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| Member functions | ||||
| Modifiers | ||||
| Observers | ||||
weak_ptr::owner_before | ||||
(C++26) | ||||
(C++26) | ||||
| Non-member functions | ||||
| Helper classes | ||||
(C++20) | ||||
| Deduction guides(C++17) |
template<class Y> bool owner_before(const weak_ptr<Y>& other)constnoexcept; | ||
template<class Y> bool owner_before(conststd::shared_ptr<Y>& other)constnoexcept; | ||
Checks whether thisweak_ptr precedesother in implementation defined owner-based (as opposed to value-based) order. The order is such that two smart pointers compare equivalent only if they are both empty or if they both own the same object, even if the values of the pointers obtained byget() are different (e.g. because they point at different subobjects within the same object).
This ordering is used to make shared and weak pointers usable as keys in associative containers, typically throughstd::owner_less.
Contents |
| other | - | thestd::shared_ptr orstd::weak_ptr to be compared |
true if*this precedesother,false otherwise. Common implementations compare the addresses of the control blocks.
#include <iostream>#include <memory> struct Foo{int n1;int n2; Foo(int a,int b): n1(a), n2(b){}}; int main(){auto p1=std::make_shared<Foo>(1,2);std::shared_ptr<int> p2(p1,&p1->n1);std::shared_ptr<int> p3(p1,&p1->n2); std::cout<<std::boolalpha<<"p2 < p3 "<<(p2< p3)<<'\n'<<"p3 < p2 "<<(p3< p2)<<'\n'<<"p2.owner_before(p3) "<< p2.owner_before(p3)<<'\n'<<"p3.owner_before(p2) "<< p3.owner_before(p2)<<'\n'; std::weak_ptr<int> w2(p2);std::weak_ptr<int> w3(p3);std::cout// << "w2 < w3 " << (w2 < w3) << '\n' // won't compile// << "w3 < w2 " << (w3 < w2) << '\n' // won't compile<<"w2.owner_before(w3) "<< w2.owner_before(w3)<<'\n'<<"w3.owner_before(w2) "<< w3.owner_before(w2)<<'\n';}
Output:
p2 < p3 truep3 < p2 falsep2.owner_before(p3) falsep3.owner_before(p2) falsew2.owner_before(w3) falsew3.owner_before(w2) false
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2083 | C++11 | owner_before was not declaredconst | declaredconst |
| LWG 2942 | C++11 | owner_before was not declarednoexcept | declarednoexcept |
(C++11) | provides mixed-type owner-based ordering of shared and weak pointers (class template)[edit] |
