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我正在开发一个使用两个不同库的 C++ 项目：spdlog用于日志记录，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命名空间，但是当我尝试编写一个同时使用它们的程序时，我的编译器（g++ 6.2）给了我一些荒谬的错误，似乎表明它正在尝试从 spdlog 库中实例化一个函数模板通过使用 mutils 库中的函数模板的定义。

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是一个实现mutils::ByteRepresentable（启用序列化并引入 mutils-serialization 库的接口）的简单类，并提供一个operator<<（spdlog 能够记录它所必需的）。如有必要，我可以发布它的代码。

当我用 编译它时g++ -std=c++14 -I"./src" -I"./libraries" -I"./libraries/mutils/" -L"./libraries/" -O0 -g3 -Wall "src/LibraryCollisionTest.cpp"，我得到了这个又长又丑的错误（别担心，我会帮你解析它）：

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) };

关键线在这里：

./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]’

不知何故，g++ 已经从扩展 spdlog 库中的模板函数，在 namespacefmt::internal中，跳跃到 mutils 库中的函数模板，在 namespace 中mutils，这显然不是 spdlog 库打算做的！如果我查看第 1276 行format.h，它就是在这个模板结构中调用“convert”函数的那个​​：

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

上面几行，果然是函数“convert”：

template <typename T>
T &get();

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

这些都在 namespace 之内fmt::internal，我的 IDE 同意，如果我想要第 1276 行的函数“convert”的定义，我应该跳转到第 1248 行的函数“convert”。那么为什么 g++ 忽略这个定义，而是尝试使用 for 的定义mutils::convert()，它甚至不在正确的命名空间中？

请注意，clang 也无法编译此程序，并且会犯同样的错误，所以我认为这不是 g++ 中的错误。

4

1 回答 1

10

这绝对是一个错误spdlogfmtlib，由 spdlog 内部使用。

此常见问题解答中简要描述了该问题：什么
是“Argument-Dependent Lookup”（又名 ADL，或“Koenig Lookup”）？

因为messaging::TestSerializableObject继承自命名空间中的类型mutils，所以当从命名空间内部使用 aconvert被非限定调用时，两者和都被考虑在重载集中。可变参数函数在重载决议期间总是排在最后，因此后者中的模板参数比前者中的模板参数更匹配并且被选择。fmt::internalTestSerializableObjectfmt::internal::convert mutils::convertF...mutils::convert

这绝不是您的代码或 mutils 特有的——任何具有convert在同一命名空间或父命名空间中命名的一元函数或函数模板的类型都容易受到此问题的影响。

解决方法是限定convert调用并更改fmt::internal::ConvertToInt<T>::enable_conversionfrom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，我养成了始终限定对任何internal/detail命名空间内的所有函数调用的习惯，即使来自同一命名空间内的代码，除非明确打算使用 ADL。（Nb 调用不需要完全限定，只需限定即可。）我从看到 Boost 在 C++11 出现时必须以艰难的方式处理这个问题中学到了这一课。

于 2016-11-18T21:50:28.853 回答