lbonn / cpp-jinja-enums

C++ reflection with clang, Python and Jinja (demo)

Templates are one the distinguishing feature of C++. Feared and (sometimes?) loved, they are are a great tool for generic programming.

Still, they fall short on other types of meta programming and something which behaves like a Python enum usually involves some error-prone boilerplate. Or you use the C pre-processor and re-invent your own small language, that is how Better Enums solves this.

Meanwhile, Rust programmers have a macro system where they can manipulate the abstract tree of program instead of dumb strings.

A famous example is serde and its derive macro:

#[derive(Serialize, Deserialize)]
struct Point {
    x: i32,
    y: i32,
}

Here, the structure is defined in native rust, in contrast with the usual pre-processor solutions.

But, we also have a lot of overpowered tools like libclang and... Python?

Concept

It uses libclang binding in python to get an AST, extract information from select tree nodes and then feed this to a Jinja template to generate C++ code!

It looks like:

#include "message_generated.h"

enum class [[clang::annotate("from_string")]] Message {
     PING,
     PONG,
};

int main() {
    std::cout << Message::PING << "\n";  // "PING"
}

Advantages:

• reflection!
• type declaration is a normal C++ enum in the code
• writing (jinja) templated code is almost like writing regular C++ code, no dark pre-processor magic required
• easy to integrate in existing project with custom "templates"

Still Annoying:

• we cannot use a plain C++ attribute [[from_string]] as libclang only exposes the attributes it knows about. But we can re-purpose clang::annotate.

See also

• serde-cpp follows the same approach but uses an external parser, cppast, which does not have the limitation around unknown attributes. The generator is in C++ itself, it seems intended to be used as a standalone serialization library, not as a repurposeable building block to include in another project.
• open proposals for reflection in C++: N4856 and P1240R2