This is a proof of concept on how a path-based approach can make it possible to build a C++ project with modules, including consuming C++ modules external to the build system.

This is meant as a side-car to the Distributing C++ Module Libraries paper.

Building out-of-source is important, because the project will need to create bmi files for whatever modules that were not shipped with a compatible BMI file.

mkdir build && make -C build -f $(pwd)/Makefile

In order for this to work correctly, we make use of the VPATH feature of GNU Make, and we make it coincide with the module search path, meaning that Make itself is capable of resolving the dependency information and consume whatever bmi is already available, but produce whatever bmi is necessary.

In this sample project, the module search path is comprised of the root of the current directory (where bmi files are created), the root of the repo, where the modules meant as internal to this project, and the external-modules directory, meant to represent other modules available in the system.

VPATH is therefore set to the two directories where the local source code is going to be found as well as the directories where external modules may be found. Meanwhile the module path is set to include the current directory ahead of those, as that's where the local builds are going to be placed.

This allows an important optimization for systems that do use the same compiler across the board to ship pre-parsed module interfaces. The name of those files is made as unique as the compiler needs it to be, and the string is discovered by calling the compiler itself in this PoC, this is the BMI_UUID variable in the Makefile.

This compatibility id needs to be communicated to the dependency mapping, such that we can correctly generate the build rules.

This utility is a proof-of-concept implementation of a tool that knows how to parse your local non-module code and then identify which modules it depends on, and then uses the module search path to identify all the information necessary to parse any internal or external modules.

This is implemented to demonstrate that a build system would be able to configure any required build steps from modules that follow this convention in a straight-forward way that can be easily replicated by other tools, as long as the module search path is provided.

This utility is a proof-of-concept implementation of a tool that translates the information generated by c++-module-config into a make-compatible dependency file that can be included by the makefile.

The simple inclusion of deps.mk, and the rules to generate it are sufficient to fully configure the build in this given directory.

// ----------------------------------------------------------------------------
// Copyright 2021 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------