Compilation error with Type Traits in boost::type_traits::conditional - c++

Question

I am having a problem in some code using type_traits from boost.
It is quite a complex part of the code, but I could isolate the part that gives the compilation error:
template<const size_t maxLen>
class MyString {
public:
typedef boost::conditional<(maxLen > 0), char[maxLen+1], std::string> ObjExternal;
};
template <class T>
class APIBase {
public:
typedef T obj_type;
typedef typename T::ObjExternal return_type;
};
template <class T>
int edit(const T& field, const typename T::return_type& value)
{
return 0;
}
int myFunction()
{
APIBase<MyString<10> > b;
char c[11];
return edit(b, c);
}
This gives the following error:
test.cpp: In function ‘int myFunction()’:
tes.cpp:109: error: no matching function for call to ‘edit(APIBase >&, char [11])’
tes.cpp:100: note: candidates are: int edit(const T&, const typename T::return_type&) [with T = APIBase >]
However, if I change the line with the code
char c[11];
by
MyString<10>::ObjExternal c;
it works. Similarly, if instead I change the line
typedef boost::conditional<(maxLen > 0), char[maxLen+1], std::string> ObjExternal;
by
typedef char ObjExternal[maxLen+1];
it also works. I am thinking that it is a problem with boost::conditional, as it seems it is not being evaluated right. Is there a problem in my code, or there is an alternative that can be used instead of boost::conditional to have this functionality?
I am thinking about using the 2nd option, but then I could not use maxLen as 0.

Answer 1

You need to use the member typedef type provided by conditional and not the conditional type itself.
Change:
typedef boost::conditional<(maxLen > 0),
char[maxLen+1],
std::string> ObjExternal;
to:
typedef typename boost::conditional<(maxLen > 0),
char[maxLen+1],
std::string>::type ObjExternal;