data_view.h

/*    Copyright 2014 MongoDB Inc.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

#pragma once

#include "mongo/config.h"

#include <cstring>

#include "mongo/platform/endian.h"

#if __cplusplus >= 201103L
#include <type_traits>
#endif

namespace mongo {

class ConstDataView {
public:
    typedef const char* bytes_type;

    ConstDataView(bytes_type bytes) : _bytes(bytes) {}

    bytes_type view(std::size_t offset = 0) const {
        return _bytes + offset;
    }

    template <typename T>
    const ConstDataView& readNative(T* t, size_t offset = 0) const {
#if MONGO_HAVE_STD_IS_TRIVIALLY_COPYABLE
        static_assert(std::is_trivially_copyable<T>::value,
                      "Type for DataView::readNative must be trivially copyable");
#endif
        std::memcpy(t, view(offset), sizeof(*t));
        return *this;
    }

    template <typename T>
    T readNative(std::size_t offset = 0) const {
        T t;
        readNative(&t, offset);
        return t;
    }

    template <typename T>
    T readLE(std::size_t offset = 0) const {
        return endian::littleToNative(readNative<T>(offset));
    }

    template <typename T>
    T readBE(std::size_t offset = 0) const {
        return endian::bigToNative(readNative<T>(offset));
    }

private:
    bytes_type _bytes;
};

class DataView : public ConstDataView {
public:
    typedef char* bytes_type;

    DataView(bytes_type bytes) : ConstDataView(bytes) {}

    bytes_type view(std::size_t offset = 0) const {
        // It is safe to cast away const here since the pointer stored in our base class was
        // originally non-const by way of our constructor.
        return const_cast<bytes_type>(ConstDataView::view(offset));
    }

    template <typename T>
    DataView& writeNative(const T& value, std::size_t offset = 0) {
#if MONGO_HAVE_STD_IS_TRIVIALLY_COPYABLE
        static_assert(std::is_trivially_copyable<T>::value,
                      "Type for DataView::writeNative must be trivially copyable");
#endif
        std::memcpy(view(offset), &value, sizeof(value));
        return *this;
    }

    template <typename T>
    DataView& writeLE(const T& value, std::size_t offset = 0) {
        return writeNative(endian::nativeToLittle(value), offset);
    }

    template <typename T>
    DataView& writeBE(const T& value, std::size_t offset = 0) {
        return writeNative(endian::nativeToBig(value), offset);
    }
};

}  // namespace mongo