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README.md

Experimental Clang support for contract based programming in C++ (P0542R5 TS)

HitCount Compiler Explorer (godbolt) site that can be used for testing (regularly updated with master branch)

Low Level Virtual Machine (LLVM)

This directory and its subdirectories contain source code for LLVM, as it looked on Thu Oct 5 2017. This is the SVN development revision 314972. The rest of this section contains the original README.txt of the LLVM project.

LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt.

Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup.

If you are writing a package for LLVM, see docs/Packaging.rst for our suggestions.

Clang C/C++/ObjectiveC frontend

This repository also contains a clone of the Clang repository (in its SVN revision 314964, dated Thu Oct 5 2017) under the tools/clang directory.

The Clang code included in this repository includes a prototype implementation of the P0542R5 techinical specification, for Support for contract based programming in C++, recently approved by the ISO C++ comitee to be part of C++20. To see a summary of changes since this project was forked:

$ git diff --stat master{^{/SVN},}

As of Mon Jul 9 2018, this is a work in progress, however it is quite complete (see below for a list of missing features).

If you find a bug/unexpected behaviour, please file a bug in the project page. You can also try to fix it and post a patch or pull request.

Build instructions of LLVM + Clang

Provided that you have the required dependencies already installed in your machine, just run the following commands:

$ git clone https://github.com/arcosuc3m/clang-contracts/
$ mkdir -p clang-contracts/build/ && cd clang-contracts/build/
$ cmake -G "Unix Makefiles" -DLLVM_USE_LINKER=gold -DBUILD_SHARED_LIBS=ON -DLLVM_USE_SPLIT_DWARF=ON
  -DLLVM_OPTIMIZED_TABLEGEN=ON ../
$ make -j8

For additional build instruction see the Clang project page and the Getting Started guide.

Command line options

Four new options were added to the Clang driver: -build-level=, -axiom-mode=, -contract-violation-handler= and -fcontinue-after-violation, e.g.

$ clang++ -std=c++14 [-build-level=(off|default|audit)] [-axiom-mode=(off|on)]
  [-contract-violation-handler=my_handler]
  [-fcontinue-after-violation] ...

Because the contract header is contained in the tools/clang/lib/Headers/ directory, you will probably need to add the -idirafter path-to-clang-contracts-repo/tools/clang/lib/Headers/ option.

The -build-level= option allows to specify the build level of the translation (P0542R5 Proposed Wording, Section 10.6.11.12). If unspecified, it defaults to default.

The -axiom-mode= option allows changing the axiom mode of the translation. If set, the compiler assumes any [[expects]], [[ensures]] or [[assert]] tagged as axiom as if __builtin_assume() was there. If unspecified, it defaults to on.

The other two options allow specifying a custom violation handler and the violation continuation mode, as per Section 10.6.11.16 and 10.6.11.18 of the current wording.

MWE with C++ contracts

Remember that contract attribute spelling is quite different from that of CXX11, e.g. [[attribute contract-level-opt identifier-opt: conditional-expression]].

As of May 22 2018, templates are completely supported. Try this MWE that uses templates!

template <typename T>
T func(T a, T b)
[[expects: a > b]] [[ensures r: r > 0]] {
  return a + b;
}

int main(int argc, char *argv[]) {
  return func<int>(argc, 0);
}

Known problems

See the Issues page. If you have found a new (undocumented) issue, please add it to the tracker.

TO DO

As of Jul 9 2018, the implementation is quite complete only missing a few features, namely:

  • P0542R5 Section 2.3, Contracts repetition: currently, expects/ensures attributes are merged into the most recent redeclaration.
  • Late parsing of C++11 attributes is required to accomodate the modification of Section 12.2 [class.mem], p.6. See also issue #3.

Within the class member-specification, the class is regarded as complete within function bodies, default arguments, noexcept-specifiers, and default member initializers, and contract conditions (10.6.11 [dcl.attr.contracts]).

  • Handle inheritance of [[expects]] and [[ensures]] attributes as per the P0542R5 TS.

Also, these features may be added to improve the quality of the implementation (not required by the TS):

  • Static evaluation of contracts (if possible).

Authors

This implementation of the P0542R5 TS was authored and maintained by Javier López-Gómez (jalopezg [at] inf.uc3m.es), with notable contributions from José Cabrero Holgueras (jcabrero [at] inf.uc3m.es).

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