ecpz - Evaluate C++ using Python and Zig

Do you need to evaluate some simple C++ code from inside your application, and it should work cross-platform (Linux, Mac and Windows) without causing a headache?

You've seen it all, quine-relay stopped being amusing years ago, and you feel the itch for something new?

No problem, easy-peasy with ecpz!

This little package combines the ubiquity of Python and ingeniuity of the Zig toolchain to give you the ability to compile C++ snippets without pain.

If you have a non-trivial project consisting of more than one source file, you should probably configure and build it properly using e.g. CMake.

If you need an interactive C++ code execution environment (i.e. a REPL), check out cling.

But if for some reason you need to produce and execute some ad-hoc throw-away C++ snippets as a part of your workflow, ecpz might be just what you need!

Usage

Install ecpz using pip or uv and check ecpz --help for all options.

In the following, the features of ecpz are illustrated by some examples.

ecpz run

Compile and run a single source file provided either as argument or via standard input.

For example, create hello.cpp:

#include <print>

int main() {
  std::println("Hello world!");
}

And run it:

$ cat hello.cpp | ecpz --clang-arg -std=c++23 run
Hello world!

ecpz print

Evaluates some expressions and pretty-print them using std::print(ln) (note that this automatically implies -std=c++23).

For example, create a header prelude.hpp:

#include <numbers>
#include <type_traits>

inline double tau() {
  return 2 * std::numbers::pi;
}

And now run:

$ ecpz --prelude prelude.hpp print "{:.3f} {} {} {}" "tau()" "[](){ int i=0; ++i; return i; }()" "std::is_same_v<int, double>" "std::is_same_v<int, int32_t>"
6.283 1 false true

You can set the ECPZ_PRELUDE environment variable to the path of your custom header to make it always included by default. Note that as usual, CLI arguments override equivalent environment variables.

Creating a Quine

Usually, using I/O to let code open itself directly is forbidden and considered bad sportsmanship. Everybody agrees that the following things are clear cases of cheating:

• reading a hard-coded string filename
• using the argv[0] trick to reflect back the name of the current file/executable
• reading the file using some compile- or runtime reflection capabilities of the programming language
• adding and using a special quine command in a custom programming language or environment

We will do none of these things. Create some source file FILE with the following content:

#include "ecpz/subprocess.hpp"
int main(int i, char** a) {
    using b = std::istreambuf_iterator<char>; using c = std::string;
    auto d = [](auto e){ std::ifstream f(e, std::ios::binary); return c(b(f), b()); };
    std::vector<c> e(i+3, a[i-1]); e[0]="ecpz"; e[1]="run";
    auto f = i%2 == 0 ? subprocess::run(e).output : d(a[i-1]);
    subprocess::set_bin(); std::cout<<f;
}

Now run it with ecpz run FILE FILE and convince yourself that we do not cheat in such despicable and reprehensible ways as the ones listed above!

Limitations

The following issues are fixable, i.e. only a matter of more effort and time:

• currently only synchronous execution is supported, no I/O buffering or streaming
• currently it is not possible to pipe input data into ecpz run to pass it as stdin

Acknowledgements

Thanks to sheredom for creating subprocess.h, the only true header-only C(++) subprocess library I could find!