C++ 如何使用 std::copy 将一张地图复制到另一张地图？
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How can I copy one map into another using std::copy?
提问by Frank
I would like to copy the content of one std::map into another. Can I use std::copyfor that? Obviously, the following code won't work:
我想将一个 std::map 的内容复制到另一个。我可以用std::copy它吗？显然，以下代码将不起作用：
int main() {
typedef std::map<int,double> Map;
Map m1;
m1[3] = 0.3;
m1[5] = 0.5;
Map m2;
m2[1] = 0.1;
std::copy(m1.begin(), m1.end(), m2.begin());
return 0;
}
This won't work because copywill call operator*on m2.begin()to "dereference" it and assign a value (all values are of type std::pair<const int, double>). Then it will call operator++to move to the next space in m2. Both of these operations don't work because of the constin const intand there is no space reserved for any new elements.
这不会工作，因为copy会叫operator*上m2.begin()为“解引用”，并分配一个值（所有值的类型的std::pair<const int, double>）。然后它会调用operator++移动到 中的下一个空间m2。由于constinconst int并且没有为任何新元素保留空间，因此这两个操作都不起作用。
Is there any way to make it work with std::copy?
有什么办法可以让它工作std::copy吗？
Thanks!
谢谢！
回答by Billy ONeal
You can use GMan's answer --- but the question is, whydo you want to use std::copy? You should use the member function std::map<k, v>::insertinstead.
您可以使用 GMan 的答案 --- 但问题是，您为什么要使用std::copy？您应该改用成员函数std::map<k, v>::insert。
m2.insert(m1.begin(), m1.end());
回答by GManNickG
You need a variant of an insert iterator:
您需要一个插入迭代器的变体：
std::copy(m1.begin(), m1.end(), std::inserter(m2, m2.end()) );
inserteris defined in <iterator>. It requires a place to insert into (hence the m2.end()), and returns an insert_iterator.
inserter中定义<iterator>。它需要一个插入的地方（因此是m2.end()），并返回一个insert_iterator.
