在C中将内存分配给二维数组的最佳方法? [英] Best way to allocate memory to a two-dimensional array in C?

问题描述

从memory-management和speed的两个角度来看，为C中的two-d array分配内存的最佳方法是什么?

What is the best way to allocate memory to a two-d array in C,from both the perspectives : memory-management and speed ?

还更好地使用two-d array(并为其分配内存)还是double pointer?有人可以详细解释一下，里面发生了什么，为什么一种方法比另一种更好?

Also, which is better to use, a two-d array (and allocate memory to it) or a double pointer ? Can someone explain in detail,what happens inside,why a method is better than the other one ?

推荐答案

为获得最佳性能和最佳可读性，应始终将此类数组分配为连续的内存块:

To get best performance and best readability, such arrays should always be allocated as a contiguous chunk of memory:

type (*array) [X][Y] = malloc( sizeof(type[X][Y]) );

您应该避免这种情况:

// BAD METHOD, not a real array

type** lookup_table = malloc( X*sizeof(type*) );
for(size_t i=0; i<Y; i++)
{
  lookup_table[i] = malloc( Y*sizeof(type) );
}

由于许多原因，前者速度更快.它分配在连续的内存块中，而不是在整个堆中分段.分段版本会阻止所有形式的代码优化和有效的片上数据缓存使用，而且实际分配也要慢得多.

The former is faster for many reasons. It is allocated in a contiguous chunk of memory and not segmented all over the heap. Segmented versions block all forms of code optimizations and efficient on-chip data cache use, plus the actual allocation is also much slower.

尽管上面的坏"版本有一个优点，那就是当您希望各个尺寸具有可变长度时，例如在为字符串创建查找表时.然后，您必须使用该表格.但是，如果您想要一个真正的2D数组，则没有理由不使用前者.

The "bad" version above has one advantage though, and that is when you want individual dimensions to have variable length, such as when making a look-up table for strings. Then you have to use that form. But if you want a real 2D array, there is never a reason not to use the former.

请注意，第一个版本通常写为

Note that the first version is usually written as

type (*array) [Y] = malloc( sizeof(type[X][Y]) );

以便更方便地使用:array[i][j]，而不是不太易读的(*array)[i][j].

to allow more convenient use: array[i][j], rather than the less readable (*array)[i][j].

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