My journey through programming at compile time began in the last posts with template metaprogramming. Today, I jumped from C++98 to C++11. This is a jump to the type-traits library which is template metaprogramming in a standardized way.
The type-traits library has been part of C++ since C++11. Its origin is in boost. The type traits support type checks, comparisons, and type modifications at compile time. The library has over 100 functions, but each C++ standard adds new ones.
I will not go into the details of the type-traits library because I have already written posts about it: type-traits library. On the contrary, I cannot skip the type-traits library to continue my introduction to programming at compile time. In the end, here is my short introduction to the type-traits library. For more information, I add links to my older posts.
First of all, what’s inside the type-traits library?
The Type-Traits Library
The library consists of type checks, type comparisons, and type modifications. Let me start with the type checks.
Each type belongs precisely to one primary type category.
Primary type categories
Here are they:
The following program gives you each primary type category a type.
Here is the output of the program:
If you want to know how this magic works, my post Check Types provides more information.
Based on the primary type categories are the composite type categories.
Composite Type Categories
The following table shows the relation between the primary type categories and the composite type categories.
There are more type checks possible with the type traits.
Many of the function templates like is_trivially_copyable have the name component trivially. That means that these methods are not implemented by you but by the compiler. Requesting a method from the compiler with the keyword default is also trivial.
The type-traits library supports three kinds of comparisons:
- is_base_of<Base, Derived>
- is_convertible<From, To>
- is_same<T, U>
The following example uses all three functions
and has the expected outcome.
Programming At Compile Time
Okay, let’s step back and think about the functions of the type-traits library. Here are a few observations.
- The functions from the type-traits library are metafunctions because they run at compile time. Metafunctions are class templates
- The metafunctions’ arguments that go into the sharp brackets (<…>) are metadata. Metadata are (in this case) types.
- The return value of the functions is the (::value). This is just an alias. Since C++17, there is a more straightforward form for getting the result: instead of std::is_void<void>::value, you type std::is_void_v<void>.
I hope these three observations remind you of my last post. These are precisely the conventions I presented in my previous post to template metaprogramming: C++ Core Guidelines: Programming at Compile Time.
If a function from the type-traits library wants to return a type, not a value, you must ask for it with ::type. My following post shows which type of modifications the type-traits library supports at compile time. Ultimately, the type-traits library has two goals: correctness and optimization.
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- C++ – The Core Language
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