时间：2019-05-16 标签：c++unordered_map ofvectors
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c++ unordered_map of vectors
提问by user1463416
So conceptually I'm trying to create a hash of arrays of pointers to my object.
所以从概念上讲，我试图创建指向我的对象的指针数组的散列。
The hash key is an int for the type of the object, and the array is a list of the objects to render.
哈希键是对象类型的 int，数组是要呈现的对象列表。
What I'm trying to do is :
我想要做的是：
unordered_map<int, vector<Object*> > drawQueue;
drawQueue.clear(); // new empty draw queue
for ( ... ) {
drawQueue.at(type).push_back(my_obj);
}
So I'm not familiar enough with the nuances of the STL stuff, since I get an exception saying out_of_bounds, which is what happens when the key doesn't exist.
所以我对 STL 内容的细微差别还不够熟悉，因为我收到一个异常，说 out_of_bounds，当密钥不存在时会发生这种情况。
So I figured I need to create the key first, and then add to the vector :
所以我想我需要先创建密钥，然后添加到向量中：
if (drawQueue.count(type)) {
// key already exists
drawQueue.at(type).push_back(my_obj);
} else {
//key doesn't exist
drawQueue.insert(type, vector<Object*>); // problem here
drawQueue.at(type).push_back(my_obj);
}
But now I'm really lost, as I don't know how to create/initialise/whatever an empty vectorto the insert of the unordered_map...
但现在我真的迷路了，因为我不知道如何创建/初始化/任何空vector的插入unordered_map......
Or am I doing this the entirely wrong way?
还是我这样做完全错误？
Any hints wise peoples?
有智慧的人有什么提示吗？
回答by betabandido
You are not using insertin the proper way. This should work:
您没有insert以正确的方式使用。这应该有效：
drawQueue.insert(std::make_pair(type, std::vector<Object*>()));
If using C++11, the previous statement can be simplified to:
如果使用C++11，前面的语句可以简化为：
drawQueue.emplace(type, std::vector<Object*>());
By using this approach the element is constructed in-place (i.e., no copy or move operations are performed).
通过使用这种方法，元素就地构建（即，不执行复制或移动操作）。
I also include links to the documentation for insertand emplace.
回答by Venkata Gogu
I think this is an easy approach. My example will create an unordered_map string as key and integer vector as values.
我认为这是一个简单的方法。我的示例将创建一个 unordered_map 字符串作为键和整数向量作为值。
unordered_map<string,vector<int>> keys;
keys["a"] = vector<int>(); // Initialize key with null vector
keys["a"].push_back(1); // push values into vector.
keys["a"].push_back(5);
for(int i : keys["a"] ){
cout << i << "\t";
}
