Variable Template Restrictions

While variable templates provide a lot of flexibility and power, there are a few limitations and restrictions to keep in mind:

• Variable templates are a compile-time feature, so the arguments passed to the template must be known at compile-time. This means that the arguments must be constant expressions or constexpr variables.
• The type used in a variable template must be a complete type at the point of instantiation. If the type is incomplete or not yet defined, it will result in a compilation error.
• Variable templates cannot be specialized in the same way as function templates or class templates. The template arguments must match exactly for the variable template to be instantiated.
• Variable templates are not allowed to have default template arguments. Each template parameter must be explicitly provided when instantiating the variable template.
• The initialization of a variable template must be done with a constant expression. Non-constant expressions or runtime values cannot be used to initialize a variable template.

It's important to be aware of these limitations when using variable templates to ensure that your code compiles successfully and behaves as expected.