当初始值设定项是基类名称时出现错误“初始值设定项未命名非静态数据成员或基类”

Question

I am facing the following problem. In the file my_exception.h I have defined my own exception class inheriting from std::exception :

// File "my_exception.h"
#include <exception>
#include <string>

namespace proj { namespace exception {

struct Exception : public std::exception {
    explicit Exception(const std::string& msg) noexcept : msg_(msg) { }

    inline const char* what() const noexcept override { return msg_.c_str(); }

private:
    std::string msg_;
};

} }

Then I defined a derived exception class called BadParameterAccess in another namespace, splitting the declaration and the implementation in a .h and a .cpp files, respectively:

// File parameter_exception.h
#include "exception.h"

namespace proj { namespace parameter {

struct BadParameterAccess final : public exception::Exception
{
    BadParameterAccess() noexcept;
};

} }

// File parameter_exception.cpp
#include "parameter_exception.h"

namespace proj { namespace parameter {

BadParameterAccess::BadParameterAccess() noexcept
    : exception::Exception("[BadParameterAccess] parameter not set yet."){ }

} }

I tried to compile this code using several compilers. With clang 6.0 I get the following error:

parameter_exception.cpp:7:18: error: initializer 'Exception' does not name a non-static data member or base class; did you mean the base class 'Exception'?
    : exception::Exception("[BadParameterAccess] parameter not set yet."){ }
                 ^~~~~~~~~
                 Exception
./parameter_exception.h:11:35: note: base class 'exception::Exception' specified here
struct BadParameterAccess final : public exception::Exception
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~

g++ 7 gives an equivalent error, and Visual Studio 2017 gives the following:

parameter_exception.cpp(8): error C2039: 'Exception': is not a member of 'std::exception'

The code compiles perfectly when either:

1. in the file parameter_exception.cpp I specify the full path for the base class initializer ( proj::exception::Exception ), or
2. in the file parameter_exception.cpp I remove the namespaces from the base class initializer ( Exception ), or
3. in the file my_exception.h I remove the inheritance from std::exception , or
4. I rename my namespace exception in some other way.

As far as I have understood from the different errors I got, the compiler expects to find a member called Exception inside the class std::exception rather than inside the namespace exception , but I don't understand why this happens. Moreover, I would have expected the compiler to give me an error when I inherit from exception::Exception in the header file parameter_exception.h first, but it does not.

Can someone explain me the reason?

Thank you in advance.

Answer 1

as @molbdnilo suggests there is a problem of name lookup. the problem is whith the name "exception" beeing used for namespace exception and the standard::exception struct. I removed code and comments from the code you posted.

namespace standard {
    struct exception{
        explicit exception() noexcept { }
    };
}
namespace exception {
    struct A: public standard::exception {
        explicit A() noexcept { }
    };
}
namespace parameter {
    struct BadParameterAccess final : public exception::A
    {
        //BadParameterAccess() noexcept : exception::A() { }; // KO
        BadParameterAccess() noexcept : ::exception::A() { }; // OK
    };
}

namespace standard1 {
    struct exception1{
        explicit exception1() noexcept { }
    };
}
namespace exception2 {
    struct A: public standard1::exception1 {
        explicit A() noexcept { }
    };
}
namespace parameter1 {
    struct BadParameterAccess1 final : public exception2::A
    {
        BadParameterAccess1() noexcept : exception2::A() { }; // OK
        //BadParameterAccess1() noexcept : ::exception2::A() { }; // OK
    };
}