On resolving the type vs member conflict in C++, revisited

Raymond Chen

Some time ago, I wrote about the type vs. member conflict, known informally as The Color Color problem. I may have started in the deep end of the pool, so here’s a little bit of getting-up-to-speed so that article might make more sense.

namespace Windows::UI::Xaml
  enum class Visibility { Collapsed, Visible };

  struct Style { /* ... */ };

  namespace Controls
    struct UIElement
      /* ... */

      // returns current visibility
      Windows::UI::Xaml::Visibility Visibility();

      // change visibility
      void Visibility(Windows::UI::Xaml::Visibility value);

      // returns current style
      Windows::UI::Xaml::Style Style();

      // change style
      void Style(Windows::UI::Xaml::Style value);

The fundamental problem here is that there is a name conflict between the type Style and the method Style. There is also a name conflict between the type Visibility and the method Visibility.

When used from within the UIElement class, or any class derived from it, the names Style and Visibility refer to the methods UIElement::Style and UIElement::Visibility, rather than to the types.

In language-speak, these are unqualified names, meaning that the name is just hanging out by itself without any clues as to where to find it. You’re asking the compiler to figure out what you’re referring to. And if you are using the name in the context of a class, the members of the class have priority over names outside the class.

In other words, the method names Style and Visibility cause the type names to become hidden. (Another name for this is shadowing.)

Some people tut-tut at this problem and declared, “You silly Windows people, using Pascal case for your names. If you had followed the language standard naming pattern, this problem wouldn’t even exist!”

The C++ language standard naming convention has the same problem. In the C++ standard library, type names are snake_case, and method names are also snake_case. The method

mutex_type* std::shared_lock::mutex() const noexcept;

has a name mutex that shadows the type name std::mutex. If you derive from std::shared_lock and try to use a mutex, you’re going to get the method, not the type.

Even outside of Windows, type hiding is not a purely theoretical problem: The sys/stat.h header file defines a structure called struct stat, as well as a function stat(). As a result, you are forced to say struct stat in order to get the structure. Writing stat by itself gets you the function.

So keep your eye open for the Color Color problem, even if your use case doesn’t involve Color.



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  • Matteo Italia 0

    Every time I have to write `struct stat` I cry a little. I always feel like someone 30+ years ago was so smug that C had different namespaces for functions and structures that they felt compelled to embed this gem in the Unix API, and now we are stuck with it.

  • cheong00 0

    IMO, even when the programming language allows, using the same name for both type and variable as asking for trouble, much like when the programming language is case sensitive, you use variable and type name in different casing. (I’ve seen people use ALL-CAPS for type names, and PascalCase for variable names)
    I choose to use TClassName or typeClassName convension.

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