c++ - 一个库中的功能与另一个库中的模板相匹配

我正在一个使用两个不同库的C++项目中:spdlog用于记录日志，mutils-serialization用于将对象序列化为字节(用于通过网络发送)。这两个库都正确使用了 namespace ，但是当我尝试编写同时使用它们的程序时，我的编译器(g++ 6.2)给了我无意义的错误，似乎表明它正试图从spdlog库实例化函数模板。通过使用mutils库中的功能模板的定义。

这是我的简单测试程序:

#include <spdlog/spdlog.h>
#include <spdlog/fmt/ostr.h>
#include "TestSerializableObject.h"

int main(int argc, char** argv) {
    auto global_logger = spdlog::rotating_logger_mt("global_logger", "log", 1024 * 1024 * 500, 3);
    global_logger->set_pattern("[%H:%M:%S.%e] [%l] %v");
    global_logger->set_level(spdlog::level::trace);

    std::shared_ptr<spdlog::logger> logger(spdlog::get("global_logger"));

    auto message = std::make_shared<messaging::TestSerializableObject>(
        1, 2, "A message!");

    logger->trace("Received a message: {}", *message);
}

TestSerializableObject是一个简单的类，实现mutils::ByteRepresentable(启用序列化并拉入mutils-serialization库的接口(interface))，并提供operator<<(spdlog能够对其进行记录是必需的)。如有必要，我可以为其发布代码。

当我用g++ -std=c++14 -I"./src" -I"./libraries" -I"./libraries/mutils/" -L"./libraries/" -O0 -g3 -Wall "src/LibraryCollisionTest.cpp"编译它时，我得到了这个长而丑陋的错误(不用担心，我会帮助您解析它):

In file included from ./libraries/mutils/mutils.hpp:3:0,
                 from ./libraries/mutils-serialization/SerializationSupport.hpp:2,
                 from src/TestSerializableObject.h:10,
                 from src/LibraryCollisionTest.cpp:10:
./libraries/mutils/args-finder.hpp: In instantiation of ‘struct mutils::function_traits<messaging::TestSerializableObject>’:
./libraries/mutils/args-finder.hpp:75:41:   required from ‘auto mutils::convert(F) [with F = messaging::TestSerializableObject; ignore = void]’
./libraries/spdlog/fmt/bundled/format.h:1276:46:   required from ‘struct fmt::internal::ConvertToInt<messaging::TestSerializableObject>’
./libraries/spdlog/fmt/bundled/format.h:1485:5:   required by substitution of ‘template<class T> fmt::internal::MakeValue<Formatter>::MakeValue(const T&, typename fmt::internal::EnableIf<fmt::internal::Not<fmt::internal::ConvertToInt<T>::value>::value, int>::type) [with T = messaging::TestSerializableObject]’
./libraries/spdlog/fmt/bundled/format.h:2465:12:   required from ‘static fmt::internal::Value fmt::internal::ArgArray<N, true>::make(const T&) [with Formatter = fmt::BasicFormatter<char>; T = messaging::TestSerializableObject; unsigned int N = 1u]’
./libraries/spdlog/fmt/bundled/format.h:2898:5:   required from ‘void fmt::BasicWriter<Char>::write(fmt::BasicCStringRef<CharType>, const Args& ...) [with Args = {messaging::TestSerializableObject}; Char = char]’
./libraries/spdlog/details/logger_impl.h:69:9:   required from ‘void spdlog::logger::log(spdlog::level::level_enum, const char*, const Args& ...) [with Args = {messaging::TestSerializableObject}]’
./libraries/spdlog/details/logger_impl.h:127:5:   required from ‘void spdlog::logger::trace(const char*, const Args& ...) [with Args = {messaging::TestSerializableObject}]’
src/LibraryCollisionTest.cpp:21:53:   required from here
./libraries/mutils/args-finder.hpp:12:37: error: ‘operator()’ is not a member of ‘messaging::TestSerializableObject’
   : public function_traits<decltype(&T::operator())>
                                     ^~
./libraries/mutils/args-finder.hpp: In instantiation of ‘auto mutils::convert(F) [with F = messaging::TestSerializableObject; ignore = void]’:
./libraries/spdlog/fmt/bundled/format.h:1276:46:   required from ‘struct fmt::internal::ConvertToInt<messaging::TestSerializableObject>’
./libraries/spdlog/fmt/bundled/format.h:1485:5:   required by substitution of ‘template<class T> fmt::internal::MakeValue<Formatter>::MakeValue(const T&, typename fmt::internal::EnableIf<fmt::internal::Not<fmt::internal::ConvertToInt<T>::value>::value, int>::type) [with T = messaging::TestSerializableObject]’
./libraries/spdlog/fmt/bundled/format.h:2465:12:   required from ‘static fmt::internal::Value fmt::internal::ArgArray<N, true>::make(const T&) [with Formatter = fmt::BasicFormatter<char>; T = messaging::TestSerializableObject; unsigned int N = 1u]’
./libraries/spdlog/fmt/bundled/format.h:2898:5:   required from ‘void fmt::BasicWriter<Char>::write(fmt::BasicCStringRef<CharType>, const Args& ...) [with Args = {messaging::TestSerializableObject}; Char = char]’
./libraries/spdlog/details/logger_impl.h:69:9:   required from ‘void spdlog::logger::log(spdlog::level::level_enum, const char*, const Args& ...) [with Args = {messaging::TestSerializableObject}]’
./libraries/spdlog/details/logger_impl.h:127:5:   required from ‘void spdlog::logger::trace(const char*, const Args& ...) [with Args = {messaging::TestSerializableObject}]’
src/LibraryCollisionTest.cpp:21:53:   required from here
./libraries/mutils/args-finder.hpp:75:41: error: ‘as_function’ is not a member of ‘mutils::function_traits<messaging::TestSerializableObject>’
   return function_traits<F>::as_function(f);
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from ./libraries/spdlog/fmt/fmt.h:21:0,
                 from ./libraries/spdlog/common.h:41,
                 from ./libraries/spdlog/spdlog.h:12,
                 from src/LibraryCollisionTest.cpp:8:
./libraries/spdlog/fmt/bundled/format.h: In instantiation of ‘struct fmt::internal::ConvertToInt<messaging::TestSerializableObject>’:
./libraries/spdlog/fmt/bundled/format.h:1485:5:   required by substitution of ‘template<class T> fmt::internal::MakeValue<Formatter>::MakeValue(const T&, typename fmt::internal::EnableIf<fmt::internal::Not<fmt::internal::ConvertToInt<T>::value>::value, int>::type) [with T = messaging::TestSerializableObject]’
./libraries/spdlog/fmt/bundled/format.h:2465:12:   required from ‘static fmt::internal::Value fmt::internal::ArgArray<N, true>::make(const T&) [with Formatter = fmt::BasicFormatter<char>; T = messaging::TestSerializableObject; unsigned int N = 1u]’
./libraries/spdlog/fmt/bundled/format.h:2898:5:   required from ‘void fmt::BasicWriter<Char>::write(fmt::BasicCStringRef<CharType>, const Args& ...) [with Args = {messaging::TestSerializableObject}; Char = char]’
./libraries/spdlog/details/logger_impl.h:69:9:   required from ‘void spdlog::logger::log(spdlog::level::level_enum, const char*, const Args& ...) [with Args = {messaging::TestSerializableObject}]’
./libraries/spdlog/details/logger_impl.h:127:5:   required from ‘void spdlog::logger::trace(const char*, const Args& ...) [with Args = {messaging::TestSerializableObject}]’
src/LibraryCollisionTest.cpp:21:53:   required from here
./libraries/spdlog/fmt/bundled/format.h:1276:38: warning: invalid application of ‘sizeof’ to a void type [-Wpointer-arith]
     enum { enable_conversion = sizeof(convert(get<T>())) == sizeof(Yes) };

关键行在这里:

./libraries/mutils/args-finder.hpp: In instantiation of ‘auto mutils::convert(F)
 [with F = messaging::TestSerializableObject; ignore = void]’:
./libraries/spdlog/fmt/bundled/format.h:1276:46:   required from ‘struct
 fmt::internal::ConvertToInt<messaging::TestSerializableObject>’
./libraries/spdlog/fmt/bundled/format.h:1485:5:   required by substitution of
 ‘template<class T> fmt::internal::MakeValue<Formatter>::MakeValue(const
 T&, typename fmt::internal::EnableIf<fmt::internal::Not<
 fmt::internal::ConvertToInt<T>::value>::value, int>::type) [with T =
 messaging::TestSerializableObject]’

某种程度上，g++已从在spdlog库中扩展命名空间fmt::internal中的模板化函数跃迁到在mutils库中命名空间mutils中的函数模板，这显然不是spdlog库打算执行的操作!如果我看format.h的第1276行，它是在此模板结构内调用“convert”功能的代码:

template<typename T>
struct ConvertToInt
{
    enum { enable_conversion = sizeof(convert(get<T>())) == sizeof(Yes) };
    enum { value = ConvertToIntImpl2<T, enable_conversion>::value };
};

上面的几行肯定是函数“转换”:

template <typename T>
T &get();

Yes &convert(fmt::ULongLong);
No &convert(...);

这些都在命名空间fmt::internal内，并且我的IDE同意，如果我想在第1276行中定义函数“convert”，我应该跳到第1248行中的函数“convert”。那么，为什么g++会忽略此定义，并且而是尝试使用mutils::convert()的定义，即使在正确的 namespace 中也没有？

请注意，clang也无法编译该程序，并且会犯同样的错误，因此我认为这不是g++中的错误。

最佳答案

这绝对是spdlog中的错误。

该常见问题概述了该问题:
What is “Argument-Dependent Lookup” (aka ADL, or “Koenig Lookup”)?

因为messaging::TestSerializableObject继承自命名空间mutils中的类型，所以当使用convert从命名空间fmt::internal内部调用TestSerializableObject不合格时，在重载集中会同时考虑fmt::internal::convert和mutils::convert。在重载解析期间，可变参数函数始终排在最后，因此，后者中的模板参数F比前者中的...更好地匹配，因此选择了mutils::convert。

这绝不是您的代码或mutils所特有的-在同一 namespace 或父 namespace 中具有一元函数或名为convert的函数模板的任何类型都容易受到此问题的影响。

解决方法是限定convert调用并从以下位置更改fmt::internal::ConvertToInt<T>::enable_conversion的定义:

enum { enable_conversion = sizeof(convert(get<T>())) == sizeof(Yes) };

至

enum { enable_conversion = sizeof(internal::convert(get<T>())) == sizeof(Yes) };

在我自己的代码中，我习惯于始终限定对任何internal/detail命名空间内的函数的所有调用，即使是来自同一命名空间内的代码，除非明确指定使用ADL。 (注:调用不需要完全合格，而只是合格。)我从观看Boost到C++ 11兴起时必须用困难的方式来解决这个问题的经验教训。 :-]