Why it is a segmentation fault with this code fragment?

May 2, 2022

Here is my code about Matrix (I decided to practise OOP writing own Matrix class)

Matrix.hpp

#ifndef MATRIX_HEADER
#define MATRIX_HEADER

typedef unsigned int u_int;

class Matrix
{
    double **mtrx;
    u_int x, y;

public:
    Matrix(u_int a, u_int b);
    Matrix(const Matrix &);
    ~Matrix();

    double det();

    Matrix operator+(const Matrix &) const;
    Matrix operator-(const Matrix &) const;                           
    Matrix operator*(const Matrix &) const;
    friend Matrix operator*(const Matrix &, const double &);
    Matrix operator/(const Matrix &) const;
    double *operator[](const u_int idx) const { return mtrx[idx]; }

    bool IsEqual(const Matrix &) const;
    u_int GetX() const { return x; }
    u_int GetY() const { return y; }
};

#endif

Matrix.cpp

#include "Matrix.hpp"

Matrix::Matrix(u_int a, u_int b) : x(a), y(b)
{
    mtrx = new double *[x];
    for (u_int i = 0; i < x; i++)
        mtrx[i] = new double[y];
}

Matrix::Matrix(const Matrix &ref)
{
    if (mtrx)
    {
        for (u_int i = 0; i < x; i++)
            delete[] mtrx[i];
        delete[] mtrx;
    }
    x = ref.x;
    y = ref.y;
    *mtrx = new double[x];
    for (u_int i = 0; i < x; i++)
        mtrx[i] = new double[y];
}

Matrix::~Matrix()
{
    if (mtrx)
    {
        for (u_int i = 0; i < x; i++)
            delete[] mtrx[i];
        delete[] mtrx;
    }
}

bool Matrix::IsEqual(const Matrix &a) const     // If sizes of matrixes are equal
{                                               // matrixes are equal
    return (a.GetX() == x) && (a.GetY() == y);
}

Matrix Matrix::operator+(const Matrix &a) const
{
    if (!IsEqual(a))                            // Check on equality matrixes
        return Matrix(1,1);                     // I have not any idea yet what          
    Matrix matrix(x, y);                        // should it give is sizes 
    for (u_int i = 0; i < x; i++)               // of Matrix are not equal
        for (u_int j = 0; j < y; j++)
            matrix[i][j] = a.mtrx[i][j] + mtrx[i][j];
    return matrix;
}

main.cpp

#include <stdio.h>
#include "Matrix.hpp"

int main()
{
    Matrix a(2, 5);
    Matrix b(2, 5);
    for (u_int i = 0; i < a.GetX(); i++)
    {
        for (u_int j = 0; j < a.GetY(); j++)
        {
            a[i][j] = i + j;
        }
    }
    for (u_int i = 0; i < b.GetX(); i++)
    {
        for (u_int j = 0; j < b.GetY(); j++)
        {
            b[i][j] = i + j;
        }
    }
    Matrix c = a + b;
    return 0;
}

When I turn on my program, it throws me a segmentation fault after adding two matrixes. The more interesting thing for me in that situation is first 2 lines in Matrix.cpp, Matrix Matrix::operation+....

When I delete these 2 lines (check on equality 2 matrixes), when I turn on my program, it doesn’t throw me a segmentation fault, but when I add these 2 lines, the program throws me a s.f. Can you tell me why does it work like that?

>Solution :

You might consider using a 1D double[x*y] array instead of a 2D double*[x] of double[y] arrays. It will make memory management a bit easier, since you will have only 1 array to deal with, instead of multiple arrays.

In any case, your Matrix(const Matrix &) copy constructor should not be delete[]‘ing anything, because nothing has been initialized yet. And the rest of that constructor is not allocating the arrays correctly. You allocate them correctly in the Matrix(u_int, u_int) constructor, so copy that logic into the copy constructor. And then finish off the copy constructor by, you know, actually copying values from the input Matrix.

As for operator+, if you don’t know what you should return for different sized matrices, then I would suggest throw‘ing an exception instead.

You are also missing an operator= to finish off the Rule of 3/5/0.

Try this:

#ifndef MATRIX_HEADER
#define MATRIX_HEADER

typedef unsigned int u_int;

class Matrix
{
    double **mtrx;
    //or: double *mtrx;
    u_int x, y;

public:
    Matrix(u_int a, u_int b);
    Matrix(const Matrix &);
    ~Matrix();

    double det();

    Matrix operator+(const Matrix &) const;
    Matrix operator-(const Matrix &) const;                           
    Matrix operator*(const Matrix &) const;
    friend Matrix operator*(const Matrix &, const double &);
    Matrix operator/(const Matrix &) const;
    double* operator[](const u_int idx) const {
        return mtrx[idx];
        //or: return &mtrx[idx*x];
    }

    bool IsEqualSize(const Matrix &) const;
    u_int GetX() const { return x; }
    u_int GetY() const { return y; }
};

#endif

#include "Matrix.hpp"

Matrix::Matrix(u_int a, u_int b)
    : x(a), y(b)
{
    mtrx = new double*[x];
    for (u_int i = 0; i < x; ++i) {
        mtrx[i] = new double[y];
    }

    // or: mtrx = new double[x*y];
}

Matrix::Matrix(const Matrix &ref)
    : x(ref.x), y(ref.y)
{
    mtrx = new double*[x];
    for (u_int i = 0; i < x; ++i) {
        mtrx[i] = new double[y];
        for (u_int j = 0; j < y; ++j) {
            mtrx[i][j] = ref.mtrx[i][j];
        }
    }

    /* or:
    u_int size = x*y;
    mtrx = new double[size];
    for (u_int i = 0; i < size; ++i) {
        mtrx[i] = ref.mtrx[i];
    }
    */
}

Matrix::~Matrix()
{
    // this loop is not needed for a 1D array...
    for (u_int i = 0; i < x; ++i) {
        delete[] mtrx[i];
    }

    delete[] mtrx;
}

bool Matrix::IsEqualSize(const Matrix &a) const
{
    return (a.GetX() == x) && (a.GetY() == y);
}

Matrix Matrix::operator+(const Matrix &a) const
{
    if (!IsEqualSize(a))
        throw ...;

    Matrix matrix(x, y);

    for (u_int i = 0; i < x; ++i) {
        for (u_int j = 0; j < y; ++j)
            matrix.mtrx[i][j] = mtrx[i][j] + a.mtrx[i][j];
    }

    /* or:
    u_int size = x*y;
    for (u_int i = 0; i < size; ++i) {
        matrix.mtrx[i] = mtrx[i] + a.mtrx[i];
    }
    */

    return matrix;
}

...

#include <stdio.h>
#include "Matrix.hpp"

int main()
{
    Matrix a(2, 5);
    Matrix b(2, 5);

    for (u_int i = 0; i < a.GetX(); ++i)
    {
        for (u_int j = 0; j < a.GetY(); ++j)
        {
            a[i][j] = i + j;
        }
    }

    for (u_int i = 0; i < b.GetX(); ++i)
    {
        for (u_int j = 0; j < b.GetY(); ++j)
        {
            b[i][j] = i + j;
        }
    }

    Matrix c = a + b;

    return 0;
}