这是找到动态分配的数组长度的好方法吗? [英] Is this a good way to find the length of a dynamically allocated array?

问题描述

我有一个Matrix类，我想将一个数组传递给构造函数，以便动态设置矩阵的值.我发现如果分配这样的数组:

I have a Matrix class and I want to pass an array to the constructor in order to set the values of the matrix dynamically. I found that if I allocate an array like this:

double **array;
array = new double*[3];
array[0] = new double[2];
array[1] = new double[2];
array[2] = new double[2];
array[0][0] = 1;
array[0][1] = 1;
array[1][0] = 1;
array[1][1] = 1;
array[2][0] = 1;
array[2][1] = 1;

我可以使用如下方法获取行数和列数:

I can get the number of rows and cols using a method like this:

int getNRows(double **data){
  int size = 0;
  while(*(data+size)){size++;}
  return size;
}

int getNCols(double **data){
  int size = 0;
  while(**(data+size)){size++;}
  return size;
}

这样可以吗，还是应该坚持使用向量声明?

Is this ok or should I stick to the vector declaration?

推荐答案

您的假设完全错误；您无法以接近您建议的任何方式获得尺寸.这是完全不确定的危险行为.

Your assumption is totally wrong; you cannot obtain the size in any way close to what you propose. That's utterly undefined and dangerous behaviour.

这是伪规则(即不正确，但是除非您理解为什么不正确，否则它适用于您):

Here's a pseudo-rule (i.e. it's not true, but unless you understand why it's not true, it applies to you):

不要使用指针.不要使用new和delete. (不要说using namespace std;.)

Don't use pointers. Don't use new and delete. (And don't say using namespace std;.)

使用C ++容器是唯一的方法.

The one and only way in which you should be doing this is with C++ containers.

向量的向量将是第一个镜头，尽管跨步访问的平面向量可能会更好，并且Boost.multi_array甚至可能是最好的:

A vector of vectors would be the first shot, though a flat vector accessed in strides may be better, and Boost.multi_array may even be the best:

• std::vector< std::vector<double> > v (3, std::vector<double>(2));

std::array<std::array<double, 2>, 3>

std::vector<double> v(6);，并使用v[i + 2 *j]等.

Boost.MultiArray

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