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    <p>Two languages that come to mind which support both the object-oriented paradigm and non-nullable types are (arguably) C++ and (definitely) <a href="http://research.microsoft.com/en-us/projects/specsharp/" rel="nofollow noreferrer">Spec#</a>.</p> <blockquote> <p><em>Spec# is a formal language for API contracts (influenced by JML, AsmL, and Eiffel), which extends C# with constructs for non-null types, preconditions, postconditions, and object invariants.</em> <br/>&mdash; from the Spec# homepage on Microsoft Research</p> </blockquote> <p>The latter is probably not in widespread use (if it's used at all). There you get a non-nullable type by suffixing a type with <code>!</code> (e.g. <code>object!</code> or <code>string![]!</code>).</p> <p>Concerning C++, I'm sure someone clever will find plenty of arguments why C++ should not be mentioned here; being the complex, delicate language that it is, I'm sure that such reasons exist. My argument for mentioning C++ anyway is that apart from pointers (<code>*</code>) (which can very well be <code>0</code>), C++ also has references (<code>&amp;</code>), which have to be initialized to something sensible. The main problem with references is, AFAIK, that they cannot be used for everything. There's probably certain situations, e.g. with memory management or smart pointers, where you cannot get completely rid of pointers.</p> <p>Finally, you could always try to build your own non-nullable wrapper types, e.g. in C#. Probably like many others, <a href="https://stackoverflow.com/users/22656/jon-skeet">Jon Skeet</a> has done so and <a href="http://msmvps.com/blogs/jon_skeet/archive/2008/10/06/non-nullable-reference-types.aspx" rel="nofollow noreferrer">blogged about it</a>. It's a workable solution, but such a <code>NotNull&lt;T&gt;</code> type can feel a bit bulky since it's not truly integrated into the language, as with Spec#.</p>
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    1. POIn what languages references are non-nullable by default?
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    1. COThanks but from what I heard if you say try to use Spec# assemblies in C#, you would be able to set their types to null, right? Like the runtime itself isn't able to enforce this nor does it have the ability to truly have something as non-nullable. From what I read, it still stores them as null, because the types are still class.
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    2. COAlso that NotNull, even though tries to do this behavior, will still not be able to save itself from null, right?
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    3. CO@Joan, **1.** you are correct that Spec#'s non-nullable types are not directly supported by the CLI, but are a "feature" added on top of it by the language itself. Which means that the benefits won't be huge in real-world scenarios because you would likely make heavy use of the Base Class Library, which uses nullable (ie. reference) types everywhere. Still... the language mentioned supports non-nullable references, which is what you asked for. :) **2.** Declaring such a `NotNull<T>` type as a `struct` (value type) would help somewhat, as passing `null` in its place would then not be allowed.
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