C++ STL 映射 - 插入或更新
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STL map - insert or update
提问by CodeJunkie
I have a map of objects and I want to update the object mapped to a key, or create a new object and insert into the map. The update is done by a different function that takes a pointer to the object (void update(MyClass *obj))
我有一个对象映射，我想更新映射到一个键的对象，或者创建一个新对象并插入到映射中。更新由一个不同的函数完成，该函数采用指向对象的指针 (void update(MyClass *obj))
What is the best way to "insert or update" an element in a map?
在地图中“插入或更新”元素的最佳方法是什么？
回答by Terry Mahaffey
The operator[]
回答by CB Bailey
With something like the following snippet:
使用类似于以下代码段的内容：
std::map<Key, Value>::iterator i = amap.find(key);
if (i == amap.end())
amap.insert(std::make_pair(key, CreateFunction()));
else
UpdateFunction(&(i->second));
If you want to measure something that might improve performance you might want to use .lower_bound()to find where an entry and use that as a hint to insert in the case where you need to insert a new object.
如果您想衡量一些可能会提高性能的东西，您可能希望使用它.lower_bound()来查找条目的位置，并将其用作在需要插入新对象的情况下插入的提示。
std::map<Key, Value>::iterator i = amap.lower_bound(key);
if (i == amap.end() || i->first != key)
amap.insert(i, std::make_pair(key, CreateFunction()));
// Might need to check and decrement i.
// Only guaranteed to be amortized constant
// time if insertion is immediately after
// the hint position.
else
UpdateFunction(&(i->second));
回答by ablaeul
The operator[]already does, what you want. See the referencefor details.
在operator[]已经这样做，你想要什么。有关详细信息，请参阅参考资料。
回答by RvdK
回答by InsanityPants
The return value of insert is "a pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool denoting whether the insertion took place."
insert 的返回值是“一对由插入元素（或阻止插入的元素）的迭代器和指示是否发生插入的布尔值组成。”
Therefore you can simply do
因此你可以简单地做
auto result = values.insert({ key, CreateFunction()});
if (!result.second)
UpdateFunction(&(result.first->second));
NOTE: Since your question involved raw pointers, and you said you wanted your Update function to take a pointer, I have made that assumption in my snippet. Assume that CreateFunction() returns a pointer and UpdateFunction() expects a pointer.
注意：由于您的问题涉及原始指针，并且您说您希望 Update 函数采用指针，因此我在我的代码片段中做出了这个假设。假设 CreateFunction() 返回一个指针，而 UpdateFunction() 需要一个指针。
I'd strongly advise against using raw pointers though.
不过，我强烈建议不要使用原始指针。
