如何使用 new 在 C++ 中声明一个二维数组?

问题描述

如何使用 new 声明一个二维数组?

比如，对于普通"数组，我会:

int* ary = new int[Size]

但是

int** ary = new int[sizeY][sizeX]

a) 不能工作/编译，b) 不能完成什么:

int ary[sizeY][sizeX]

确实如此.

解决方案

如果您的行长度是编译时常量，C++11 允许

auto arr2d = new int [nrows][CONSTANT];

见中的示例.

How do i declare a 2d array using new?

Like, for a "normal" array I would:

int* ary = new int[Size]

but

int** ary = new int[sizeY][sizeX]

a) doesn't work/compile and b) doesn't accomplish what:

int ary[sizeY][sizeX]

does.

解决方案

If your row length is a compile time constant, C++11 allows

auto arr2d = new int [nrows][CONSTANT];

See this answer. Compilers like gcc that allow variable-length arrays as an extension to C++ can use new as shown here to get fully runtime-variable array dimension functionality like C99 allows, but portable ISO C++ is limited to only the first dimension being variable.

Another efficient option is to do the 2d indexing manually into a big 1d array, as another answer shows, allowing the same compiler optimizations as a real 2D array (e.g. proving or checking that arrays don't alias each other / overlap).

---

Otherwise, you can use an array of pointers to arrays to allow 2D syntax like contiguous 2D arrays, even though it's not an efficient single large allocation. You can initialize it using a loop, like this:

int** a = new int*[rowCount];
for(int i = 0; i < rowCount; ++i)
    a[i] = new int[colCount];

The above, for colCount= 5 and rowCount = 4, would produce the following:

Don't forget to delete each row separately with a loop, before deleting the array of pointers. Example in another answer.

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