Скачать или смотреть Why Isn't std::hash const std::string Specialized in std?

Discover the reasons behind the non-specialization of `std::hash const std::string ` in the C+ + standard library and learn how to properly implement unordered maps for string keys.
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Why Isn't std::hash<const std::string> Specialized in std?

When diving into C+ + programming, particularly when working with associative containers like std::unordered_map, you might encounter the query: Why isn't std::hash<const std::string> specialized in the C+ + Standard Library? This seemingly straightforward question can lead to compile errors that may confuse even seasoned developers.
In this guide, we will unravel the purpose behind the design choice of C+ + 's hash functions and provide clear guidance on how to resolve common issues related to string keys in unordered maps.

The Issue at Hand

Suppose you try to implement the following code snippet:

[[See Video to Reveal this Text or Code Snippet]]

Running this code will lead to compilation errors that might leave you scratching your head. At the core of this issue lies the lack of specialization for std::hash<const std::string> in the C+ + Standard Library. But why has it been designed this way?

Understanding std::hash Specialization

To grasp the rationale behind this design choice, let's break down a few crucial concepts.

No Need for Specialization

C+ + has not specialized the std::hash<const T> because it likely isn't necessary. The template structure for std::hash<T> is designed such that the primary function it contains is operator(), which already accepts a const T& as an argument. This setup implies that even had std::hash<const Foo> been specialized, its behavior would remain identical to std::hash<Foo>.

Key Takeaway:

All std::hash<T> implementations work directly with non-const types and utilize references internally.

Correcting the Code

To resolve the initial error in our example, you simply need to revise the unordered map declaration to use std::string as the key type, rather than const std::string:

[[See Video to Reveal this Text or Code Snippet]]

Even though the unordered map keys are defined as std::string, the internal mechanisms can understand the reference as const std::string. This distinction clarifies how hash functions operate.

Insights on Specialization

Efficient Usage of Hash Functions

While it's reasonable to argue that keys should not change, this does not inherently require their specialization. This leads us to consider an alternative approach: using std::hash<const T&>. The objective here remains efficiency; minimizing unnecessary copies while respecting the immutability of key values.

Template Argument vs. Argument Type

It's also essential to note that the types on which std::hash specializes do not have to equal the argument types. The template argument mainly indicates the type related to the hash function, rather than what gets passed to the function call. For instance, numeric_limits<T> involves non-const numeric types while still accommodating the limits of const int and const double.

The Role of std::hash

Lastly, std::hash serves as a utility class, particularly designed to operate with the unordered_XXX family in C+ + and primarily used as a default hash function. This adds a layer of abstraction that emphasizes generic programming, allowing developers to define custom hash functions effortlessly.

Conclusion

In conclusion, the absence of specialization for std::hash<const std::string> in the C+ + Standard Library stems from design choices prioritizing efficiency and practicality over redundancy. Understanding how to correctly declare and use unordered maps will empower you to write cleaner and error-free code. By grasping the foundational concepts of hash specialization, you can become more adept at leveraging C+ + 's powerful