How to check that a function is constexpr

I’ve been implementing quite a few simple C++11 constexpr functions in my ail (Avid Insight Library) project. While writing unit tests alongside it, I quickly realised that it would be very helpful to have code which verifies that a constexpr function is actually being evaluated at compile-time. It turns out that it’s remarkably easy to do.
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C++ constexpr functions

I’ve been very excited to see that Visual Studio 2015 supports the constexpr keyword in C++. It was introduced in the C++11 standard, and is being taken further in upcoming revisions.

There are a number of uses for the keyword, but the one which excites me the most is using it for writing functions which can be executed during compilation, potentially saving a lot of runtime overhead. In this post, I’ll show a couple of quick examples.
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Using C++ templates for size-based type selection

The standardisation of size-specific integer types in C/C++ is extremely useful for portability. That is, when you use types like uint16_t and int32_t, you know exactly what size of data type you’re getting (assuming your compiler supports it, which is usually the case unless you’re working with very specialist/embedded systems). This isn’t the case with the more traditional types like short and int whose sizes can vary from one compiler to another. However, the size is obviously part of the type name, meaning you need to alter declarations directly if you want to use a type of a different size.

Using the std::conditional template from C++11 lets you change this. Instead, you can have a template parameter which specifies the size you want (e.g. in bytes). The corresponding type declaration can be automatically deduced from there.
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Pure virtual (abstract) final functions in C++

Today I ran across an interesting little quirk of C++11. You can declare a member function which is pure virtual, has no implementation, and which is final. That means the class can never be instantiated or inherited, and the function will never have a body.

For example:

class Widget
    virtual void foo() final = 0;

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Understanding C++11 move semantics

If you’re a programmer, you’ll be familiar with the difference between making a copy of an object, and making a reference (or a pointer) to it. If implemented correctly, the former duplicates the data, resulting in two (or more) independent instances. The latter allows the same original data to be accessed in two (or more) different ways. These concepts are common to many languages, and are essential to passing and returning data in your program.

As of C++11, you can think of ‘move’ as being a third alternative. It’s expanded the core features of the language, adding move constructors and move-assign operators to your class design arsenal. It’s required the introduction of a new type of reference as well, known as “R-value references”, which we’ll look at later.

To quote from Scott Meyers’ excellent C++11 training materials, “move is an optimization of copy”. In practical terms it’s very similar to copying, but can offer a number of advantages, especially where copying and referencing are impossible or undesirable. In this post, I’ll try to cover the basics of moves, so you can understand what they are and how you can start using them yourself.
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