矢量载体，备用 [英] Vector of vectors, reserve

问题描述

假设我想将 int 的二维矩阵表示为向量的向量：

Suppose I want to represent a two-dimensional matrix of int as a vector of vectors:

std::vector<std::vector<int> > myVec;

内部尺寸为常数，例如5，外部尺寸小于或等于 N 。为了最大限度地减少重新分配，我想保留空间：

The inner dimension is constant, say 5, and the outer dimension is less than or equal to N. To minimize reallocations I would like to reserve space:

myVec.reserve(N);

内部向量的大小是多少？这纯粹取决于实现吗？这如何影响数据的空间局部性？由于内部尺寸是一个常量，有没有办法告诉编译器使用这个常量大小？如果内部向量的大小发生变化，这些答案将如何改变？

What size is assumed for the inner vector? Is this purely implementation dependent? How does this effect the spatial locality of the data? Since the inner dimension is a constant is there a way to tell the compiler to use this constant size? How do these answers change if the inner vector's size changes?

推荐答案

由于内部维度是恒定的，我想您想要

Since your inner dimension is constant, I think you want

std::vector< std::array<int, 5> > vecs;
vecs.reserve(N);

这将为您提供预分配的连续存储，这对于性能而言是最佳的。

This will give you preallocated contiguous storage, which is optimal for performance.

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