TransientFunction: A light-weight alternative to std::function [C++11]

TransientFunction.h

// TransientFuction: A light-weight alternative to std::function [C++11]
// Pass any callback - including capturing lambdas - cheaply and quickly as a 
// function argument
// 
// Based on: 
// https://deplinenoise.wordpress.com/2014/02/23/using-c11-capturing-lambdas-w-vanilla-c-api-functions/
//
//  - No instantiation of called function at each call site
//  - Simple to use - use TransientFunction<> as the function argument
//  - Low cost: cheap setup, one indirect function call to invoke
//  - No risk of dynamic allocation (unlike std::function)
//
//  - Not C-friendly, unlike the original Reverse - but easy to adapt
//  - NOT PERSISTENT: Use for synchronous calls only
//  - No thought has been given to argument forwarding in this implementation 
//
// Compiles as C++11 using recent g++, clang, and msvc compilers
// Try it at https://godbolt.org
//
// Prior art:
// A proposal to the C++ ISO committee has been written by Vittorio Romeo,
// which lists a number of other instances of this same pattern:
// function_ref: a non-owning reference to a Callable
// https://vittorioromeo.info/Misc/p0792r1.html
// https://www.youtube.com/watch?v=6EQRoqELeWc
// https://vittorioromeo.info/index/blog/passing_functions_to_functions.html
//

#pragma once

// For std::decay
#include <type_traits>

template<typename>
struct TransientFunction; // intentionally not defined

template<typename R, typename ...Args>
struct TransientFunction<R(Args...)>
{
  using Dispatcher = R(*)(void*, Args...);
  
  Dispatcher m_Dispatcher; // A pointer to the static function that will call the 
                           // wrapped invokable object
  void* m_Target;          // A pointer to the invokable object

  // Dispatch() is instantiated by the TransientFunction constructor,
  // which will store a pointer to the function in m_Dispatcher.
  template<typename S>
  static R Dispatch(void* target, Args... args)
  {
    return (*(S*)target)(args...);
  }
  
  template<typename T>
  TransientFunction(T&& target)
    : m_Dispatcher(&Dispatch<typename std::decay<T>::type>)
    , m_Target(&target)
  {
  }

  // Specialize for reference-to-function, to ensure that a valid pointer is 
  // stored.
  using TargetFunctionRef = R(Args...);
  TransientFunction(TargetFunctionRef target)
    : m_Dispatcher(Dispatch<TargetFunctionRef>)
  {
    static_assert(sizeof(void*) == sizeof target, 
    "It will not be possible to pass functions by reference on this platform. "
    "Please use explicit function pointers i.e. foo(target) -> foo(&target)");
    m_Target = (void*)target;
  }

  R operator()(Args... args) const
  {
    return m_Dispatcher(m_Target, args...);
  }
};


#if defined(TRANSIENTFUNCTION_DEMO)

// Example usage:
void vv(TransientFunction<void()>);
void vi(TransientFunction<void(int)>);
int iiii(TransientFunction<int(int, int, int)>);

static void g() {}

int demonstration()
{
  // Non-capturing lambda, void()
  vv([]{});

  // Capturing lambda, void()
  int x = 3;
  vv([&]{x++;}); // x == 4

  // Free function, void()
  // both pointer to function and reference to function are valid
  vv(&g);
  vv(g);

  // std::function, void(); commented out because it wrecks codegen
  //   std::function<void()> f = [&]{ x += 21; };
  //   vv(f);

  vi([&](int i){x += i;}); // x == 11

  // x == 11 + 3 + 4 * 5 == 34
  return iiii([&](int a, int b, int c) { return x + a + b * c; });
}

/*
By defining vv(), vi() and iiii() as follows, clang and gcc are able to inline
and simplify all code in demonstration(). Mark those functions static for even
less generated code.

demonstration():
        mov     eax, 34
        ret
*/

void vv(TransientFunction<void()> f)
{
  f();
}

void vi(TransientFunction<void(int)> f)
{
  f(7);
}

int iiii(TransientFunction<int(int, int, int)> f)
{
  return f(3, 4, 5);
}

#endif


#if 0
#include <stdint.h>
#include <stdlib.h>
class MemAllocStack;

// For a more "real-world" perspective, we can look at the examples from
// https://deplinenoise.wordpress.com/2014/02/23/using-c11-capturing-lambdas-w-vanilla-c-api-functions/


// QuickSort without TransientFunction:

typedef int64_t          (*QuickCompareFunc)(const void*, const void*);
typedef int64_t (*QuickCompareFuncReentrant)(const void*, const void*, void* arg);

void QuickSort (void* __restrict elems, int64_t elem_count, size_t elem_size, 
                QuickCompareFunc cf);
void QuickSort (void* __restrict elems, int64_t elem_count, size_t elem_size, 
                QuickCompareFuncReentrant cf, void* arg);

template <typename T, typename Func>
void QuickSort(T* __restrict elems, int64_t elem_count, Func f)
{
  auto compare_closure = [](const void* ll, const void* rr,
                            void* context) -> int64_t {
    const T* l = (const T*) ll;
    const T* r = (const T*) rr;
    return (*(Func*)context)(l, r);
  };
  QuickSort(elems, elem_count, sizeof elems[0], compare_closure, &f);
}

// QuickSort with TransientFunction

// Use TransientFunction in comparison function typedef
using QuickCompareTFunc = TransientFunction<int64_t (const void*, const void*)>;

// Only one external function required - no overload required for reentrant 
// comparison function: pass any closure
void QuickSortTF(void* __restrict elems, int64_t elem_count, size_t elem_size, 
                 QuickCompareTFunc cf);

template <typename T, typename Func>
void QuickSortTF(T* __restrict elems, int64_t elem_count, Func f)
{
  // capture function rather than pass as arg
  auto compare_closure = [&f](const void* ll, const void* rr) -> int64_t { 
    const T* l = (const T*) ll;
    const T* r = (const T*) rr;
    return f(l, r); // simpler dispatch
  };

  // Pass the closure directly
  QuickSortTF(elems, elem_count, sizeof elems[0], compare_closure); 
}


// DecompressFile
// Without TransientFunction:
typedef bool (ZReadFn)(void* target_buffer, size_t read_size, void* user_data);

bool DecompressFile(void* buffer, size_t compressed_size, size_t decompressed_size, 
                    MemAllocStack* scratch, void *user_data, ZReadFn* read_fn);

template <typename Fn>
bool DecompressFile(void* buffer, size_t cs, size_t ds, 
                    MemAllocStack* scratch, Fn fn)
{
 auto closure = [](void* target_buffer, size_t read_size, void* user_data) -> bool
 {
   return (*(Fn*)user_data)(target_buffer, read_size);
 };

 return DecompressFile(buffer, cs, ds, scratch, &fn, closure);
}


// Using TransientFunction:

using ZReafTFn = TransientFunction<bool (void* target_buffer, size_t read_size)>; 
bool DecompressFile(void* buffer, size_t compressed_size, size_t decompressed_size,
                    MemAllocStack* scratch, ZReadFn read_fn);

// That's it. No forwarding template required.

#endif



/*
Copyright (c) 2018 Jonathan Adamczewski
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, 
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE 
SOFTWARE.
*/