在 C++ std::vector 和 C 数组之间转换而不复制 [英] Converting between C++ std::vector and C array without copying

问题描述

我希望能够在 std::vector 与其底层 C 数组 int* 之间进行转换，而无需显式复制数据.

I would like to be able to convert between std::vector and its underlying C array int* without explicitly copying the data.

std::vector 是否提供对底层 C 数组的访问?我正在寻找这样的东西

Does std::vector provide access to the underlying C array? I am looking for something like this

vector<int> v (4,100)
int* pv = v.c_array();

另外，是否可以反过来，即如何在不复制的情况下从 C 数组初始化 std::vector?

Also, is it possible to do the converse, i.e. how would I initialize an std::vector from a C array without copying?

int pv[4] = { 4, 4, 4, 4};
vector<int> v (pv);

推荐答案

你可以得到一个指向第一个元素的指针，如下所示:

You can get a pointer to the first element as follows:

int* pv = &v[0];

这个指针只有在向量没有被重新分配时才有效.如果插入的元素多于向量剩余容量的容量(即，如果 v.size() + NumberOfNewElements > v.capacity()，则重新分配会自动发生.您可以使用 v.reserve(NewCapacity) 以确保向量至少具有 NewCapacity 的容量.

This pointer is only valid as long as the vector is not reallocated. Reallocation happens automatically if you insert more elements than will fit in the vector's remaining capacity (that is, if v.size() + NumberOfNewElements > v.capacity(). You can use v.reserve(NewCapacity) to ensure the vector has a capacity of at least NewCapacity.

还要记住，当向量被破坏时，底层数组也会被删除.

Also remember that when the vector gets destroyed, the underlying array gets deleted as well.

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