fu2::function an improved drop-in replacement to std::function

Provides improved implementations of std::function:

• copyable fu2::function
• move-only fu2::unique_function (capable of holding move only types)
• non-owning fu2::function_view (capable of referencing callables in a non owning way)

that provide many benefits and improvements over std::function:

• [x] const, volatile, reference and noexcept correct (qualifiers are part of the operator() signature)
• [x] convertible to and from std::function as well as other callable types
• [x] adaptable through fu2::function_base (internal capacity, copyable and exception guarantees)
• [x] overloadable with an arbitrary count of signatures (fu2::function<bool(int), bool(float)>)
• [x] full allocator support in contrast to std::function, which doesn't provide support anymore
• [x] covered by many unit tests and continuous integration services (GCC, Clang and MSVC)
• [x] header only, just copy and include function.hpp in your project
• [x] permissively licensed under the boost license

Table of Contents

• Documentation
  • How to use
  • Constructing a function
  • Non copyable unique functions
  • Convertibility of functions
  • Adapt function2
• Performance and optimization
  • Small functor optimization
  • Compiler optimization
  • std::function vs fu2::function
• Coverage and runtime checks
• Compatibility
• License
• Similar implementations

Documentation

How to use

function2 is implemented in one header (function.hpp), no compilation is required. Just copy the function.hpp header in your project and include it to start. It's recommended to import the library as git submodule using CMake:

# Shell:
git submodule add https://github.com/Naios/function2.git

# CMake file:
add_subdirectory(function2)
# function2 provides an interface target which makes it's
# headers available to all projects using function2
target_link_libraries(my_project function2)

Use fu2::function as a wrapper for copyable function wrappers and fu2::unique_function for move only types. The standard implementation std::function and fu2::function are convertible to each other, see the chapter convertibility of functions for details.

A function wrapper is declared as following:

fu2::function<void(int, float) const>
// Return type ~^   ^     ^     ^
// Parameters  ~~~~~|~~~~~|     ^
// Qualifier ~~~~~~~~~~~~~~~~~~~|

• Return type: The return type of the function to wrap.
• Arguments: The argument types of the function to wrap. Any argument types are allowed.
• Qualifiers: There are several qualifiers allowed:
  • no qualifier provides ReturnType operator() (Args...)
    • Can be assigned from const and no const objects (mutable lambdas for example).
  • const provides ReturnType operator() (Args...) const
    • Requires that the assigned functor is const callable (won't work with mutable lambdas),
  • volatile provides ReturnType operator() (Args...) volatile
    • Can only be assigned from volatile qualified functors.
  • const volatile provides ReturnType operator() (Args...) const volatile
    • Same as const and volatile together.
  • r-value (one-shot) functions ReturnType operator() (Args...) &&
    • one-shot functions which are invalidated after the first call (can be mixed with const, volatile and noexcept). Can only wrap callable objects which call operator is also qualified as && (r-value callable). Normal (C) functions are considered to be r-value callable by default.
  • noexcept functions ReturnType operator() (Args...) noexcept
    • such functions are guaranteed not to throw an exception (can be mixed with const, volatile and &&). Can only wrap functions or callable objects which call operator is also qualified as noexcept. Requires enabled C++17 compilation to work (support is detected automatically). Empty function calls to such a wrapped function will lead to a call to std::abort regardless the wrapper is configured to support exceptions or not (see adapt function2).
• Multiple overloads: The library is capable of providing multiple overloads:

  fu2::function<int(std::vector<int> const&),
                int(std::set<int> const&) const> fn = [] (auto const& container) {
                  return container.size());
                };
  

Constructing a function

fu2::function and fu2::unique_function (non copyable) are easy to use:

fu2::function<void() const> fun = [] {
  // ...
};

// fun provides void operator()() const now
fun();

Non copyable unique functions

fu2::unique_function also works with non copyable functors/ lambdas.

fu2::unique_function<bool() const> fun = [ptr = std::make_unique<bool>(true)] {
  return *ptr;
};

// unique functions are move only
fu2::unique_function<bool() const> otherfun = std::move(fun):

otherfun();

Non owning functions

A fu2::function_view can be used to create a non owning view on a persistent object. Note that the view is only valid as long as the object lives.

auto callable = [ptr = std::make_unique<bool>(true)] {
  return *ptr;
};

fu2::function_view<bool() const> view(callable);

Convertibility of functions

fu2::function, fu2::unique_function and std::function are convertible to each other when:

• The return type and parameter type match.
• The functions are both volatile or not.
• The functions are const correct:
  • noconst = const
  • const = const
  • noconst = noconst
• The functions are copyable correct when:
  • unique = unique
  • unique = copyable
  • copyable = copyable
• The functions are reference correct when:
  • lvalue = lvalue
  • lvalue = rvalue
  • rvalue = rvalue
• The functions are noexcept correct when:
  • callable = callable
  • callable = noexcept callable
  • noexcept callable = noexcept callable

fu2::function<void()> fun = []{};
// OK
std::function<void()> std_fun = fun;
// OK
fu2::unique_function<void()> un_fun = fun;

// Error (non copyable -> copyable)
fun = un_fun;
// Error (non copyable -> copyable)
fun = un_fun;

Adapt function2

function2 is adaptable through fu2::function_base which allows you to set:

• IsOwning: defines whether the function owns its contained object
• Copyable: defines if the function is copyable or not.
• Capacity: defines the internal capacity used for sfo optimization:

struct my_capacity {
  static constexpr std::size_t capacity = sizeof(my_type);
  static constexpr std::size_t alignment = alignof(my_type);
};

• IsThrowing defines if empty function calls throw an fu2::bad_function_call exception, otherwise std::abort is called.
• HasStrongExceptGuarantee defines whether the strong exception guarantees shall be met.
• Signatures: defines the signatures of the function.

The following code defines an owning function with a variadic signature which is copyable and sfo optimization is disabled:

template<typename Signature>
using my_function = fu2::function_base<true, true, fu2::capacity_none, true, false, Signature>;

The following code defines a non copyable function which just takes 1 argument, and has a huge capacity for internal sfo optimization. Also it must be called as r-value.

template<typename Arg>
using my_consumer = fu2::function_base<true, false, fu2::capacity_fixed<100U>,
                                       true, false, void(Arg)&&>;

// Example
my_consumer<int, float> consumer = [](int, float) { }
std::move(consumer)(44, 1.7363f);

Performance and optimization

Small functor optimization

function2 uses small functor optimization like the most common std::function implementations which means it allocates a small internal capacity to evade heap allocation for small functors.

Smart heap allocation moves the inplace allocated functor automatically to the heap to speed up moving between objects.

It's possible to disable small functor optimization through setting the internal capacity to 0.

Coverage and runtime checks

Function2 is checked with unit tests and valgrind (for memory leaks), where the unit tests provide coverage for all possible template parameter assignments.

Compatibility

Tested with:

• Visual Studio 2017+ Update 3
• Clang 3.8+
• GCC 5.4+

Every compiler with modern C++14 support should work. function2 only depends on the standard library.

License

function2 is licensed under the very permissive Boost 1.0 License.

Similar implementations

There are similar implementations of a function wrapper:

• pmed/fixed_size_function
• stdex::function - A multi-signature function implementation.
• multifunction - Example from Boost.TypeErasure, another multi-signature function.
• std::function - Standard.
• boost::function - The one from Boost.
• func::function - From this blog.
• generic::delegate - Fast delegate in C++11, also see here.
• ssvu::FastFunc - Another Don Clugston's FastDelegate, as shown here.
• cxx_function::function - By David Krauss

Also check out the amazing CxxFunctionBenchmark which compares several implementations.