Why Can't `std::string` Be a Non-Type Template Parameter in C ?

Why Can't std::string Be Used as a Non-Type Template Parameter?

In C , non-type template parameters play a crucial role in generic programming. However, they have a specific requirement: they must be constant integral expressions. This restriction is essential for several reasons.

One of the primary reasons stems from the nature of non-type template parameters. They are used to specify parameters during template instantiation, allowing the compiler to generate specialized versions of the template based on provided values. This process occurs at compile time, so the values of non-type template parameters must be known and constant throughout compilation.

However, types like std::string do not meet this requirement. They are not constant integral expressions and can be modified during runtime. Allowing them as non-type template parameters would introduce a situation where the value of the parameter could change after instantiation. This would require the generation of new template instances during runtime, which is not feasible within the C template system.

The C standard explicitly defines the permissible types for non-type template parameters in section 14.1 [temp.param] p4. According to this section, non-type template parameters can only be of the following types:

• Integral or enumeration type
• Pointer to object or pointer to function
• Lvalue reference to object or lvalue reference to function
• Pointer to member
• std::nullptr_t

These types satisfy the requirement of being constant and known at compile time, ensuring the consistent generation of template instances throughout the compilation process. By restricting non-type template parameters to these specific types, C maintains its efficiency and flexibility in generic programming.

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