Unable to insert smart pointer into map - c++

Question

I am trying to insert my smart pointer, p_pointer<foo> pointing towards a foo class into map, however the program would not compile. Therefore I tried to use a normal pointer foo* and the program does compile. So far, I have been able to use the p_pointer like a normal pointer without any issue so I am surprised that this does not work. If anyone can explain to me why this would not work...
#include <iostream>
#include<map>
#include<string>
using namespace std;
template <class T>
class p_pointer
{
public:
T* cp;
size_t* refptr;
size_t* counter()
{
return refptr;
}
//default constructor
p_pointer():cp(0),refptr(new size_t(1)) {}
p_pointer(T*t):cp(t),refptr(new size_t(1)) {}
//copy constructor
p_pointer (const p_pointer&s):cp(s.cp),refptr(s.refptr)
{
refptr=s.refptr;
cp=s.cp;
*refptr=*refptr+1;
}
//destructor
~p_pointer()
{
if(--*refptr==0)
{
delete cp;
delete refptr;
}
}
//assignment operator
p_pointer&operator=(const p_pointer&s)
{
++*s.refptr;
//freeing the left hand size if it is the last one
if(--*refptr==0)
{
delete cp;
delete refptr;
}
cp=s.cp;
refptr=s.refptr;
}
operator bool()
{
return cp;
}
T*&operator->()
{
if(cp)
return cp;
else throw std::runtime_error("uninitialized player");
}
T operator*()
{
if(cp)
return *cp;
else throw std::runtime_error("uninitialized player");
}
};
class foo
{};
Method 1 (works)
int main()
{
map<foo*,int> x;
foo* y=new foo();
x[y]=1;
}
Method 2 (does not work)
int main()
{
map<p_pointer<foo>,int> x;
p_pointer<foo> y=new foo();
x[y]=1;
}
The error message is:
This is my first time posting an error message. I just copied all the content from build messages and paste it into here. Please let me know if this is not the optimal way
||=== Build: Debug in trial 821 (compiler: GNU GCC Compiler) ===|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h||In instantiation of 'bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = p_pointer<foo>]':|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_map.h|498|required from 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = p_pointer<foo>; _Tp = int; _Compare = std::less<p_pointer<foo> >; _Alloc = std::allocator<std::pair<const p_pointer<foo>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = p_pointer<foo>]'|
C:\trial 821\main.cpp|80|required from here|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|error: no match for 'operator<' (operand types are 'const p_pointer<foo>' and 'const p_pointer<foo>')|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: candidates are:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: operator<(int, int) <built-in>|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: no known conversion for argument 2 from 'const p_pointer<foo>' to 'int'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_pair.h|220|note: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_pair.h|220|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::pair<_T1, _T2>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|298|note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|298|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::reverse_iterator<_Iterator>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|348|note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|348|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::reverse_iterator<_Iterator>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|1072|note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_IteratorR>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|1072|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::move_iterator<_Iterator>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|1078|note: template<class _Iterator> bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_iterator.h|1078|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::move_iterator<_Iterator>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\basic_string.h|2588|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>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\basic_string.h|2588|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::basic_string<_CharT, _Traits, _Alloc>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\basic_string.h|2600|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\basic_string.h|2600|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::basic_string<_CharT, _Traits, _Alloc>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\basic_string.h|2612|note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\basic_string.h|2612|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: mismatched types 'const _CharT*' and 'p_pointer<foo>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_tree.h|980|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>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_tree.h|980|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\array|242|note: template<class _Tp, unsigned int _Nm> bool std::operator<(const std::array<_Tp, _Nm>&, const std::array<_Tp, _Nm>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\array|242|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::array<_Tp, _Nm>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\tuple|857|note: template<class ... _TElements, class ... _UElements> constexpr bool std::operator<(const std::tuple<_Args1 ...>&, const std::tuple<_Args2 ...>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\tuple|857|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::tuple<_Args1 ...>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_map.h|1017|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>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_map.h|1017|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::map<_Key, _Tp, _Compare, _Alloc>'|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_multimap.h|920|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>&)|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_multimap.h|920|note: template argument deduction/substitution failed:|
C:\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_function.h|371|note: 'const p_pointer<foo>' is not derived from 'const std::multimap<_Key, _Tp, _Compare, _Alloc>'|
||=== Build failed: 1 error(s), 3 warning(s) (0 minute(s), 0 second(s)) ===|

Answer 1

Your problem can be found in the error message: no match for 'operator<' (operand types are 'const p_pointer' and 'const p_pointer')|. In order to use map with a custom type (p_pointer in this case), that type needs to declare a comparison operator < which is used to sort the keys in the map. You'll need to implement the < operator, and that should fix this error. (Other errors may show up to replace it though.)
Again, I would recommend just using the existing std::shared_ptr rather than re-implementing it yourself, unless you have some reason not to do so.