C++ matrix multiplication warning appears

vector<double>MatrixVector(vector<vector>double>A,
    vector<double>b){
    unsigned longm = A.size();
    unsigned long n = A.front().size();
    vector<vector<double>>d(m);

    for(inti=0;i<m;i++){
        for(int j=0;j<n;j++){
            d[i][j] = 0;

            for(intk=0;k<n;k++){
                d[i][j]+=A[i][k]*b[j];
            }
        }
    }
}