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

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.