c++ functor usage, e.g. std::hash

最新推荐文章于 2025-11-29 20:55:25 发布

原创最新推荐文章于 2025-11-29 20:55:25 发布 · 405 阅读

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文章标签：

#哈希算法 #c++ #算法 #functor

博客介绍了C++中哈希算法的使用，通过代码示例展示了两种计算哈希值的方式等价，还深入解释了代码中使用{}的原因，指出std::hash是类模板而非函数，调用时会创建类的实例并调用其方法。

This line :

size_t hashVal0 = std::hash<std::string>{}(strInput);

equal these two lines:

std::hash<std::string> szHash;
size_t hashVal = szHash(strInput);

The hashVal0 == hashVal == true.

this is why need {} in first line:

std::hash<KeyType>{}: This creates an instance of the std::hash<KeyType> class. The {} is used to call the default constructor.
(key): This calls the operator() method on the instance of std::hash<KeyType>, passing key as the argument.

more deep dive:

The syntax std::hash<KeyType>(key) might look like it’s invoking a function, but std::hash<KeyType> is not a function—it’s a class template. Therefore, std::hash<KeyType>(key) would be trying to call a constructor of std::hash<KeyType> that takes an argument of type KeyType, but no such constructor exists.