Using C++ type aliasing to avoid the ODR problem with conditional compilation

a year ago

The article discusses the C++ concept of 'ill-formed no diagnostic required' (IFNDR), particularly focusing on violations of the One Definition Rule (ODR).
A common source of ODR violations is defining a class or function differently based on compile-time switches, such as `#ifdef` directives.
The article provides an example where `Widget` is defined differently in different translation units, leading to ODR violations.
Type aliases are highlighted as a solution since they are not subject to the ODR, allowing different definitions in different translation units without introducing new types.
The article introduces a template-based approach (`WidgetT<true>` and `WidgetT<false>`) to avoid ODR violations by using type aliases (`using Widget = WidgetT<true/false>`).
The template approach involves explicit template instantiation and boilerplate code but ensures type safety across different compilation units.
Potential issues with static members in the template approach are discussed, suggesting that they could lead to surprising behavior if not handled carefully.
The article hints at a future discussion on resolving these issues, possibly by moving static members to a shared base class.
A bonus section mentions a similar technique used to handle multiple definitions of `wchar_t`.

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