不是 lambda 函数中的常量表达式

Question

我有以下代码：

#include <boost/hana.hpp>
#include <array>
#include <iostream>
#include <utility>

namespace hana = boost::hana;

#define HEADER_CONNECT 0b00010000
#define HEADER_CONNACK 0b00001000

struct ConnectFrame
{
    uint8_t header = 16;
    uint8_t variable = 2;
};

struct ConnackFrame
{
    uint8_t header = 8;
    uint8_t variable = 3;
};

constexpr auto FramesMap = hana::make_tuple(
    hana::make_pair(hana::type_c<ConnectFrame>, hana::integral_c<std::uint8_t, HEADER_CONNECT>),
    hana::make_pair(hana::type_c<ConnackFrame>, hana::integral_c<std::uint8_t, HEADER_CONNACK>));

//! Runtime deserialization switch based on FramesMap
template <typename InputIterator>
auto deserializeByFrameHeader(const std::uint8_t frameHeader, const InputIterator buffer)
{
    auto found = hana::index_if(FramesMap, [&frameHeader = std::as_const(frameHeader)](auto const &pair) {
        return hana::second(pair) == hana::integral_c<std::uint8_t, frameHeader>;
    });
    auto FrameType = hana::first(hana::at(FramesMap, found.value()));
    using T = typename decltype(FrameType)::type;
    T var;
    //deserialize(buffer, var);
    return var;
}

int main()
{
    std::array<std::byte, 128> buffer;
    // for dummy purposes we assume that the first byte of the buffer array after serialization is 8
    const uint8_t header = 8;
    ConnackFrame frameOut = deserializeByFrameHeader(header, buffer.begin());
}

Live demo

我试图在元组中找到与变量 frameHeader 匹配的对的索引。不幸的是，我得到一个编译错误：

../include/minimalMQTT.hpp:178:43: error: 'this' is not a constant expression
  178 |                 return hana::second(pair) == hana::integral_c<std::uint8_t, frameHeader>;
      |                        ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

如何将变量 frameHeader 声明为常量表达式才能使其正常工作？

Answer 1

hana::integral_c<std::uint8_t, frameHeader>

integral_c 是一种编码静态已知值的类型。但是，您尝试使用静态未知的 frameHeader 实例化它。

要将运行时值映射到编译时间值，最好的办法是映射（有时使用二进制搜索）。但是，您也可以检查是否需要评估此编译时。

解决方法

在您的具体示例中，您可以通过使用 constexpr lambda（如果编译器足够新！）来解决问题。

在实践中，我怀疑这是否会满足您的需求，但只是为了让您了解其中的技巧：

Live On Wandbox

#include <boost/hana.hpp>
#include <boost/core/ignore_unused.hpp>
#include <array>
#include <iostream>
#include <utility>

namespace hana = boost::hana;

#define HEADER_CONNECT 0b00010000
#define HEADER_CONNACK 0b00001000

struct ConnectFrame
{
    uint8_t header = 16;
    uint8_t variable = 2;
};

struct ConnackFrame
{
    uint8_t header = 8;
    uint8_t variable = 3;
};

constexpr auto FramesMap = hana::make_tuple(
    hana::make_pair(hana::type_c<ConnectFrame>, hana::integral_c<std::uint8_t, HEADER_CONNECT>),
    hana::make_pair(hana::type_c<ConnackFrame>, hana::integral_c<std::uint8_t, HEADER_CONNACK>));

//! Runtime deserialization switch based on FramesMap
template <typename FrameHeader, typename InputIterator>
auto deserializeByFrameHeader(FrameHeader const frameHeader, const InputIterator buffer)
{
    auto found = hana::index_if(FramesMap, [=](auto const &pair) constexpr {
        return hana::second(pair) == hana::integral_c<std::uint8_t, frameHeader()>;
    });
    auto FrameType = hana::first(hana::at(FramesMap, found.value()));
    using T = typename decltype(FrameType)::type;
    T var;
    boost::ignore_unused(buffer);
    //deserialize(buffer, var);
    return var;
}

int main()
{
    std::array<std::byte, 128> buffer;
    // for dummy purposes we assume that the first byte of the buffer array after serialization is 8
    ConnackFrame frameOut = deserializeByFrameHeader(
            []() constexpr { return 8; },
            buffer.begin());

    boost::ignore_unused(frameOut);
}

更新

Technically, the header is the first byte of the buffer array, i.e. uint8_t header = (uint8_t)buffer[0]. Would it be possible to omit the header argument and extract the header from the buffer as a constexpr directly?

没有。

return类型固定。输入是动态的。没有办法解决这个问题 (usefully/efficiently)。

By the way, what would be a solution if I don't need it compile time evaluated?

由于您正在解析协议消息，因此您自然会打开类型 ID（因为这就是它们在线路上的存在方式）。作为一个 认真的 C++ 程序员 你自然希望尽快将抽象层跳到 proper type-switching。

老式的技术是动态多态（虚拟接口和继承）
现代机制包括 std::variant<...> 探视。

根据您的使用模式和处理需求，两者可能更适用。 std::variant 有一个很好的特性，它以可切换的方式对类型进行编码，但访问保留了静态类型信息。这意味着：从技术上讲，您可以利用静态类型信息、内联和所有优化优点。

看来这就是你想要的。所以我建议：

Live On Coliru

#include <array>
#include <iostream>
#include <variant>

constexpr uint8_t HEADER_CONNECT = 0b00010000;
constexpr uint8_t HEADER_CONNACK = 0b00001000;

struct ConnectFrame {
    uint8_t header = 16;
    uint8_t variable = 2;
};

struct ConnackFrame {
    uint8_t header = 8;
    uint8_t variable = 3;
};

// Static typed land
void handler(ConnectFrame const&) { std::cout << "Handling ConnectFrame\n"; }
void handler(ConnackFrame const&) { std::cout << "Handling ConnackFrame\n"; }

template <typename InputIterator>
void deserialize(InputIterator&, ConnectFrame&) { /*TODO*/ }

template <typename InputIterator>
void deserialize(InputIterator&, ConnackFrame&) { /*TODO*/ }

template <typename Frame, typename InputIterator>
Frame deserialize(InputIterator& buffer) {
    Frame frame;
    deserialize(buffer, frame);
    return frame;
}

// Type-swithcing land
template <typename InputIterator>
constexpr inline std::uint8_t frameHeader(InputIterator& buffer) {
    return static_cast<std::uint8_t>(*buffer++);
}

using AnyFrame = std::variant<ConnectFrame, ConnackFrame>;

template <typename InputIterator>
AnyFrame deserializeByFrameHeader(InputIterator&& buffer) {
    switch (uint8_t h = frameHeader(buffer)) {
        case HEADER_CONNECT: return deserialize<ConnectFrame>(buffer);
        case HEADER_CONNACK: return deserialize<ConnackFrame>(buffer);
    }
    throw std::range_error("frameHeader");
}

int main() {
    constexpr auto process = [](auto const& frame) { handler(frame); };
    using Buffer = std::array<std::byte, 128>;

    for (auto buffer : { Buffer 
        { std::byte(HEADER_CONNECT), std::byte(0x12), std::byte(0x34), },
        { std::byte(HEADER_CONNACK), std::byte(0xab), std::byte(0xcd), } })
    {
        auto frameOut = deserializeByFrameHeader(buffer.begin());
        std::visit(process, frameOut);
    }
}

打印

Handling ConnectFrame
Handling ConnackFrame

使用 Hana 映射

如果您真的认为从映射 table 开始工作很重要，您可以使用更多代码和编译器工作：

constexpr auto FramesMap = hana::make_tuple(
    hana::make_pair(hana::type_c<ConnectFrame>, HEADER_CONNECT),
    hana::make_pair(hana::type_c<ConnackFrame>, HEADER_CONNACK)
);

Note how I dropped the integral_c because we don't need it

让我们制作 AnyFrame 框架类型的变体：

constexpr auto FrameTypes = hana::transform(FramesMap, hana::first);

using AnyFrame = decltype(
        hana::unpack(FrameTypes, hana::template_<std::variant>))
    ::type;

现在，让我们使用它重新实现 deserializeByFrameHeader：

template <typename InputIterator>
AnyFrame deserializeByFrameHeader(InputIterator&& buffer) {
    AnyFrame retval;

    hana::for_each(FramesMap,
        [&, frameHeader = frameHeader(buffer)](auto const &pair) {
            auto first = hana::first(pair);
            using T = typename decltype(first)::type;

            if (hana::second(pair) == frameHeader) {
                retval.emplace<T>();
                deserialize(buffer, std::get<T>(retval));
            }
        });

    return retval;
}

请注意简化：我们根据多态 lambda 中元组元素 (pair) 的静态类型保留了所有内容，我们始终可以使用框架类型。

完整演示

Live On Coliru

#include <cstdint>
constexpr uint8_t HEADER_CONNECT = 0b00010000;
constexpr uint8_t HEADER_CONNACK = 0b00001000;

struct ConnectFrame {
    uint8_t header = 16;
    uint8_t variable = 2;
};

struct ConnackFrame {
    uint8_t header = 8;
    uint8_t variable = 3;
};

#include <boost/hana.hpp>
#include <stdexcept>
#include <variant>
#include <iostream>

namespace {
    namespace hana = boost::hana;

    constexpr auto FramesMap = hana::make_tuple(
        hana::make_pair(hana::type_c<ConnectFrame>, HEADER_CONNECT),
        hana::make_pair(hana::type_c<ConnackFrame>, HEADER_CONNACK)
    );

    constexpr auto FrameTypes = hana::transform(FramesMap, hana::first);

    using AnyFrame = decltype(
            hana::unpack(FrameTypes, hana::template_<std::variant>))
        ::type;
}

// Static typed land
void handler(ConnectFrame const&) { std::cout << "Handling ConnectFrame\n"; }
void handler(ConnackFrame const&) { std::cout << "Handling ConnackFrame\n"; }

template <typename InputIterator>
void deserialize(InputIterator&, ConnectFrame&) { /*TODO*/ }

template <typename InputIterator>
void deserialize(InputIterator&, ConnackFrame&) { /*TODO*/ }

// Type-swithcing land
template <typename InputIterator>
constexpr inline std::uint8_t frameHeader(InputIterator& buffer) {
    return static_cast<std::uint8_t>(*buffer++);
}

template <typename InputIterator>
AnyFrame deserializeByFrameHeader(InputIterator&& buffer) {
    AnyFrame retval;

    hana::for_each(FramesMap,
        [&, frameHeader = frameHeader(buffer)](auto const &pair) {
            auto first = hana::first(pair);
            using T = typename decltype(first)::type;

            if (hana::second(pair) == frameHeader) {
                retval.emplace<T>();
                deserialize(buffer, std::get<T>(retval));
            }
        });

    return retval;
}

#include <array>
int main() {
    constexpr auto process = [](auto const& frame) { handler(frame); };
    using Buffer = std::array<std::byte, 128>;

    for (auto buffer : { Buffer 
        { std::byte(HEADER_CONNECT), std::byte(0x12), std::byte(0x34), },
        { std::byte(HEADER_CONNACK), std::byte(0xab), std::byte(0xcd), } })
    {
        auto frameOut = deserializeByFrameHeader(buffer.begin());
        std::visit(process, frameOut);
    }
}

版画

Handling ConnectFrame
Handling ConnackFrame

不是 lambda 函数中的常量表达式

Not a constant expression in lambda function

c++

boost

boost-hana

c++17

解决方法

更新

使用 Hana 映射

完整演示