Can You Specialize Every Template in the `std` Namespace?

Specialized Templates in the std Namespace

Users can add explicit specializations to the std namespace, but certain templates are off-limits. These restrictions ensure that the behavior of the standard library remains consistent and well-defined.

Prohibited Specializations:

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

• numeric_limits: Prohibited for non-arithmetic standard types (e.g., complex).
• shared_ptr: Specializations must be CopyConstructible, CopyAssignable, LessThanComparable, and convertible to bool.
• weak_ptr: Specializations must be CopyConstructible and CopyAssignable.
• hash: Specializations must meet the requirements of class template hash.
• Class templates in : Program behavior is undefined if specializations are added for these templates.
• Complex: Undefined for types other than float, double, or long double.
• Atomic: Specializations must have a deleted copy constructor, copy assignment operator, and constexpr value constructor.
• Unary_function and binary_function: Deprecated and should not be specialized.

Additional Restrictions:

Furthermore, the standard prohibits the following:

• Adding declarations or definitions to the std namespace or any sub-namespaces without meeting specific requirements.
• Explicit specialization of member functions or member templates of standard library classes.
• Explicit or partial specialization of member class templates of standard library classes.

Allowed Specializations:

Template specializations are only allowed if:

• They depend on a user-defined type.
• They adhere to the standard library requirements for the original template.
• They are not explicitly prohibited as outlined above.

By following these restrictions, programmers can ensure the integrity and predictability of the std namespace and the standard library as a whole.

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