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How To Use void_t Alias Template in C++ 17?

How To Use void t Alias Template in C++ 17

In C++ 17, there is a very useful alias template for metaprogramming that can be used to simplify use of SFINAE. The void_t is a meta-function that is used to map any types ( or types) to type void. In this post, we explain what is void_t, how you can use it in different examples.

What is alias template void_t in C++ 17?

The void_t is an alias template which is introduced with C++17, it is defined in <type_traits> header and it is a metafunction that is used to map any types (or a sequence of any types) to type void. The main purpose of void_t is to make easy writing of type traits. It is used to solve SFINAE (Substitution Failure Is Not An Error) prior to concepts of C++20 standard. SFINAE rule says that If the deduced type or explicitly specified type for the template parameter fails, the specialization is discarded from the overload set instead of causing an error in compilation.

Since C++17, the std::void_t is defined as below.

Now, let’s see how we can use it in template definitions.

How to use alias template void_t to check a typename in a template?

We can use void_t to if a class has a certain typename using at compile time, here we check if it has ‘type’ typename.

here is a full example about this, here we check ‘typeB‘ typename if it has or not.

and the output will be as follows.

How to use alias template void_t to check a member in a template?

We can use void_t to check if there is a member in a template, i.e. we can check if it has ‘varB’ member or not, as given below.

and the output will be as follows.

How to use alias template void_t to check if a class has a function name?

We can use void_t to check if a class has a method. Here is an example that checks if myfB() function exists in that class.

Here is a full example.

and the output will be as follows.

How to use alias template void_t to check whether a class is iterable?

We can use void_t to check whether a class is iterable or not. Here is a simple example.

Here is a full example.

and the output will be as follows.

As you see, interestingly the void_t is very useful to simplify use of SFINAE for metaprogramming.

For more details about this feature in C++17 standard, please see these papers; N3911

How To Use void t Alias Template in C++ 17

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About author

Dr. Yilmaz Yoru has 35+ years of coding with more than 30+ programming languages, mostly C++ on Windows, Android, Mac-OS, iOS, Linux, and some other operating systems. He graduated and received his MSc and PhD degrees from the Department of Mechanical Engineering of Eskisehir Osmangazi University. He is the founder and CEO of ESENJA LLC Company. His interests are Programming, Thermodynamics, Fluid Mechanics, Artificial Intelligence, 2D & 3D Designs, and high-end innovations.
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