        //Enter the X data
        mat = input_x(rows,cols);
/*
        for(int i=0;i<rows;i++){
            for(int j=0;j<cols;j++){
                cout<<mat[i][j]<<" ";
            }
            cout << endl;
        }
*/



        return 0;
    }