I am trying to allocate a 2D array inside AllocateMatrix and return it but when I use the returned 2D array I get a segfault. My question is, why is it seg faulting and how do I fix it to where I can pass it and use it in other functions? fileTest is just a txt file with row next line column next line and then the matrix with one space on each column and a new line for each row.

class Matrix 
{
public:
    Matrix();
    ~Matrix();
    double** AllocateMatrix(unsigned rows, unsigned columns);
    Matrix(std::string filePath);

private:
    unsigned rows_;
    unsigned columns_;
};

Matrix::Matrix(std::string filePath)
{
    std::ifstream matrixFile;

    try
    {
        matrixFile.open(filePath);
        if(matrixFile.fail()) 
            throw "Failed to open file.";
        std::cout << "Starting read file...\n";

        matrixFile >> rows_;
        std::cout << "rows: " <<  rows_ << "\n";
        if(rows_ < 0) 
            throw "Number of rows cannot be negative.";
        matrixFile >> columns_;
        std::cout << "columns: " << columns_ << "\n";
        if(columns_ < 0)
            throw "Number of columns cannot be negative.";

        double** array2D;
        array2D = AllocateMatrix(rows_, columns_);

        for(uint64_t i = 0; i < rows_; i  ) 
        {
            for(uint64_t j = 0; j < columns_; j  )
            {
                std::cout << i << " " << j << " " << array2D[i][j]  << "\n"; // seg fault here 
            }
        }
        std::cout << "Finished file read" << std::endl;
        matrixFile.close();
    }
    catch(const char* message) 
    {
        std::cerr << message << std::endl;
        rows_ = 0;
        columns_ = 0;
        return;
    }
}

double** Matrix::AllocateMatrix(unsigned rows, unsigned columns)
{
    try 
    {
        double** matrix = new double *[rows];
        for(unsigned i = 1; i <= rows; i  )
        {
            matrix[i] = new double[columns];
        }
        for(unsigned i = 1; i <= rows; i  )
        {
            for(unsigned j = 1; j <= columns; j  )
            {
                matrix[i][j] = 0;
                std::cout << "i: " << i << " j: " << j << " value: " << matrix[i][j]  << "\n";
            }
        }
        matrix = result_;
        std::cout << "Returning 2D Array\n";
        return matrix;
    }
    catch(const std::bad_alloc&) {
        std::cerr << "Bad allocation."  << std::endl;
        return nullptr;
    }
}

int main()
{
    Matrix matrixOne("fileTest.txt");
    return 0;
}

CodePudding user response：

try not using C style pointers, dont use C style pointers. your problem is a usre error. use std::vector instead of using double pointer because is big issue, if have use it then do this: double* array2d[array_sz]; as for not memleak.

CodePudding user response：

I suspect this line is the wrong way round

matrix = result_;

I assume result_ is meant to be a calls field (it isnt but it looks like it)

But here you move unitialized value to 'matrix' which is a pointer the memory you just allocated and initlaized. So you lost any use of that new matrix.

You surely mean

result_ = matrix;

but you should be using

  std::vector<std::vector<int>>

life will be a lot simpler then