C++ Custom user defined attributes

[baderouaich]

Channel
C++Weekly

Topics

• Custom User-Defined Attributes

Since the C++ standard does not yet allow us to define our own attributes like [[metadata]], what are the alternatives to achieve this? Is it even possible?

Here's a use case where I wished C++ could have custom attributes. Imagine you want to create a SQL ORM library object User and mark its class members with specific metadata, such as User::id as the primary key and User::email as unique .. etc, using custom attributes such as:

struct User {
   [[primary_key]]              int id;
   [[unique]]                   std::string email;
   [[length(24)]]               std::string name;
   [[not_null]]                 std::string gender;
   [[check(between(18, 80))]]   int age;
};

Then, we could check if a class member has an attribute with something like this:

if constexpr (has_attribute(&User::id, "primary_key")) {
    ...
}
if constexpr (has_attribute(&User::name, "length")) {
    constexpr auto name_length = get_attribute_value(&User::name, "length");
    ...
}

After little research, I found that this can be achieved by modifying Clang's source code to handle custom attributes, but it's quite a task, really! Another approach is to use cppast to parse the C++ files with the attributes and generate additional C++ files before building.

I think this topic would be really fun to cover in an episode. The ability to create your own custom attributes and handle them at compile time is interesting.

Some useful resources on c++ custom attributes:

• Fun with (User-Defined) Attributes - Jonathan Müller - Meeting C++ 2017: A great explanation about the limitations and possible approaches using cppast.
• Supporting User-Defined Attributes Paper