C++: overriding function from templated base class - c++

Question

Here's the simple version. Below, I fully explain the real-world use case.
I've got two classes:
template<class T>
class Base {
protected:
virtual void foo();
std::map<std::string*, T*> things;
};
class Derived : public Base<int> {
protected:
void foo();
};
template<class T>
void Base<T>::foo() {
for (auto itr = things.begin(); itr < things.end(); ++itr) {
}
}
void Derived::foo() {
for (auto itr = things.begin(); itr < things.end(); ++itr) {
}
}
I run into problems with the for loop condition in the Derived implementation of foo:
example.cpp: In member function ‘virtual void Derived::foo()’:
example.cpp:23:53: error: no match for ‘operator<’ in ‘itr < ((Derived*)this)->Derived::<anonymous>.Base<int>::things.std::map<_Key, _Tp, _Compare, _Alloc>::end<std::basic_string<char>*, int*, std::less<std::basic_string<char>*>, std::allocator<std::pair<std::basic_string<char>* const, int*> > >()’
example.cpp:23:53: note: candidates are:
In file included from /usr/include/c++/4.7/string:54:0,
from example.cpp:2:
/usr/include/c++/4.7/bits/basic_string.h:2590:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.7/bits/basic_string.h:2590:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: mismatched types ‘const _CharT*’ and ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’
In file included from /usr/include/c++/4.7/string:54:0,
from example.cpp:2:
/usr/include/c++/4.7/bits/basic_string.h:2578:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
/usr/include/c++/4.7/bits/basic_string.h:2578:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
In file included from /usr/include/c++/4.7/string:54:0,
from example.cpp:2:
/usr/include/c++/4.7/bits/basic_string.h:2566:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.7/bits/basic_string.h:2566:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
In file included from /usr/include/c++/4.7/map:62:0,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_multimap.h:822:5: note: template<class _Key, class _Tp, class _Compare, class _Alloc> bool std::operator<(const std::multimap<_Key, _Tp, _Compare, _Alloc>&, const std::multimap<_Key, _Tp, _Compare, _Alloc>&)
/usr/include/c++/4.7/bits/stl_multimap.h:822:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::multimap<_Key, _Tp, _Compare, _Alloc>’
In file included from /usr/include/c++/4.7/map:61:0,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_map.h:906:5: note: template<class _Key, class _Tp, class _Compare, class _Alloc> bool std::operator<(const std::map<_Key, _Tp, _Compare, _Alloc>&, const std::map<_Key, _Tp, _Compare, _Alloc>&)
/usr/include/c++/4.7/bits/stl_map.h:906:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::map<_Key, _Tp, _Compare, _Alloc>’
In file included from /usr/include/c++/4.7/map:60:0,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_tree.h:873:5: note: template<class _Key, class _Val, class _KeyOfValue, class _Compare, class _Alloc> bool std::operator<(const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&, const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&)
/usr/include/c++/4.7/bits/stl_tree.h:873:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/stl_tree.h:63,
from /usr/include/c++/4.7/map:60,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template<class _Iterator> bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&)
/usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::move_iterator<_Iterator>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/stl_tree.h:63,
from /usr/include/c++/4.7/map:60,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_IteratorR>&)
/usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::move_iterator<_Iterator>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/stl_tree.h:63,
from /usr/include/c++/4.7/map:60,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
/usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::reverse_iterator<_Iterator>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/stl_tree.h:63,
from /usr/include/c++/4.7/map:60,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
/usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::reverse_iterator<_Iterator>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:65:0,
from /usr/include/c++/4.7/bits/stl_tree.h:63,
from /usr/include/c++/4.7/map:60,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
/usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const std::pair<_T1, _T2>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/stl_tree.h:63,
from /usr/include/c++/4.7/map:60,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:838:5: note: template<class _Iterator, class _Container> bool __gnu_cxx::operator<(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)
/usr/include/c++/4.7/bits/stl_iterator.h:838:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const __gnu_cxx::__normal_iterator<_Iterator, _Container>’
In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0,
from /usr/include/c++/4.7/bits/stl_tree.h:63,
from /usr/include/c++/4.7/map:60,
from example.cpp:1:
/usr/include/c++/4.7/bits/stl_iterator.h:832:5: note: template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator<(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)
/usr/include/c++/4.7/bits/stl_iterator.h:832:5: note: template argument deduction/substitution failed:
example.cpp:23:53: note: ‘std::_Rb_tree_iterator<std::pair<std::basic_string<char>* const, int*> >’ is not derived from ‘const __gnu_cxx::__normal_iterator<_IteratorL, _Container>’
Why doesn't this work, and what should I be doing to fix it?
P.S.: I know that this is a prime opportunity to use template specialization, but I don't think that is appropriate for my use case.
Here's the full story.
I'm making a scene graph with several types of nodes. Some of the important ones for this issue are:
SceneParent
SceneRegion
SceneGroup
SceneComponent
They each play the following roles:
SceneParent contains a set of children, of template type T.
SceneRegion extends SceneParent, specifying T as SceneParent
SceneGroup extends SceneParent, specifying T as SceneComponent
The function I am trying to implement is getVisibleChildren. Given a viewport, I want a receive a set of SceneComponents that should be rendered.
SceneParents implementation is recursively called on all children:
template< class T >
void SceneParent< T >::getVisibleChildren(const util::Camera& camera,
std::vector< SceneComponent* >& visibleChildren) const {
if (!this->isVisible(camera)) {
return;
}
// Get ids into a flat container so the next loop can be parallelized.
std::vector< std::string* > childIds;
for (auto itr = this->children.cbegin(); itr < this->children.cend();
++itr) {
childIds.push_back(itr->first);
}
// TODO: parallelize this.
for (auto itr = childIds.cbegin(); itr < childIds.cend(); ++itr) {
this->children[*itr]->getVisibleChildren(camera, visibleChildren);
}
}
SceneGroups implementation adds all of its SceneComponents to the accumulator (the assumption being that if a group is visible, so are its components). So the SceneGroup node acts as the base case for the recursion:
void SceneGroup::getVisibleChildren(const util::Camera& camera,
std::vector< SceneComponent* >& visibleChildren) const {
if (!this->isVisible(camera)) {
return;
}
// If this group is visible, assume all of its components are as well.
for (auto itr = this->children.cbegin(); itr < this->children.cend();
++itr) {
visibleChildren.push_back(itr->second);
}
}
Now that you have made it to this point in my post, I'm assuming you are interested (at least mildly) in my real-world application, not just the compiler error it is causing. So, if you feel so inclined, please offer any advice about how I can improve my implementation of the scene graph / things I should be looking out for while working on this project.

Answer 1

The iterators of std::map<K, V> are bidirectional iterators. It doesn't make any sense to use the less than operator for bidirectional iterator. Just write your loop using the equality or inequality operator:
for (auto itr = things.begin(); itr != things.end(); ++itr) {
}