Question

I’ve been given some code that compiles fine on MSVC, and I’m trying to get it to compile on Clang in Xcode. I’m currently coming across an issue where a class is being redefined using the following typedef:

typedef std::map<MyNS::istring, EntityState> Entity;

Looking at the preprocessed output, I can see that there are two forward declarations of class Entity before this typedef. However, the actual definition of class Entity is not in the preprocessed output, but it is in the same namespace as the new Entity map (not MyNS though…). Is it the forward declarations that are causing this error? And is there some way that this could be valid in MSVC and not working due to the pedantry of Clang?

EDIT: I don’t have MSVC to hand, but here’s a snippet I put together to demonstrate the kind of error I’m getting (I’ve simplified the definitions so that it all fits in a small space). This causes the same error as I get when I try to compile it with Clang. Would this work in MSVC?

namespace TheNS {

    class Entity;

    struct EntityState
    {
        std::string aString, anotherString;
        int anInt;

        EntityState() {}

        EntityState(std::string a, std::string b, int i)
        {
            // constructor
        }
    };

    typedef std::map<std::string, EntityState> Entity;

    class Entity
    {
    public:
        void SomeFunction();

    private:
        int m_aVar;

    };

}    

Answer 1

Yes, it’s not correct. Should never compile, if it compiles on MSVC – probably it’s a bug of compiler. Forward declaration tells compiler, that TheNS::Entity will be class and nothing else (not enum, union, or typedef). Really, your code is same as

class Entity;   
typedef int Entity;

Of course it’s incorrect.

n3337 9.1/2

A declaration consisting solely of class-key identifier; is either a redeclaration of the name
in the current scope or a forward declaration of the identifier as a class name. It introduces the class name
into the current scope.

So, after this

class Entity;

compiles knows, that Entity will be used as class-name. This name can be redeclared as function (in the same scope), in this case you should use class Entity, when you want to use Entity class (or redeclare Entity name by typedef as says in comments).

7.1.3/6

In a given scope, a typedef specifier shall not be used to redefine the name of any type declared in that
scope to refer to a different type. [ Example:

class complex { /∗ ... ∗/ };
typedef int complex; // error: redifinition

— end example ]