Segmentation fault implementing matrix class c++

Question

I wanted to implement the strassen's algorithm and I though to implement a matrix class. However I get a segmentation fault with the following code:

class Matrix {
    public:
        vector < vector <int> > m;
        int r;
        int c;
        Matrix() {;}
        Matrix(int _r, int _c) {
            r = _r;
            c = _c;
            m.resize(r);
            for (int i = 0; i < r; i++) {
                m[i].resize(c);
            }
            for(int i =0; i < r; i++){
                for(int j=0; j< c; j++){
                    m[i][j] = 0;
                }
            }
        }
        friend ostream& operator<<(ostream &out, Matrix &A) {
            for(int i =0; i<A.r; i++){
                out << endl;
                for(int j=0; j<A.c; j++){
                    out << A.m[i][j] << "\t";
                }
            }
            out<< endl;
            return out;
        }

        Matrix(const Matrix &A) {
            c = A.c; 
            r = A.r;
            m.resize(r);
            for (int i = 0; i < r; i++) {
                m[i].resize(c);
            }
            for(int i =0; i<r; i++){
                for(int j=0; j<c; j++){
                    m[i][j] =  A.m[i][j];
                }
            }
        }

        Matrix& operator-= (Matrix &A) {
            assert(A.r == r);
            assert(A.c == c);
            for(int i =0; i<r; i++){
                for(int j=0; j<c; j++){
                    m[i][j] -= A.m[i][j];
                }
            }
            return *this;
        }
        Matrix& operator- (Matrix &A) {
            Matrix C(*this);
            return C-=A;
        }
        Matrix& operator+= (Matrix &A) {
            assert(A.r == r);
            assert(A.c == c);
            for(int i =0; i<r; i++){
                for(int j=0; j<c; j++){
                    m[i][j] += A.m[i][j];
                }
            }
            return *this;
        }
        Matrix& operator+ (Matrix &A) {
            Matrix C (*this);
            (C)+=A;
            return C;
        }


        Matrix getBlock(int sR, int eR, int sC, int eC) {
            assert(sR > eR);
            assert(sC > eC);
            Matrix C(eR-sR, eC-sC);
            for(int i = 0; i < C.r; i++) {
                for(int j=0; j < C.c; j++) {
                    C.m[i][j] = m[sR+i][sC+j];
                }
            }
            return C;
        }

        friend void swap(Matrix& first, Matrix& second) {
            using std::swap; 
            swap(first.r, second.r); 
            swap(first.c, second.c); 
            swap(first.m, second.m);
        }

        Matrix& operator=(Matrix other){
            return *this;
        }

        friend Matrix& operator*(const Matrix& A, const Matrix &B) {
            assert(A.r == B.c);
            Matrix C(A.r, B.c);
            for(int i =0; i<C.r; i++){
                for(int j=0; j<C.c; j++){
                    for(int k = 0; k < A.r; k++) {
                        C.m[i][j] += A.m[i][k] * B.m[k][j];
                    }
                }
            }
            return C;
        }

};

int main (void) 
{
    Matrix A(2,2), B(2,2);
    A.m[0][0] = 1;  A.m[0][1] = 2;
    A.m[1][0] = 3;  A.m[1][1] = 4;
    B.m[0][0] = 1;  B.m[0][1] = 2;
    B.m[1][0] = 3;  B.m[1][1] = 4;
    Matrix C(2,2);
    C =A+B;
    cout << C << endl;
    return 0;

}

If I try A+=B; it works... I do not understand what it is the difference with A+B. I tried to print C before returning from

Matrix& operator+ (Matrix &
    Matrix C (*this);
    (C)+=A;
    return C;
}

And it is correct. When the code hit the return, my program crashes. I would like to understand what I am doing wrong. Thanks a lot. Davide

Change the return type of `operator +` from `Matrix&` to `Matrix`, then think about the lifetime of the referenced variable you *were* returning. — WhozCraig, May 03 '14 at 23:58
possible duplicate of [Can a local variable's memory be accessed outside its scope?](http://stackoverflow.com/questions/6441218/can-a-local-variables-memory-be-accessed-outside-its-scope) — chris, May 04 '14 at 00:07
Consider replacing `assert(A.r == B.c);` with throwing an exception; that gives the caller the option to catch and continue, instead of having the program abort. — M.M, May 04 '14 at 02:13
WhozCraig, Craig, thanks for hint. The tutorial was indeed very good. Matt, which assert are you referring to, the one in the "*"? This should check the row Matrix A are the same number of the row of matrix B. I am not sure it is wrong. — Kude, May 04 '14 at 07:46

vsoftco · Answer 1 · 2014-05-04T00:05:07.100

3

In operator+ you are returning a reference to a locally constructed object. The object will be destroyed at the exit of the function, so you end up with a dangling reference. Same story with operator*. Here is a nice tutorial about operator overloading

http://courses.cms.caltech.edu/cs11/material/cpp/donnie/cpp-ops.html

edited May 04 '14 at 00:05

answered May 03 '14 at 23:57

vsoftco

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Thanks, for the tutorial it is very useful. – Kude May 04 '14 at 07:47

score 0 · Answer 2 · answered May 04 '14 at 00:52

You can't use a local variable as an l-value return value.

Matrix& operator=(Matrix && other){
            this->m = other.m;
            return *this;
        }

Matrix operator+ (Matrix &A) {
            Matrix C (*this);
            (C)+=A;
            return std::move(C);
        }

This would fix this problem.

score 0 · Answer 3 · answered May 04 '14 at 02:11

0

The problem , as others have pointed out, is that you return a reference to a local variable, which is destroyed when that function exits.

The usual way to implement operator+ is as a non-member function:

Matrix operator+(Matrix A, Matrix const &B)
{
    return A += B;
}

This approach is better than the member function in that it allows conversions to occur on the left-hand operand.

Your operator- and operator* have the same issue.

answered May 04 '14 at 02:11

M.M

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Thanks, after reading a tutorial suggested by, this make perfect sense. – Kude May 04 '14 at 07:48

Segmentation fault implementing matrix class c++

3 Answers3