How can I use a std::map with std::weak_ptr as key?
C++11 provides appropriate mechanisms for comparisons of std::weak_ptr
, namely: std::owner_less
.
This should be the default for maps and sets. If the C++ compiler you are using is having a hard time, try using std::owner_less
if it is available. If it is not available, you will need to provide a similar mechanism as std::owner_less
so you can appropriately compare std::weak_ptr
objects.
As explained in Jody Hagins' answer, you should use std::owner_less
as comparison function object of an associative container if you use a std::weak_ptr
as key. I'd like to expand on that answer by providing the following full solution for your code:
int main() {
std::map<std::weak_ptr<int>, bool, std::owner_less<std::weak_ptr<int>>> myMap;
std::shared_ptr<int> sharedptr(new int(5));
std::weak_ptr<int> weakptr = sharedptr;
myMap[weakptr] = true;
return 0;
}
Since C++17 you can omit the template parameter for owner_less
, resulting in shorter code as follows:
std::map<std::weak_ptr<int>, bool, std::owner_less<>> myMap;
And if you plan to use a weak_ptr
to a custom class instead of an int
, then you can simply replace the int
by the name of your class as shown in this example code on Coliru.
In general, apart from providing a proper comparison function (or overloading operator<
) for a (custom) key type, nothing needs to be done for a std::map
. Overloading operator==
for the key type is not required.