为稀疏矢量重载operator [] [英] Overloading operator [] for a sparse vector

问题描述

我试图在C ++中创建一个稀疏的向量类，如下：

I'm trying to create a "sparse" vector class in C++, like so:

template<typename V, V Default>
class SparseVector {
    ...
}

它将由 std :: map （其中 V 是存储的值的类型）。如果地图中没有元素，我们将假设它等于来自模板参数的值 Default 。

Internally, it will be represented by an std::map<int, V> (where V is the type of value stored). If an element is not present in the map, we will pretend that it is equal to the value Default from the template argument.

但是，我无法重载下标运算符 [] 。我必须重载 [] 运算符，因为我将这个类中的对象传递到Boost函数，期望 []

However, I'm having trouble overloading the subscript operator, []. I must overload the [] operator, because I'm passing objects from this class into a Boost function that expects [] to work correctly.

const 版本很简单：检查索引是否在地图中，return它的值如果是这样，或默认否则。

The const version is simple enough: check whether the index is in the map, return its value if so, or Default otherwise.

然而，非const版本要求我返回一个引用，这就是我遇到麻烦。如果值只是读取，我不需要（也不想）添加任何东西到地图;但如果正在写入 ，我可能需要在地图中添加一个新条目。问题是重载的 [] 不知道值是读取还是写入。它只是返回一个引用。

However, the non-const version requires me to return a reference, and that's where I run into trouble. If the value is only being read, I do not need (nor want) to add anything to the map; but if it's being written, I possibly need to put a new entry into the map. The problem is that the overloaded [] does not know whether a value is being read or written. It merely returns a reference.

有没有办法解决这个问题？

Is there any way to solve this problem? Or perhaps to work around it?

推荐答案

可能有一些非常简单的技巧，但是否则我认为 operator [] 只需要返回可以从V（并转换为V）赋值的东西，不一定是V&。所以我认为你需要返回一个对象与重载的 operator =（const V&），它创建了您的稀疏容器中的条目。

There may be some very simple trick, but otherwise I think operator[] only has to return something which can be assigned from V (and converted to V), not necessarily a V&. So I think you need to return some object with an overloaded operator=(const V&), which creates the entry in your sparse container.

您必须使用模板参数来检查Boost函数的作用，但是 - 用户定义的V转换会影响可能的转换链，例如通过阻止更多的用户定义转换在同一个链中。

You will have to check what the Boost function does with its template parameter, though - a user-defined conversion to V affects what conversion chains are possible, for example by preventing there being any more user-defined conversions in the same chain.

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