functionRef.h

//
// Copyright 2020 Pixar
//
// Licensed under the Apache License, Version 2.0 (the "Apache License")
// with the following modification; you may not use this file except in
// compliance with the Apache License and the following modification to it:
// Section 6. Trademarks. is deleted and replaced with:
//
// 6. Trademarks. This License does not grant permission to use the trade
//    names, trademarks, service marks, or product names of the Licensor
//    and its affiliates, except as required to comply with Section 4(c) of
//    the License and to reproduce the content of the NOTICE file.
//
// You may obtain a copy of the Apache License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the Apache License with the above modification is
// distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the Apache License for the specific
// language governing permissions and limitations under the Apache License.
//
#ifndef PXR_BASE_TF_FUNCTION_REF_H
#define PXR_BASE_TF_FUNCTION_REF_H

#include "pxr/pxr.h"

#include <memory>
#include <type_traits>
#include <utility>

PXR_NAMESPACE_OPEN_SCOPE

template <class Sig>
class TfFunctionRef;

template <class Ret, class... Args>
class TfFunctionRef<Ret (Args...)>
{
    // Type trait to detect when an argument is a potentially cv-qualified
    // TfFunctionRef.  This is used to disable the generic constructor and
    // assignment operator so that TfFunctionRef arguments are copied rather
    // than forming TfFunctionRefs pointing to TfFunctionRefs.
    template <typename Fn>
    using _IsFunctionRef = std::is_same<
        std::remove_cv_t<std::remove_reference_t<Fn>>, TfFunctionRef>;

public:
    template <class Fn, class = std::enable_if_t<!_IsFunctionRef<Fn>::value>>
    constexpr TfFunctionRef(Fn &fn) noexcept
        : _fn(static_cast<void const *>(std::addressof(fn)))
        , _invoke(_InvokeFn<Fn>) {}

    TfFunctionRef(TfFunctionRef const &rhs) noexcept = default;

    TfFunctionRef &
    operator=(TfFunctionRef const &rhs) noexcept = default;

    template <class Fn>
    std::enable_if_t<!_IsFunctionRef<Fn>::value,
                     TfFunctionRef &>
    operator=(Fn &fn) noexcept {
        *this = TfFunctionRef(fn);
        return *this;
    }
    
    void swap(TfFunctionRef &other) noexcept {
        std::swap(_fn, other._fn);
        std::swap(_invoke, other._invoke);
    }
    
    inline Ret operator()(Args... args) const {
        return _invoke(_fn, std::forward<Args>(args)...);
    }

private:
    template <class Fn>
    static Ret _InvokeFn(void const *fn, Args...args) {
        using FnPtr = typename std::add_pointer<
            typename std::add_const<Fn>::type>::type;
        return (*static_cast<FnPtr>(fn))(std::forward<Args>(args)...);
    }

    void const *_fn;
    Ret (*_invoke)(void const *, Args...);
};

template <class Sig>
inline void
swap(TfFunctionRef<Sig> &lhs, TfFunctionRef<Sig> &rhs)
{
    lhs.swap(rhs);
}

PXR_NAMESPACE_CLOSE_SCOPE

#endif // PXR_BASE_TF_FUNCTION_REF_H