Hi everyone, I would like to introduce a project I'm working on and a mechanism that emerged from the projects needs and, as far as I could feel, may be useful in a general case. By lack of words (let say), I named this Type Aspects, as an analogy to Type Traits. The project I'm working on, for quite a while now, is about creating a statically, strongly typed, compile-time checked and heavily simplified interface to OpenGL. I know that "Boost.OpenGL" is a subject that has been discussed many times on this list and that every time the outcome of the discussion is that Boost is not going to support a mere wrapper around some existing API. Well, I perfectly understand this and I'm not looking for the possibility to have this included in Boost. The project itself and its functional goal isn't actually the subject of my post, but it is a good introduction to type aspects, as it is what led me writting this as a general supporting mechanism. So, to introduce the idea, if some people do not know OpenGL, it is a typical C API, full of enumerations (well not even enumerations, integers preprocessor constants), loosely typed, and full of bloated function names with fixed number of parameters of various flavors. Obviously, every single function call takes an average of two of those constants as parameters. And obviously most combinations of values are leading to undefined operations, whithout having anyone complaining about anything, except a black window on your screen and a poor-man error code whenever there is one. Probably the most hideous nightmare you could ever have to use as a programmer. That was my observation. So I decided to take all these constants, that any OpenGL programmer is usually giving directly to feed the function calls, as parameters, to take all these functions, wrapp them all, rename related functions to use nice C++ overloading... but above all, convert these constants as types. This way I was able to write compile time constraints, checking the combination of constants provided to these functions. The goal was (and I must admit that it's working pretty much fine), that I would certainly be able to write the constraints described in the OpenGL specifications in term of templated C++ type constraints, making good use of static assertions with nice messages referring to the spec itself, and of metaprogramming techniques to achieve this. Then it appeared, after struggling a while trying to abstract all the data in use in OpenGL, that I could extract some fundamental properties of all these types and that the constraints described in the specifications would be experessed as simple and clear constraints on these properties. That's what I defined as Type Aspects. Let say OpenGL is able to handle 10 different type of textures. These types are different by the way color components are packed together, by their order, by their precision or numeric type (floating point or integral). All these are aspects of the texture type. Some OpenGL function might then be used to load data to a given texture, but depending on the texture type, it will only accept a certain type of source data. For example, integral textures only accept integral data, whereas floating point textures accept any data. This would be implemented as a simple constraint checking that the "Numeric" aspect of the source data type is compatible with the "Numeric" aspect of the texture type. Moreover, this also allows the user to provide a limited set of these aspects that he desires to have, and not others that he doesn't care of, and the compiler would be able to resolve the more relevant type to use depending on the use case. Type Aspects, as I defined it for the use in my projects, are like axises describing a type, every axis taking a value in a given set of possible values. This is not limited: as these values are types as well, they can also have aspects and be described. As an example that's what I did to describe the "Numeric" aspects of OpenGL data. Numeric aspect takes a value in a given set of predefined numeric archetypes. Every single archetype then has some values the three aspects describing a number type: its signness (signed, unsigned, any), its integralness (floating, fixed point, integral, boolean, any), and its width (number of bits/bytes, any). Well to conclude, I can see that this is certainly not related to OpenGL anymore and that this is a general mechanism that can be used to describe types and create relationships between types. I don't know if it is a subject that already has been discussed or if there's anything like that in Boost already (I don't think so but I don't know all the Boost libraries by heart). Do you see any interest in this kind of thing? Do you think that this could be something that worth working on deeper? TL;DR: 1. OpenGL sucks, I implemented compile-time constraints wrapping the functions calls. 2. Thinking about compile-time constraints made me qualify the types in use with a general mechanism describing the type's "aspects". 3. Aspects are axises and a given set of predefined values on these axises using which a type and its properties can be described. 4. Constraints can then be easily written as MPL checks on aspects axises and values as long as type aspects are formalized. 5. Does it already exist? Is that interesing? Should I try to go deeper in the abstraction? Kind regards, -- Beren Minor