将`std :: copy()`与`std :: back_inserter()`一起使用 [英] Using `std::copy()` with `std::back_inserter()`

问题描述

我有两个班级A和B，都有一个成员，如下所示:

I have two class A and B both have a member like below:

class A {
  ...
  std::vector<std::vector<std::vector<size_t>>> grid;
}

class B {
  ...
  std::vector<std::vector<std::vector<size_t>>> grid;
}

我发现当我使用 std :: copy()从 A :: grid 复制到 B :: grid 时，它将失败.这是我的工作:

I found when I use std::copy() to copy from A::grid to B::grid, it will fail. Here is what I do:

// Here is in B's constructor.
// I initialize B::grid with the same size of A::grid
grid = vector<vector<vector<size_t>>>(GetSetting().grid_cols());
for (int i = 0; i < GetSetting().grid_cols(); i++) {
  grid[i] = vector<vector<size_t>>(GetSetting().grid_rows());
  for (int j = 0; j < GetSetting().grid_rows(); j++) {
    grid[i][j].reserve(a.grid[i][j].size());
  }
}

// Copy from A to B
std::copy(a.grid.begin(), a.grid.end(), std::back_inserter(grid));

但是，如果我删除了初始化部分，那么std :: copy将可以正常工作.初始化部分怎么了?

But if I remove initialize part, then the std::copy will work fine. What's wrong for the initialize part?

推荐答案

让我为您展示一个简化的示例.

Let me show you with a simplified example.

std::vector<int> v = {1, 2, 3};
std::vector<int> v1;
std::copy(v.begin(), v.end(), std::back_inserter(v1));

在这种情况下，v1将按预期为1、2、3.现在考虑一下:

In this scenario v1 will be 1, 2, 3, as expected. Now consider this:

std::vector<int> v = {1, 2, 3};
std::vector<int> v1(3); //v1 has initial size!!
std::copy(v.begin(), v.end(), std::back_inserter(v1));

现在v1将为0、0、0、1、2、3，因为 back_inserter push_back s.如果您已经在目标中分配了必要的大小，请使用 begin()迭代器，而不要使用 back_insert_iterator :

Now v1 will be 0, 0, 0, 1, 2, 3, because back_inserter push_backs. If you have already allocated the necessary size in the destination, then use the begin() iterator and not the back_insert_iterator:

std::vector<int> v = {1, 2, 3};
std::vector<int> v1(3); //v1 has initial size!!
std::copy(v.begin(), v.end(), v1.begin()); //use begin here

v1符合预期，是1，2，3.

v1 is 1, 2, 3, as expected.

这篇关于将`std :: copy()`与`std :: back_inserter()`一起使用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！