最简单的二维数组，使用g ++，一个可变维度？ [英] Simplest 2D array, using g++, with one variable dimension?

问题描述

我想创建一个这样的数组：

I would like to make an array like this:

double array[variable][constant];

只有第一个维度是可变的。使用变量作为第一维声明此变量会产生初始化错误。使用指针或其他基本类型是否有一个简单的方法？

Only the first dimension is variable. Declaring this with a variable as the first dimension gives initialization errors. Is there a simple way to do this with pointers or other basic types?

推荐答案

可变长度数组没有在最新的C ++标准。您可以使用 std :: vector 代替

Variable length arrays didn't make it in the latest C++ standard. You can use std::vector instead

std::vector<std::vector<double> > arr;

或者，为了修复例如第二维（在本示例中为10），您可以

Or, to fix for example the second dimension (to 10 in this example), you can do

std::vector<std::array<double, 10> > arr1; // to declare a fixed second dimension

否则，您需要使用旧的指针

Otherwise you need to use old pointers,

double (*arr)[10]; // pointer to array-of-10-doubles

根据@ Chiel的评论，你可以做

In light of @Chiel's comment, to increase performance you can do

typedef double (POD_arr)[10];
std::vector<POD_arr> arr;

以这种方式，所有数据都连续存储在内存中，因此访问应该使用一个简单的 C 数组。

In this way, you have all data stored contiguously in the memory, so access should be as fast as using a plain old C array.

PS：看起来最后一个声明是违反标准， @juanchopanza提到，POD数组不满足要存储在STL数组中的数据的要求（它们是不可分配的）。但是， g ++ 编译上面的2个声明没有任何问题，并且可以在程序中使用它们。但 clang ++ 失败。

PS: it seems that the last declaration is against the standard, since as @juanchopanza mentioned, POD arrays do not satisfy the requirements for data to be stored in an STL array (they are not assignable). However, g++ compiles the above 2 declarations without any problems, and can use them in the program. But clang++ fails though.

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