动态数组

Question

我做的时刻下面的练习：

的通用矩阵类（15磅）

a) Create a class called Matrix, it should contain storage for M*N numbers of type double. Just like earlier, when choosing how to store your data it is often useful to know what we are going to use the data for later. In matrix operations we are going to access the different elements of the matrix based on their column and/or row, therefore it is useful to order the members of Matrix as an array. Also, we are going to need to change the size of the data stored in the matrix, therefore it should be dynamically allocated.

b) Create constructors for the matrix.

Create the following three constructors: Matrix() • Default constructor, should initialize the matrix into the invalid state.

explicit Matrix(unsigned int N) • Should construct a valid NxN Matrix, initialized as an identity matrix. (The explicit keyword is not in the syllabus, but it should be used here.)

Matrix(unsigned int M, unsigned int N) • Should construct a valid MxN Matrix, initialized as a Zero matrix. (All elements are zero.)

~Matrix() • The destructor of Matrix, should delete any dynamically allocated memory.

我班这么远如下：

class Matrix{ 
    private: 
     int rows; 
     int columns; 
     double* matrix; 
    public: 
     Matrix(); 
     explicit Matrix(int N); 
     Matrix(int M, int N); 
     ~Matrix(); 
}; 

和我的代码的其余部分：

Matrix::Matrix(){ 
    double * matrix = NULL; 
} 

Matrix::Matrix(int N){ 
    double * matrix = new double[N * N]; 
    this->rows = N; 
    this->columns = N; 

    for(int i = 0; i < N; i++){ 
     for(int j = 0; j < N; j++){ 
      if(i==j) 
       matrix[i * N + j] = 1; 
      else 
       matrix[i * N + j] = 0; 
     } 
    } 
} 

Matrix::Matrix(int M, int N){ 
    double * matrix = new double[M * N]; 
    this->rows = M; 
    this->columns = N; 

    for(int i = 0; i < M; i++){ 
     for(int j = 0; j < N; j++) 
      matrix[i * N + j] = 0; 
    } 
} 

Matrix::~Matrix(){ 
    delete [] matrix; 
} 

我是否正确创建了动态数组和构造函数？ 我后来在练习中使用三种不同的构造函数创建三个不同的数组。我如何做到这一点？如果我尝试这样的事情

Matrix::Matrix(); 
Matrix::Matrix(3); 

或

Matrix::Matrix(3,4) 

我得到以下错误：

Unhandeled exception at 0x773c15de in Øving_6.exe: 0xC0000005: Access violation reading location 0xccccccc0.

我到底做错了什么？

Answer 1

在你的构造函数，你定义一个局部变量

double * matrix = new double[N * N]; 

其阴影同名的成员变量，那么该成员从来没有初始化。

所有你应该需要的是将其更改为

matrix = new double[N * N]; 

而且这非常不-C++使用this->的成员访问，除非它是绝对必要的消歧（这是几乎从来没有）

Answer 2

在你的三个构造函数中，你用本地构造函数掩盖了实例变量矩阵。试试这个：

Matrix::Matrix(){ 
this->matrix = NULL; 
} 

Matrix::Matrix(int N){ 
this->matrix = new double[N * N]; 
this->rows = N; 
this->columns = N; 

for(int i = 0; i < N; i++){ 
    for(int j = 0; j < N; j++){ 
     if(i==j) 
      matrix[i * N + j] = 1; 
     else 
      matrix[i * N + j] = 0; 
    } 
} 
} 

Matrix::Matrix(int M, int N){ 
this->matrix = new double[M * N]; 
this->rows = M; 
this->columns = N; 

for(int i = 0; i < M; i++){ 
    for(int j = 0; j < N; j++) 
     matrix[i * N + j] = 0; 
} 

}

Answer 3

你会找到更多的“C++”（并且有时只有这样，才能初始化成员）：

Matrix::Matrix(int M, int N): rows (M), 
           columns (N), 
           matrix (new double[M * N]) 
{ 
    for(int i = 0; i < M; i++) 
     for(int j = 0; j < N; j++) 
      matrix[i * N + j] = 0; 

} 

现在尝试明白这一点：

Matrix::Matrix(  int N): rows (N), 
           columns (N), 
           matrix (new double[N * N]) 
{ 
    for(int i = 0; i < N; i++) 
     for(int j = 0; j < N; j++) 
      matrix[i * N + j] = (i==j); 

} 

如果你使用：

class Matrix{ 
private: 
    int rows; 
    int columns; 
    std::unique_ptr<double[]> matrix; 

你会发现，你不需要析构函数，以及其他一些inerest事情。另外，请阅读我的其他答案。