Where is type traits

This is the general pattern you will see for almost all type traits. However, there are a few saving graces. There are traits to tell you the basic datatypes, whether something is a pointer, whether something is an array, whether something is const, etc.

There are even traits to remove traits, like turn a pointer type into a value type, or a const type into a non-const type. Most of the time you are able to make use of these built-in type traits for your own code. So we check to make sure the size of the type passed in is at least two bytes.

The boost library comes with many type traits, as well as its own implementation of static assserts. Regardless, type traits are an incredibly powerful tool to use when doing template programming. It allows you to have more fine-tuned control over your template functionality, ensuring that no one accidentally misuses it with silent errors.

Another thing to keep in mind is that type traits can make use of other type traits. Generally, if there is a sensible, harmless fallback for template functionality, I will use specialization to perform the fallback.

The same is true when I wish to optimize a function for a particular type. However, it is not a one-size-fits-all feature. Regardless, you now have another powerful tool to use with template code! Great tutorial. Spot on. I find a lot of blogs get too technical and confusing, but you explained an advanced topic perfectly. Good examples also. I omit the if, etc. With the type typedef you can access the type itself. Capitalizing on that we might think that if you have a class MyInt then having it an integral type only means that we simply have to write such code I omit the problem of references and cv qualifications for the sake of simplicity :.

If you read attentively, probably you pointed out that I used the auxiliary "might" and it's not incidental. I learned that having such a specialization results in undefined behaviour according to the standard [meta.

What is in that subsection? Go look for a draft standard here is one if you don't have access to a paid version. In fact, all the primary or composite type categories are in there.

As soon as we are allowed to add specializations, we cannot rely on this. This rule is by no means foolproof, but it is reasonably simple to remember and close enough to the actual rule to be useful for everyday use. This class defines a single typedef-member type that is the same type as T but with any top-level array bounds removed; this is an example of a traits class that performs a transformation on a type:.

This example also shows that the number of template parameters in a partial specialisation does not have to match the number in the default template. However, the number of parameters that appear after the class name do have to match the number and type of the parameters in the default template.

As an example of how the type traits classes can be used, consider the standard library algorithm copy:. Obviously, there's no problem writing a generic version of copy that works for all iterator types Iter1 and Iter2; however, there are some circumstances when the copy operation can best be performed by a call to memcpy.

In order to implement copy in terms of memcpy all of the following conditions need to be met:. If all these conditions are met then a type can be copied using memcpy rather than using a compiler generated assignment operator. The code for an optimised version of copy that uses memcpy where appropriate is given in listing 1. This is what the version in listing 1 does.

It has often been repeated in these columns that "premature optimisation is the root of all evil" [4]. We are going to see how they can set conditions for compiling different template specializations and then how they can alter types. Just to emphasize, this has no runtime costs, all the checks and errors happen at compile-time. We can simply statically assert that T is an unsigned type. We can see that we were able to define two functions with the same signature, while only the template parameter list is different.

In order not to end up with a compilation error we default these types to nullptr. Something we are going to cover in detail in the coming weeks. With if constepxr we can evaluate conditions at compile time and we can discard branches from the compilation. With if constexpr you can significantly simplify obscure metaprogramming constructs.