Can You Specialize Every Template in the `std` Namespace? Limitations and Allowances in Explicit Specializations

Explicit Specializations in the std Namespace: Limitations and Allowances

While users are granted the ability to append explicit specializations to the std namespace, certain templates are explicitly barred from this practice.

Prohibited Specializations

According to the standard, the following templates cannot be specialized:

• numeric_limits for non-arithmetic standard types like complex
• shared_ptr, unless they meet specific criteria for copyability, assignability, and comparability
• weak_ptr, unless they are copyable and assignable
• std::hash, unless specializations adhere to its template requirements
• Class templates defined in (except common_type)
• Locales, which require specific specializations
• istreambuf_iterator, which must have a trivial copy constructor, default constructor, and destructor
• complex for types other than float, double, or long double
• atomic, unless specializations follow specific constraints such as having deleted copy constructors and value constructors
• unary_function and binary_function, which are deprecated and cannot be specialized

Overarching Considerations

In addition to these specific prohibitions, the standard provides general guidelines for adding to the std namespace:

• Declarations or definitions to std or its sub-namespaces should not be added unless explicitly specified.
• Template specializations are allowed only if they depend on a user-defined type and meet library requirements.
• Member function specializations for standard library classes, member function templates, and member class templates are prohibited.
• Explicit template instantiations are permissible only if they depend on a user-defined type and meet library requirements.

By adhering to these limitations, developers can ensure that their specializations do not conflict with the integrity and operability of the standard library in the std namespace.

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