What are the Restrictions on Specializing Templates within the std Namespace?

Specialization Restrictions within the std Namespace

The std namespace allows explicit specialization of its templates, but specific limitations exist.

Templates Prohibited from Specialization:

According to the C standard, the following templates cannot be specialized within the std namespace:

• numeric_limits for non-arithmetic types (e.g., complex): Only arithmetic types can be specialized.
• shared_ptr and weak_ptr: Must meet specific requirements for copyability, assignability, and comparisons.
• hash: Specializations must conform to the requirements of the standard hash class template.
• Type traits (): Specialization behavior is undefined unless explicitly allowed (e.g., only certain common_type specializations are permitted).
• Locales: Have required specializations.
• istreambuf_iterator: Require a trivial copy constructor, default constructor, and destructor.
• complex for non-floating-point types: Defining specializations for other types is considered pointless.
• atomic: Must have deleted copy operations and a constexpr value constructor.
• unary_function and binary_function: Specializations are deprecated and prohibited.

General Restrictions:

• Any specialization must not conflict with the original template requirements.
• Explicit specializations and instantiations of standard library templates are prohibited unless explicitly specified.
• Explicit specializations of member functions, function templates, or member class templates of standard library classes are undefined behavior.

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