用'std :: array`替换`std :: vector`

I have a code as follows:

我有一个代码如下:

int n;

int get_the_number();
void some_computations();

int main()
{
     n = get_the_number();
     some_computations()

     return(0);
}

The get_the_number function get some input and returns the integer n, which after its call will not be modified.

get_the_number函数得到一些输入并返回整数n,它在调用后不会被修改。

In the some_computation function there is the following code

在some_computation函数中有以下代码

std::vector<my_struct> my_array;

for(int i=0; i<n; i++)
{ 
     my_struct struct_temp;

     // fill struct_temp;

     my_array.push_back(struct_temp);
}

Question: Since the size of my_array is known a priori, is it possible to replace the std::vector with a std::array? Moreover, in the affirmative case, should I expect a gain in terms of efficiency?

问题:由于my_array的大小是先验已知的,是否可以用std :: array替换std :: vector?而且,在肯定的情况下,我应该期望在效率方面获得收益吗?

I tried to replace the vector declaration with

我试图用。替换矢量声明

 std::array<my_struct,n> my_array;

but I get an error: the size of the array must be constant. Is there a way to avoid it?

但我得到一个错误:数组的大小必须是常量。有没有办法避免它?

Thank you very much.

非常感谢你。

2 个解决方案

#1

std::array needs to know the size at compile time, which doesn't apply to your code. So no, you cannot simply replace std::vector with std::array here, unless get_the_number() can return a constexpr For example.

std :: array需要知道编译时的大小,这不适用于您的代码。所以不,你不能简单地用std :: array替换std :: vector,除非get_the_number()可以返回constexpr例如。

constexpr int get_the_number() { return 42; }

int main()
{
  std::array<int, get_the_number()> a;
}

But presumably in your case int get_the_number() obtains a number determined at runtime.

但是大概在你的情况下int get_the_number()获得在运行时确定的数字。

#2

If you want to use the fact that your array length is a run time constant to improve efficiency, what you want to do is to use std::vector::reserve to reserve the necessary space ahead of time to save any reallocations as the vector grows - this should make it almost as fast as an array.

如果你想使用你的数组长度是运行时常量以提高效率的事实,你想要做的是使用std :: vector :: reserve提前保留必要的空间以保存任何重新分配为向量增长 - 这应该使它几乎与数组一样快。

my_array.reserve(get_the_number());
some_computations()

Or if array is local to the function, pass in the number as a parameter.

或者,如果数组是函数的本地数组,则将数字作为参数传入。

#1