A Rust inspired Result<V,E> type for C++17.
template<typename V, typename E> struct Result;
basically a sophisticated wrapper for:
std::variant< ok<V>, error<E> >
auto fn() -> Result<std::ifstream, string>; auto splitLines(std::stringstream&) -> std::vector<string>; int main() { auto f = openFile("hello.txt") .orMap([](auto&&) { return openFile("fallback.txt"); }) // invoked only on error .andMap([](auto&& in) -> Result<std::stringstream, string> { // invoked only if successful std::stringstream out; out << in.rdbuf(); if (out.str().empty()) return error("empty file"s); return ok(move(out)); }) .andMap(&splitLines) // adds no errors .orMap([](auto) -> Result<std::vector<string>, string> { return ok(std::vector<string>{}); }); for (auto &l : f.unwrap()) std::cout << l << '\n'; // we are sure we have a value }
see Result.test.cpp for a working example.
You will need a C++17 compiler.
- GCC 8.x
- Clang 8.x + libc++
- MSVC2017/2019
- googletest is optional to run the tests
- github/oktal/result a similar result type for C++14
- doc.rust-lang/std/result the rust result type documentation
Even though this seems to work, it is only a proof of concept.
- it works
-
std::variantis unusable slow for this kind of use case. -
std::reference_wrapperdoes not allow comparisons.