Disclaimer: This project is still experimental.

Overview

Piecewise is a small C++14 library for structuring code via compile-time dependency injection. It favors explicit error handling over exceptions.

This library was born out of a desire to reduce the boilerplate associated with dependency injection in header-only types that have multiple templated members. It attempts to do so without sacrificing flexibility.

Foo::builder(
  // Perfectly forward arguments to the Bar member
  Bar::builder("abc", 42)
, // Perfectly forward arguments to the Baz member
  Baz::builder("xyzzy")
).construct(
  // A success callback. It is only called if both the Bar and the Baz instances
  // were successfully instantiated
  [](auto builder) {
    // We now know we can construct a valid Foo! We use `std::move` to indicate
    // that we are potentially stealing resources
    Foo result = std::move(builder).construct();
    // Use it here!
    std::cout << result.bar().get_string(); // "abc"
    std::cout << result.bar().get_int();    // 42
    std::cout << result.baz().get_string(); // "xyzzy"
  }
, // This is called if initialization fails at *any* point
  [](auto error) {
    std::cerr << "Validation failed: " << decltype(error)::description << std::endl;
  }
);

// Or save it for later as a std::variant
auto saved = Foo::variant<Error1, Error2>(
  Bar::builder("abc", 42)
, Baz::builder("xyzzy")
);

// Or use a std::optional
auto maybe_foo = Foo::optional(
  Bar::builder("abc", 42)
, Baz::builder("xyzzy")
).construct(
  [](auto error) {
    std::cerr << "Validation failed: " << decltype(error)::description << std::endl;
  }
);
if (maybe_foo) {
  maybe_foo->do_foo_things();
}

Test Matrix

This table attempts to document the configurations that are currently tested by CI. Assume that compiler versions are the latest offered in the listed environment. Untested configurations may still work.

Environment	Compiler	Standard Library	Standard	Build Type	Sanitizers
Arch Travis	GCC	libstdc++	C++14	Release	None
Arch Travis	GCC	libstdc++	C++14	Debug	Address/Undefined
Arch Travis	Clang	libstdc++	C++14	Release	None
Arch Travis	Clang	libstdc++	C++14	Debug	Address/Undefined
Arch Travis	Clang	libc++	C++14	Release	None
Arch Travis	Clang	libc++	C++14	Debug	Address/Undefined
Arch Travis	GCC	libstdc++	C++17	Release	None
Arch Travis	GCC	libstdc++	C++17	Debug	Address/Undefined
~~Arch Travis~~	~~Clang~~	~~libstdc++~~	~~C++17~~	~~Release~~	~~None~~
~~Arch Travis~~	~~Clang~~	~~libstdc++~~	~~C++17~~	~~Debug~~	~~Address/Undefined~~
Arch Travis	Clang	libc++	C++17	Release	None
Arch Travis	Clang	libc++	C++17	Debug	Address/Undefined
Travis OS X	Apple Clang	libc++	C++14	Release	None
AppVeyor Windows	MSVC	MS	C++14	Release	None

Note that clang C++17 builds with libstdc++ are disabled due to this bug

Build

Piecewise is header only, so you should be able to just set an appropriate include path, but if you're a meson user you can use piecewise as a subproject (currently untested). Using meson has the added benefits of both enforcing a single piecewise instance within your meson-enabled dependencies and allowing you to run piecewise's tests as part of your build.

If you'd like to build the tests manually:

Install meson with pip or your preferred package manager
Install the ninja build system
meson build generates a ninja build inside the build directory (use the CXX env var to control the compiler)
meson configure -C build will list all the knobs you can tweak
meson test -C build will build and run the test suite

Builders

Builders are piecewise's fundamental construct. They're essentially callbacks paired with references to arguments. When the construct member function is called on a Builder instance, the captured references are perfectly forwarded to the callback, along with any arguments to construct.

// This is a pre-factory builder. Its factory callable has not yet run.
Foo::builder(arg1, arg2)
// This construct call invokes the builder's factory callable
.construct(
  // This callback receives a post-factory builder. Its factory callable has
  // succeeded.
  [](auto builder) {
    // ...
  }
, // This callback gets called if the factory callable fails
  [](auto error) {
    // ...
  }
);

Since builders are entirely composed of references, you should treat them as such!

void does_not_compile() {}
  auto builder = Foo::builder("the 2 is temporary", 2)
  builder.construct([](auto) {}, [](auto) {}); // ERROR!
}

void compiles() {
  auto builder = Foo::builder("the 2 is temporary", 2)
  std::move(builder).construct([](auto) {}, [](auto) {}); // Bad!
}

void correct() {
  Foo::builder("the 2 is temporary", 2).construct([](auto) {}, [](auto) {});
}

Builders are designed so that it is impossible to construct a Foo instance without it being valid. But it is possible to return values from the success and error callbacks, as long as their types match.

int res = Foo::builder(42).construct(
  [](auto) -> int { return 1; }
, [](auto) -> int { return 2; }
);

Here's an example of a retry pattern:

bool connected = false;
while (!connected) {
  Connection::builder("https://github.com").construct(
    [&connected] (auto) { connected = true; }
  , [] (auto) { std::cerr << "Connection failed!" << std::endl; }
  );
}

Helpers

Piecewise includes a Helpers mix-in to help you design a piecewise-enabled type. If you want to keep your implementation details private, it requires friendship with your derived class:

using mp = mz::piecewise;
class Foo final : private mp::Helpers<Foo> {
  // Give the Helpers mixin access to Foo's factory function
  friend mp::Helpers<Foo>;
public:
  // Use mp::Helpers<Foo>::Private here to hide the constructor
  Foo(Private, ...);
};

The above boilerplate will enable Foo::builder, Foo::variant, and Foo::optional where appropriate.

Factory Function

In piecewise, the heavy lifting for construction moves to the static Foo::factory function. It accepts two callbacks, one for success, and one for failure, along with the other arguments passed to Foo::builder.

private:
  static constexpr auto factory() {
    return [](
      auto constructor
    , auto&& on_success, auto&& on_fail
    , std::string a_string, int an_int
    ) {
      // Validate arguments
      if (a_string.empty()) return on_fail(A::StringEmptyError{});
      if (an_int < 0) return on_fail(A::IntNegativeError{});

      // Now the success callback gets a builder which creates a *valid*
      // instance of `Foo`.
      return on_success(
        // Create a builder
        mp::builder(
          // This is the actual creation callback. It just calls Foo's
          // constructor in the normal way
          constructor
        , // The arguments to be passed to Foo's constructor
          std::move(a_string), an_int
        )
      );
    };
  }

Multifail

Piecewise also contains a helper function called multifail. This important function handles invoking pre-factory builders in succession until it either successfully collects all required post-factory builders or the failure callback has been called. If successful, it invokes the requested constructor. Note that due to an implementation detail it will pass regular arguments to the constructor before it passes builders. Use this helper if your type contains nested types that are piecewise-enabled.

  static constexpr auto factory() {
    return [](
      auto constructor
    , auto&& on_success, auto&& on_fail
    , auto builder1, auto builder2
    , int arg1, std::string arg2
    ) {
      // Error handling specific to this type would happen here
      
      // Handle creation of nested types
      return mp::multifail(
        constructor
      , on_success
      , on_fail
      , mp::builders(
          std::move(builder1), std::move(builder2)
        )
      , mp::arguments(arg1, std::move(arg2))
      );
    };
  }

Constructor

The private constructor is the final step for constructing an object of type Foo, and it is only called if Foo's factory function has succeeded. Notice that variadic arguments are not supported at this phase of construction. Again, notice that regular arguments are passed before builders.

  template <typename Builder1, typename Builder2>
  Foo(int arg1, std::string arg2, Builder1 builder1, Builder2 builder2)
    : nested_type1{std::move(sbuilder1).construct()}
    , nested_type2{std::move(builder2).construct()}
    , an_int{arg1}
    , a_string{std::move(arg2)}
  {}

Non-Piecewise Types

A compatibility wrapper is provided for passing arguments to nested types that must coexist with piecewise types. Note that this wrapper only works for types with public constructors.

struct C {
  int int_a;
  int int_b;
};

Aggregate<A, B>::builder(
  A::builder("abc", -42)
, B::builder(123)
, mp::wrapper<C>(5, 6) // Create a builder for type C
, 3
)
.construct(
// ...
);

Non-piecewise types without a public constructor can still be manually wrapped by defining a factory and passing it to mp::builder:

// Any callable will do
struct CFactory {
  template <typename OnSuccess, typename OnFail>
  auto operator()(OnSuccess&& on_success, OnFail&&, int arg1, int arg2) const {
    return on_success(
      mp::builder(
        [](int arg1_, int arg2_) {
          return C::non_piecewise_factory(arg1_, arg2_);
        }
      , arg1, arg2
      )
    );
  }
};

Aggregate<A, B>::builder(
  A::builder("abc", -42)
, B::builder(123)
, mp::builder(CFactory{}, 5, 6) // Create a builder for type C
, 3
)
.construct(
// ...
);

Example

See here for a more complete example.

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