How can I use std::weak_ptr
as key for a std::map
as shown in the following code?
#include <map>
#include <memory>
int main()
{
std::map< std::weak_ptr<int>, bool > myMap;
std::shared_ptr<int> sharedptr(new int(5));
std::weak_ptr<int> weakptr = sharedptr;
myMap[weakptr] = true;
return 0;
}
The above program doesn't build and trying to compile it gives many error messages such as:
1>C:Program Files (x86)Microsoft Visual Studio 10.0VCincludexfunctional(125): error C2784: 'bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)' : could not deduce template argument for 'const std::_Tree<_Traits> &' from 'const std::tr1::weak_ptr<_Ty>'
1> with
1> [
1> _Ty=int
1> ]
1> C:Program Files (x86)Microsoft Visual Studio 10.0VCincludextree(1885) : see declaration of 'std::operator <'
1> C:Program Files (x86)Microsoft Visual Studio 10.0VCincludexfunctional(124) : while compiling class template member function 'bool std::less<_Ty>::operator ()(const _Ty &,const _Ty &) const'
1> with
1> [
1> _Ty=std::tr1::weak_ptr<int>
1> ]
1> C:Program Files (x86)Microsoft Visual Studio 10.0VCincludemap(71) : see reference to class template instantiation 'std::less<_Ty>' being compiled
1> with
1> [
1> _Ty=std::tr1::weak_ptr<int>
1> ]
1> C:Program Files (x86)Microsoft Visual Studio 10.0VCincludextree(451) : see reference to class template instantiation 'std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,_Mfl>' being compiled
1> with
1> [
1> _Kty=std::tr1::weak_ptr<int>,
1> _Ty=bool,
1> _Pr=std::less<std::tr1::weak_ptr<int>>,
1> _Alloc=std::allocator<std::pair<const std::tr1::weak_ptr<int>,bool>>,
1> _Mfl=false
1> ]
The problem occurs due to the following line:
myMap[weakptr] = true;
The error messages seem to be related to operator<
. Do I need to define operator<
for weak_ptr
? Exactly, what operators need to be defined in order to use a data type as a key for a std::map
?
(I should note that I have already defined operator==
in the std
namespace. Also, I plan to use a weak_ptr
to a custom class type and not an int
.)
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