std::map::const_iterator template compilation error
I have a template class that contains a std::map
that stores pointers to T which refuses to compile:
template <class T>
class Foo
{
public:
// The following line won't compile
std::map<int, T*>::const_iterator begin() const { return items.begin(); }
private:
std::map<int, T*> items;
};
gcc gives me the following error:
error: type 'std::map<int, T*, std::less<int>, std::allocator<std::pair<const int, T*> > >' is not derived from t开发者_如何学Goype 'Foo<T>'
Similarly, the following also refuses to compile:
typedef std::map<int, T*>::const_iterator ItemIterator;
However, using a map that doesn't contain the template type works OK, e.g.:
template <class T>
class Foo
{
public:
// This is OK
std::map<int, std::string>::const_iterator begin() const { return items.begin(); }
private:
std::map<int, std::string> items;
};
I assume this is related to templates and begs the question - how can I return a const_iterator
to my map?
Use typename
:
typename std::map<int, T*>::const_iterator begin() const ...
When this is first passed by the compiler, it doesn't know what T
is. Thus, it also doesn't know wether const_iterator
is actually a type or not.
Such dependent names (dependent on a template parameter) are assumed to
- not be types unless prefixed by
typename
- not to be templates unless directly prefixed by
template
.
You need typename
:
typename std::map<int, T*>::const_iterator begin() const { return items.begin(); }
You need:
typename std::map<int, T*>::const_iterator begin() const { return items.begin(); }
Or simpler
typedef typename std::map<int, T*>::const_iterator const_iterator;
const_iterator begin() const { return items.begin(); }
This is because const_iterator
is dependent name on T
so you need to tell compiler that it is actually type.
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