std :: pair< int，int> vs struct with two int's [英] std::pair<int, int> vs struct with two int's

问题描述

在ACM示例中，我不得不为动态编程构建一个大表。我必须在每个单元格中存储两个整数，所以我决定去一个 std :: pair< int，int> 。然而，分配一个庞大的数组需要1.5秒：

In an ACM example, I had to build a big table for dynamic programming. I had to store two integers in each cell, so I decided to go for a std::pair<int, int>. However, allocating a huge array of them took 1.5 seconds:

std::pair<int, int> table[1001][1001];

之后，我将此代码更改为

Afterwards, I have changed this code to

struct Cell {
    int first;
    int second;
}

Cell table[1001][1001];

，分配耗时0秒。

推荐答案

std :: pair< int，int> ;: ：pair（）构造函数使用默认值（在 int 的情况下为零）和 struct Cell 不会（因为你只有一个自动生成的默认构造函数不起作用）。

std::pair<int, int>::pair() constructor initializes the fields with default values (zero in case of int) and your struct Cell doesn't (since you only have an auto-generated default constructor that does nothing).

初始化需要写入每个字段，的相对耗时的存储器访问。使用 struct Cell 没有做任何事情，不做任何事情有点快。

Initializing requires writing to each field which requires a whole lot of memory accesses that are relatively time consuming. With struct Cell nothing is done instead and doing nothing is a bit faster.

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