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

Understanding Template Parameter Restrictions for Non-Constant Types

In C , non-type template parameters serve a specific purpose in controlling template instantiation. However, it's crucial to understand why certain types, such as std::string, are prohibited as non-type template parameters.

According to the C standard, non-type template parameters must be constant integral expressions (14.1 [temp.param] p4). This means they cannot change their value during runtime. Conversely, types like std::string are non-constant and can be modified.

The reason for this restriction lies in the nature of templates. Templates are processed and instantiated at compile-time, meaning their behavior is determined before the program executes. Allowing non-constant template parameters would introduce ambiguity because the value could change at runtime.

For instance, in the following code snippet:

template <std::string temp>
void foo()
{
    // ...
}

The value of temp could potentially change during execution, making it impossible to determine the behavior of foo() at compile-time. This would create a runtime overhead and undermine the purpose of templates, which is to enhance efficiency by generating code at compile-time.

Therefore, the C standard restricts non-type template parameters to constant integral expressions, ensuring that their values remain constant throughout the compilation process. This enables templates to optimize code generation and maintain predictable behavior.

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