C++17 implementation of unique_function

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beyond functions

Build Status Build status codecov License: MIT

This repository is a C++17 implementation of various type erased callable types. It includes:

  • unique_function: A Non-copyable drop-in replacement to std::function
  • inplace_function (upcoming): A std::function replacement with fixed capacity storage, and guarantee not to allocate
  • function_ref (upcoming): A none owning function
  • unique_pseudofunction (upcoming) behave like unique_function, but store an overload set instead of an individual function


unique_function is a proposed standard library type-erased function type that improves std::function. The interface of unique_function is almost identical to std::function, with the following exceptions:

Move only

unique_function does not provides copy operations. This sounds like a restriction, but it means unique_function can support move-only function objects.


The operator() of std::function is const qualified even when the passed in a mutable function object. For example,

const std::function<int()> f {[x=0]() mutable { return ++x; }};
f(); // fine, returns 1
f(); // returns 2

unique_function does not suffer this const correctness problem std::function. In unique_function<F>, the operator() is const qualified only if the function type F is const qualified. For example,

beyond::unique_function<int()> f1 {[x=0]() mutable { return ++x; }};
beyond::unique_function<int() const> f2 {[x=0]() mutable { return ++x; }};  // Do not compile
beyond::unique_function<int() const> f3 {[x=0]() { return x; }};
const beyond::unique_function<int()> f4 {[x=0]() mutable { return ++x; }};
const beyond::unique_function<int() const> f5 {[x=0]() { return x; }};
f1(); // OK
f3(); // OK
f4(); // Do not compile
f5(); // OK

Removal of target and target_type

Not many people are using RTTI for std::function anyway.

Memory Footprint under a 64 bits machine

  • Total size: 48 bits
  • Behavior pointer: 8 bits (Function pointer to polymorphic behaviors except for invocation)
  • Function pointer: 8 bits (Function pointer to invocation)
  • Storage: 32 bits


Build Option

“beyond functions” is a header-only library, but the following CMake build options are used to enable test or benchmark during development

  • BEYOND_FUNCTIONS_BUILD_TESTS Build unit tests and benchmarks


You can either define macros before every includes or use the CMake build system setting with the same name.

  • BEYOND_FUNCTIONS_NAMESPACE: defines the namespace of the library, it is beyond by default
  • BEYOND_FUNCTIONS_NO_EXCEPTION: if defined, then the implementation of operator() will not throw and invoking a unique_function with no underlying target will result in undefined behavior. Note this will not prevent the underlying function from throwing.


Below is a benchmark on invocation overhead of beyond::unique_function. It is not rigorous, but it indicates that the overhead of beyond::unique_function is similar to std::function.

Windows 10, Intel Core i7-8650U CPU @ 1.90GHz (8 CPUs), ~2.1GHz, MSVC 2019 MSVC benchmark

Arch Linux, Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz GCC benchmark Clang benchmark

Compiler support

Tested on:

  • Windows
    • MSVC 2019
  • Linux
    • g++-9
    • g++-8
    • clang-9
    • clang-8
    • clang-7
    • clang-6.0