C'est la vie

The C'est project aims to provide a non-standard version of the C++ standard library with enhanced support for compile time evaluation (constexpr) for use in research.

GCC and Clang support all 7 C++20 proposals which are part of "Relaxations of constexpr restrictions": P1064 P1002 P1327 P1330 P1331 P1668 P0784. Of these, the last, P0784 (More constexpr containers) includes discussion on "constexpr new-expressions".

C'est supports both Clang and GCC. To pass all the tests provided, C++23 is used (either via flag -std=c++23 or -std=c++2b). A recent version of the GCC compiler is also required (e.g. GCC 12; sudo apt-get install g++-12).

The C'est library is not standalone, and requires a recent version of GCC's C++ standard library: libstdc++. C'est also includes code from libstdc++; as seen in basic_ios.hpp, ios_base.hpp, istream.hpp, ostream.hpp, sstream.hpp, streambuf.hpp and elsewhere.

Header files in C'est are named after those in the standard library; where you would normally have #include <vector>, you should use #include "cest/vector.hpp". Classes and objects of the C'est library reside within the cest namespace; and consequently where you would have had std::deque, you should use cest::deque instead when using C'est.

In time, it is likely that the C++ standard library will continue to include more and more constexpr support. Most containers will naturally accommodate this direction. IO and file access will raise more questions. For now, C'est does allow some IO commands to compile within constant expressions (e.g. cest::cout << "Hello World\n"). This is primarily to support the compile-time evaluation of existing code bases.

The C'est library has incomplete support for the following class templates: vector, string, forward_list, list, set, map, queue, deque, unique_ptr, shared_ptr and function. Given a constexpr container, most function templates from algorithm and numeric can now also be used within a constant expression.

The code below provides a basic demonstration of some functionality. Executing the resulting program will output Hello World 5:

#include "cest/algorithm.hpp"
#include "cest/deque.hpp"
#include "cest/functional.hpp"
#include "cest/iostream.hpp"
#include "cest/numeric.hpp"
#include "cest/set.hpp"
#include "cest/string.hpp"
#include "cest/vector.hpp"

// Copyright (c) 2020-2022 Paul Keir, University of the West of Scotland.

// g++-12  -std=c++20 -I include example.cpp
//   ... or:
// clang++ -std=c++20 -I include example.cpp

constexpr bool doit() {
  using namespace cest;

  string str = "Hello";
  vector<int> v{1, 2, 3};
  deque<int> dq{2, 3, 4};
  set<int> s;

  set_intersection(dq.begin(), dq.end(), v.begin(), v.end(),
                   inserter(s, s.end()));
  function<int()> f = [&]() { return accumulate(s.begin(), s.end(), 0); };
  auto x = f();
  cout << str << " World " << x << endl;

  return 5 == x;
}

int main(int argc, char *argv[]) {
  static_assert(doit());
  return doit() ? 0 : 1;
}

The code above comes from the example.cpp file; found in the same directory as this readme. The command required to build the code above is:

$CXX -std=c++20 -I include example.cpp

Compile-time and runtime tests are included within the tests directory. To ensure runtime and compile-time parity, each static_assert therein is accompanied by a runtime assert. To build and run the tests use the following commands (the Debug CMake flag ensures the runtime assert calls remain active). For the runtime component, expected console output is listed in cest_tests.cpp.

cd tests
mkdir build
cd build
cmake -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Debug ..
make
make test

Projects using C'est

• ctcheckmm: a compile-time Metamath proof database verifier
• poacher: a constexpr Brainfuck compiler, with proper AST and code generation (see examples/brainfuck)