c++ - “make_error_code”未在此范围内声明，并且在实例化时通过参数相关查找未找到任何声明

Question

我正在使用 boost::system::error_code(属于 boost 1.62.0)在 C++ 中定义自定义错误代码，如下面的缩小示例所示:

#include <boost/system/error_code.hpp>
#include <boost/system/system_error.hpp>

namespace mynamespace {

enum class MyErrorCodeEnum : int {
    ERROR1 = 1,
    ERROR2 = 2,
};


class MyErrorCategory: public boost::system::error_category
{
public:
    static const MyErrorCategory& instance() {
        static MyErrorCategory category;
        return category;
    }

    // error_category interface
public:
    virtual const char* name() const noexcept {
        return "MyErrorCategory";
    }
    virtual std::string message(int ev) const {
        return "dummy";
    }
};

}

namespace boost {
namespace system {

inline error_code make_error_code(const mynamespace::MyErrorCodeEnum e) {
    return error_code(static_cast<int>(e), mynamespace::MyErrorCategory::instance());
}

inline error_condition make_error_condition(const mynamespace::MyErrorCodeEnum e) {
    return error_condition(static_cast<int>(e), mynamespace::MyErrorCategory::instance());
}

template<>
struct is_error_code_enum<mynamespace::MyErrorCodeEnum>: public std::true_type {};

}
}


int main(int argc, char *argv[])
{
    throw boost::system::error_code(mynamespace::MyErrorCodeEnum::ERROR1);
}

这在使用 MSVC 时工作得很好。但是，尝试迁移到mingw 5.3.0，编译器报如下错误:

boost/system/error_code.hpp: In instantiation of 'boost::system::error_code::error_code(ErrorCodeEnum, typename boost::enable_if<boost::system::is_error_code_enum<ErrorCodeEnum> >::type*) [with ErrorCodeEnum = mynamespace::MyErrorCodeEnum; typename boost::enable_if<boost::system::is_error_code_enum<ErrorCodeEnum> >::type = void]':
main.cpp:52:70:   required from here
boost/system/error_code.hpp:329:32: error: 'make_error_code' was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
         *this = make_error_code(e);
                                ^
main.cpp:35:19: note: 'boost::system::error_code boost::system::make_error_code(mynamespace::MyErrorCodeEnum)' declared here, later in the translation unit
 inline error_code make_error_code(const mynamespace::MyErrorCodeEnum e) {
                   ^

我明白为什么会被报告(make_error_code 函数是在 error_code 模板类的构造函数引用它之后声明的)，但是我怎么应该解决这个问题？

Answer 1

值得庆幸的是 coliru 错误消息更具解释性，这是一个工作版本:

#include <boost/system/error_code.hpp>
#include <boost/system/system_error.hpp>

namespace mynamespace {

enum class MyErrorCodeEnum : int {
    ERROR1 = 1,
    ERROR2 = 2,
};


class MyErrorCategory: public boost::system::error_category
{
public:
    static const MyErrorCategory& instance() {
        static MyErrorCategory category;
        return category;
    }

    // error_category interface
public:
    virtual const char* name() const noexcept {
        return "MyErrorCategory";
    }
    virtual std::string message(int ev) const {
        return "dummy";
    }
};


inline boost::system::error_code make_error_code(const MyErrorCodeEnum e) {
    return boost::system::error_code(static_cast<int>(e), MyErrorCategory::instance());
}

inline boost::system::error_condition make_error_condition(const MyErrorCodeEnum e) {
    return boost::system::error_condition(static_cast<int>(e), MyErrorCategory::instance());
}

}

template<>
struct boost::system::is_error_code_enum<mynamespace::MyErrorCodeEnum>: public std::true_type {};


int main(int argc, char *argv[])
{
    throw boost::system::error_code(mynamespace::MyErrorCodeEnum::ERROR1);
}

clang 在 coliru 上生成的消息更广泛:

In file included from main.cpp:1:
/usr/local/include/boost/system/error_code.hpp:329:17: error: call to function 'make_error_code' that is neither visible in the template definition nor found by argument-dependent lookup
        *this = make_error_code(e);
                ^
main.cpp:52:11: note: in instantiation of function template specialization 'boost::system::error_code::error_code<mynamespace::MyErrorCodeEnum>' requested here
    throw boost::system::error_code(mynamespace::MyErrorCodeEnum::ERROR1);
          ^
main.cpp:35:19: note: 'make_error_code' should be declared prior to the call site or in namespace 'mynamespace'
inline error_code make_error_code(const mynamespace::MyErrorCodeEnum e) {
                  ^
1 error generated.

是的，依赖于参数的查找失败了，因为命名空间没有为 make_error_code 提供实现，因此您必须在模板实例化之前或在同一命名空间中移动它们。 Here是一个类似的问题，答案对此进行了更详细的解释。

这也适用于 gcc(在 is_error_code_enum 专门用于 boost::system 的情况下):

#include <boost/system/error_code.hpp>
#include <boost/system/system_error.hpp>

namespace mynamespace {

enum class MyErrorCodeEnum : int {
    ERROR1 = 1,
    ERROR2 = 2,
};

}

namespace boost {
namespace system {

template<>
struct is_error_code_enum<mynamespace::MyErrorCodeEnum>: public std::true_type {};

}
}

namespace mynamespace {

class MyErrorCategory: public boost::system::error_category
{
public:
    static const MyErrorCategory& instance() {
        static MyErrorCategory category;
        return category;
    }

    // error_category interface
public:
    virtual const char* name() const noexcept {
        return "MyErrorCategory";
    }
    virtual std::string message(int ev) const {
        return "dummy";
    }
};


inline boost::system::error_code make_error_code(const MyErrorCodeEnum e) {
    return boost::system::error_code(static_cast<int>(e), MyErrorCategory::instance());
}

inline boost::system::error_condition make_error_condition(const MyErrorCodeEnum e) {
    return boost::system::error_condition(static_cast<int>(e), MyErrorCategory::instance());
}

}

int main(int argc, char *argv[])
{
    throw boost::system::error_code(mynamespace::MyErrorCodeEnum::ERROR1);
}