List or container to hold types

This is a discussion on List or container to hold types within the C++ Programming forums, part of the General Programming Boards category; Hi, I'm having difficulty coming up with a solution for what I am trying to achieve and was hoping maybe ...

  1. #1
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    List or container to hold types

    Hi,

    I'm having difficulty coming up with a solution for what I am trying to achieve and was hoping maybe someone had encountered this kind of situation before.

    I am attempting to test a feature for various sorts of types, e.g. int, double, float, types that I define, etc. To do so I have created a function template that will take this type and then perform a certain operation.

    Ideally, what I would like to do is loop through the various types I have defined and pass them into this function template. But, I'm not sure what kind of container or list to use to hold these types.

    Any ideas?

    Thanks a lot for any help at all.

  2. #2
    Registered User whiteflags's Avatar
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    Lists and containers are useful because they are of flexible size; they can't hold more than one data type when they are declared.

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    Right, yeah that's why I'm confused about how I can achieve some sort of "list" of types.

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    Put them in a struct or a class. At a very basic level, a struct or class can be considered an array that can hold different types.

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    Depending on your needs, you could use also consider using the conversion operator, (), or a constructor to "convert" a given type into another. For example, you could convert an object of type int into an object of a rationalNumber class or an object of type double into an object of ComplexNumber, etc.. If you can convert one class into another, then you can use any of the standard containers, irrespective of the "original" type.

    Another possibility might be to devise a series of classes all derived from a single base class. If all types in the hierachy share a common behavior then you can use an array of pointer to base type to express the different behaviors for whatever types are actually in the array.
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    Are you going to be containing types, or instances of different types? Can you have a different container for each type you want to pass to the function, or are you just wanting to instantiate the function with each different type?

    If you know the types at compile time, and you need a container of type instances, I would just have separate containers for each type. If you just want to run the function with each different type, just call the function with the different types as template parameters (e.g. myFunc<int>(); myFunc<double>(); etc.).

    If you won't know the types until runtime, you might have to use some sort of RTTI (like with typeid) to help you identify different types.

    Lastly, if you want to have your container hold instances of different types at the same time (perhaps because you want them all mixed together or in a random order), then use a container of boost::variant objects. Boost's variant is able to hold an instance of any of a group of types, and you can have a single container of variants each holding different types.

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    http://en.wikipedia.org/wiki/Loki_(C_Plus_Plus)

    The Loki library implements something called TypeLists, which is a compile time linked list of types. It's been a while since I looked at it, but I *think* you can build the list at compile time, and then iterate through it at runtime. The code is a free download...someplace--I forget where. The code is explained in detail in the book "Modern C++ design" by Andrei Alexandrescu.
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