What are curly braces in hash function?

Question

In C++11, it is possible to get a hashed value for a string variable as follows:

std::size_t h1 = std::hash<std::string>{}("Some_String");

It is clean and simple. However, I have two questions:

Why do we need the curly braces here?
Is it possible to escape using the braces?

Possible duplicate of [What are the differences between C-like, constructor, and uniform initialization?](https://stackoverflow.com/questions/24953658/what-are-the-differences-between-c-like-constructor-and-uniform-initialization) — , Oct 19 '18 at 02:55
Note that `std::hash` instances are _types_, not functions. This, e.g., allows to specialize `std::hash` for user-defined types in the `std` namespace, which will no longer be allowed for _template functions_ in C++20. — Daniel Langr, Oct 19 '18 at 06:40

score 10 · Accepted Answer · answered Oct 19 '18 at 02:54

The curly braces are used to value-initialize an object of type std::hash<std::string>. That object can then be called, since it has an overloaded operator(). Alternatively, you could create a named object:

std::hash<std::string> H;
auto h1 = H("Some_String");

What are curly braces in hash function?

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