Adding expect<T> - a value-or-error implementation

1 files changed, 447 insertions, 0 deletions

diff --git a/src/common/expect.h b/src/common/expect.h
new file mode 100644
index 000000000..326242502
--- /dev/null
+++ b/src/common/expect.h
@@ -0,0 +1,447 @@
+// Copyright (c) 2018, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <cassert>
+#include <system_error>
+#include <type_traits>
+#include <utility>
+
+#include "common/error.h"
+
+//! If precondition fails, return `::error::kInvalidArgument` in current scope.
+#define MONERO_PRECOND(...)                            \
+    do                                                 \
+    {                                                  \
+        if (!( __VA_ARGS__ ))                          \
+            return {::common_error::kInvalidArgument}; \
+    } while (0)
+
+//! Check `expect<void>` and return errors in current scope.
+#define MONERO_CHECK(...)                           \
+    do                                              \
+    {                                               \
+        const ::expect<void> result = __VA_ARGS__ ; \
+        if (!result)                                \
+            return result.error();                  \
+    } while (0)
+
+/*! Get `T` from `expect<T>` by `std::move` as-if by function call.
+    `expect<void>` returns nothing.
+
+    \throw std::system_error with `expect<T>::error()`, filename and line
+        number when `expect<T>::has_error() == true`.*/
+#define MONERO_UNWRAP(...)                                        \
+    ::detail::expect::unwrap( __VA_ARGS__ , nullptr, __FILE__ , __LINE__ )
+
+/* \throw std::system_error with `code` and `msg` as part of the details. The
+filename and line number will automatically be injected into the explanation
+string. `code` can be any enum convertible to `std::error_code`. */
+#define MONERO_THROW(code, msg) \
+    ::detail::expect::throw_( code , msg , __FILE__ , __LINE__ )
+
+
+template<typename> class expect;
+
+namespace detail
+{
+    // Shortens the characters in the places that `enable_if` is used below.
+    template<bool C>
+    using enable_if = typename std::enable_if<C>::type;
+ 
+    struct expect
+    {
+        //! \throw std::system_error with `ec`, optional `msg` and/or optional `file` + `line`.
+        static void throw_(std::error_code ec, const char* msg, const char* file, unsigned line);
+
+        //! If `result.has_error()` call `throw_`. Otherwise, \return `*result` by move.
+        template<typename T>
+        static T unwrap(::expect<T>&& result, const char* error_msg, const char* file, unsigned line)
+        {
+            if (!result)
+                throw_(result.error(), error_msg, file, line);
+            return std::move(*result);
+        }
+
+        //! If `result.has_error()` call `throw_`.
+        static void unwrap(::expect<void>&& result, const char* error_msg, const char* file, unsigned line);
+    };
+}
+
+/*!
+    `expect<T>` is a value or error implementation, similar to Rust std::result
+    or various C++ proposals (boost::expected, boost::outcome). This
+    implementation currently has a strict error type, `std::error_code`, and a
+    templated value type `T`. `expect<T>` is implicitly convertible from `T`
+    or `std::error_code`, and one `expect<T>` object type is implicitly
+    convertible to another `expect<U>` object iff the destination value type
+    can be implicitly constructed from the source value type (i.e.
+    `struct U { ... U(T src) { ...} ... };`).
+
+    `operator==` and `operator!=` are the only comparison operators provided;
+    comparison between different value types is allowed provided the two values
+    types have a `operator==` defined between them (i.e.
+    `assert(expect<int>{100} == expect<short>{100});`). Comparisons can also be
+    done against `std::error_code` objects or error code enums directly (i.e.
+    `assert(expect<int>{make_error_code(common_error::kInvalidArgument)} == error::kInvalidArgument)`).
+    Comparison of default constructed `std::error_code` will always fail.
+    "Generic" comparisons can be done with `std::error_condition` via the `matches`
+    method only (i.e.
+    `assert(expect<int>{make_error_code{common_error::kInvalidErrorCode}.matches(std::errc::invalid_argument))`),
+    `operator==` and `operator!=` will not work with `std::errc` or
+    `std::error_condition`. A comparison with `matches` is more expensive
+    because an equivalency between error categories is computed, but is
+    recommended when an error can be one of several categories (this is going
+    to be the case in nearly every situation when calling a function from
+    another C++ struct/class).
+
+    `expect<void>` is a special case with no stored value. It is used by
+    functions that can fail, but otherwise would return `void`. It is useful
+    for consistency; all macros, standalone functions, and comparison operators
+    work with `expect<void>`.
+
+    \note See `src/common/error.h` for creating a custom error enum.
+ */
+template<typename T>
+class expect
+{
+    static_assert(std::is_nothrow_destructible<T>(), "T must have a nothrow destructor");
+
+    template<typename U>
+    static constexpr bool is_convertible() noexcept
+    {
+        return std::is_constructible<T, U>() &&
+            std::is_convertible<U, T>();
+    }
+
+    // MEMBERS
+    std::error_code code_;
+    typename std::aligned_storage<sizeof(T), alignof(T)>::type storage_;
+    // MEMBERS
+
+    T& get() noexcept
+    {
+        assert(has_value());
+        return *reinterpret_cast<T*>(std::addressof(storage_));
+    }
+
+    T const& get() const noexcept
+    {
+        assert(has_value());
+        return *reinterpret_cast<T const*>(std::addressof(storage_));
+    }
+
+    template<typename U>
+    void store(U&& value) noexcept(std::is_nothrow_constructible<T, U>())
+    {
+        new (std::addressof(storage_)) T{std::forward<U>(value)};
+        code_ = std::error_code{};
+    }
+
+    void maybe_throw() const
+    {
+        if (has_error())
+            ::detail::expect::throw_(error(), nullptr, nullptr, 0);
+    }
+
+public:
+    using value_type = T;
+    using error_type = std::error_code;
+
+    expect() = delete;
+
+    /*! Store an error, `code`, in the `expect` object. If `code` creates a
+    `std::error_code` object whose `.value() == 0`, then `error()` will be set
+    to `::common_error::kInvalidErrorCode`. */
+    expect(std::error_code const& code) noexcept
+      : code_(code), storage_()
+    {
+        if (!has_error())
+            code_ = ::common_error::kInvalidErrorCode;
+    }
+
+    //! Store a value, `val`, in the `expect` object.
+    expect(T val) noexcept(std::is_nothrow_move_constructible<T>())
+      : code_(), storage_()
+    {
+        store(std::move(val));
+    }
+
+    expect(expect const& src) noexcept(std::is_nothrow_copy_constructible<T>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(src.get());
+    }
+
+    //! Copy conversion from `U` to `T`.
+    template<typename U, typename = detail::enable_if<is_convertible<U const&>()>>
+    expect(expect<U> const& src) noexcept(std::is_nothrow_constructible<T, U const&>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(*src);
+    }
+
+    expect(expect&& src) noexcept(std::is_nothrow_move_constructible<T>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(std::move(src.get()));
+    }
+
+    //! Move conversion from `U` to `T`.
+    template<typename U, typename = detail::enable_if<is_convertible<U>()>>
+    expect(expect<U>&& src) noexcept(std::is_nothrow_constructible<T, U>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(std::move(*src));
+    }
+
+    ~expect() noexcept
+    {
+        if (has_value())
+            get().~T();
+    }
+
+    expect& operator=(expect const& src) noexcept(std::is_nothrow_copy_constructible<T>() && std::is_nothrow_copy_assignable<T>())
+    {
+        if (this != std::addressof(src))
+        {
+            if (has_value() && src.has_value())
+                get() = src.get();
+            else if (has_value())
+                get().~T();
+            else if (src.has_value())
+                store(src.get());
+            code_ = src.error();
+        }
+        return *this;
+    }
+
+    /*! Move `src` into `this`. If `src.has_value() && addressof(src) != this`
+    then `src.value() will be in a "moved from state". */
+    expect& operator=(expect&& src) noexcept(std::is_nothrow_move_constructible<T>() && std::is_nothrow_move_assignable<T>())
+    {
+        if (this != std::addressof(src))
+        {
+            if (has_value() && src.has_value())
+                get() = std::move(src.get());
+            else if (has_value())
+                get().~T();
+            else if (src.has_value())
+                store(std::move(src.get()));
+            code_ = src.error();
+        }
+        return *this;
+    }
+
+    //! \return True if `this` is storing a value instead of an error.
+    explicit operator bool() const noexcept { return has_value(); }
+
+    //! \return True if `this` is storing an error instead of a value.
+    bool has_error() const noexcept { return bool(code_); }
+
+    //! \return True if `this` is storing a value instead of an error.
+    bool has_value() const noexcept { return !has_error(); }
+
+    //! \return Error - always safe to call. Empty when `!has_error()`.
+    std::error_code error() const noexcept { return code_; }
+
+    //! \return Value if `has_value()` otherwise \throw `std::system_error{error()}`.
+    T& value() &
+    {
+        maybe_throw();
+        return get();
+    }
+
+    //! \return Value if `has_value()` otherwise \throw `std::system_error{error()}`.
+    T const& value() const &
+    {
+        maybe_throw();
+        return get();
+    }
+
+    /*! Same as other overloads, but expressions such as `foo(bar().value())`
+    will automatically perform moves with no copies. */
+    T&& value() &&
+    {
+        maybe_throw();
+        return std::move(get());
+    }
+
+    //! \return Value, \pre `has_value()`.
+    T* operator->() noexcept { return std::addressof(get()); }
+    //! \return Value, \pre `has_value()`.
+    T const* operator->() const noexcept { return std::addressof(get()); }
+    //! \return Value, \pre `has_value()`.
+    T& operator*() noexcept { return get(); }
+    //! \return Value, \pre `has_value()`.
+    T const& operator*() const noexcept { return get(); }
+
+    /*! 
+        \note This function is `noexcept` when `U == T` is `noexcept`.
+        \return True if `has_value() == rhs.has_value()` and if values or errors are equal.
+    */
+    template<typename U>
+    bool equal(expect<U> const& rhs) const noexcept(noexcept(*std::declval<expect<T>>() == *rhs))
+    {
+        return has_value() && rhs.has_value() ?
+            get() == *rhs : error() == rhs.error();
+    }
+
+    //! \return False if `has_value()`, otherwise `error() == rhs`.
+    bool equal(std::error_code const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+
+    /*!
+        \note This function is `noexcept` when `U == T` is `noexcept`.
+        \return False if `has_error()`, otherwise `value() == rhs`.
+    */
+    template<typename U, typename = detail::enable_if<!std::is_constructible<std::error_code, U>::value>>
+    bool equal(U const& rhs) const noexcept(noexcept(*std::declval<expect<T>>() == rhs))
+    {
+        return has_value() && get() == rhs;
+    }
+
+    //! \return False if `has_value()`, otherwise `error() == rhs`.
+    bool matches(std::error_condition const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+};
+
+template<>
+class expect<void>
+{
+    std::error_code code_;
+
+public:
+    using value_type = void;
+    using error_type = std::error_code;
+
+    //! Create a successful object.
+    expect() = default;
+
+    expect(std::error_code const& code) noexcept
+      : code_(code)
+    {
+        if (!has_error())
+            code_ = ::common_error::kInvalidErrorCode;
+    }
+
+    expect(expect const&) = default;
+    ~expect() = default;
+    expect& operator=(expect const&) = default;
+
+    //! \return True if `this` is storing a value instead of an error.
+    explicit operator bool() const noexcept { return !has_error(); }
+
+    //! \return True if `this` is storing an error instead of a value.
+    bool has_error() const noexcept { return bool(code_); }
+
+    //! \return Error - alway
+    std::error_code error() const noexcept { return code_; }
+
+    //! \return `error() == rhs.error()`.
+    bool equal(expect const& rhs) const noexcept
+    {
+        return error() == rhs.error();
+    }
+
+    //! \return `has_error() && error() == rhs`.
+    bool equal(std::error_code const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+
+    //! \return False if `has_value()`, otherwise `error() == rhs`.
+    bool matches(std::error_condition const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+};
+
+//! \return An `expect<void>` object with `!has_error()`.
+inline expect<void> success() noexcept { return expect<void>{}; }
+
+template<typename T, typename U>
+inline
+bool operator==(expect<T> const& lhs, expect<U> const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator==(expect<T> const& lhs, U const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator==(T const& lhs, expect<U> const& rhs) noexcept(noexcept(rhs.equal(lhs)))
+{
+    return rhs.equal(lhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator!=(expect<T> const& lhs, expect<U> const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return !lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator!=(expect<T> const& lhs, U const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return !lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator!=(T const& lhs, expect<U> const& rhs) noexcept(noexcept(rhs.equal(lhs)))
+{
+    return !rhs.equal(lhs);
+}
+
+namespace detail
+{
+    inline void expect::unwrap(::expect<void>&& result, const char* error_msg, const char* file, unsigned line)
+    {
+        if (!result)
+            throw_(result.error(), error_msg, file, line);
+    }
+}
+