C++17 Result
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> >Usage synopsis
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.
Requirements
You will need a C++17 compiler.
- GCC 8.x
- Clang 8.x + libc++
- MSVC2017/2019
- googletest is optional to run the tests
Links
- github/oktal/result a similar result type for C++14
- doc.rust-lang/std/result the rust result type documentation
Status
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.