operator==, !=, <, <=, >, >=, <=> (std::shared_ptr)

Defined in header <memory>.

C++20

Compare two shared_ptr objects

// 1)
template < class T, class U >
bool operator==( const std::shared_ptr<T>& lhs,
                 const std::shared_ptr<U>& rhs ) noexcept;
// 7)
template< class T, class U >
std::strong_ordering operator<=>( const std::shared_ptr<T>& lhs,
                                  const std::shared_ptr<U>& rhs ) noexcept;

Compare a shared_ptr with a null pointer

// 8)
template< class T >
bool operator==( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 20)
template< class T >
std::strong_ordering operator<=>( const std::shared_ptr<T>& lhs,
                                  std::nullptr_t ) noexcept;

C++11

Compare two shared_ptr objects

// 1)
template < class T, class U >
bool operator==( const std::shared_ptr<T>& lhs,
                 const std::shared_ptr<U>& rhs ) noexcept;
// 2)
template< class T, class U >
bool operator!=( const std::shared_ptr<T>& lhs,
                 const std::shared_ptr<U>& rhs ) noexcept;
// 3)
template< class T, class U >
bool operator<( const std::shared_ptr<T>& lhs,
                const std::shared_ptr<U>& rhs ) noexcept;
// 4)
template< class T, class U >
bool operator>( const std::shared_ptr<T>& lhs,
                const std::shared_ptr<U>& rhs ) noexcept;
// 5)
template< class T, class U >
bool operator<=( const std::shared_ptr<T>& lhs,
                 const std::shared_ptr<U>& rhs ) noexcept;
// 6)
template< class T, class U >
bool operator>=( const std::shared_ptr<T>& lhs,
                 const std::shared_ptr<U>& rhs ) noexcept;

Compare a shared_ptr with a null pointer

//8)
template< class T >
bool operator==( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 9)
template< class T >
bool operator==( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
// 10)
template< class T >
bool operator!=( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 11)
template< class T >
bool operator!=( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
// 12)
template< class T >
bool operator<( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 13)
template< class T >
bool operator<( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
// 14)
template< class T >
bool operator>( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 15)
template< class T >
bool operator>( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
// 16)
template< class T >
bool operator<=( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 17)
template< class T >
bool operator<=( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
// 18)
template< class T >
bool operator>=( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 19)
template< class T >
bool operator>=( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;

Compares two shared_ptr<T> objects or compares shared_ptr<T> with a null pointer.

Note that the comparison operators for shared_ptr simply compare pointer values; the actual objects pointed to are not compared. Having operator< defined for shared_ptr allows shared_ptrs to be used as keys in associative containers, like std::map and std::set.

The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively (since C++20).

Parameters

lhs - the left-hand shared_ptr to compare rhs - the right-hand shared_ptr to compare

Return value

1. lhs.get() == rhs.get()
   
2. !(lhs == rhs)
   
3. std::less<V>()(lhs.get(), rhs.get()), where V is the composite pointer type of std::shared_ptr<T>::element_type* and std::shared_ptr<U>::element_type*
   
4. rhs < lhs
   
5. !(rhs < lhs)
   
6. !(lhs < rhs)
   
7. std::compare_three_way{}(x.get(), y.get())
   
8. !lhs
   
9. !rhs
   
10. (bool)lhs
    
11. (bool)rhs
    
12. std::less<std::shared_ptr<T>::element_type*>()(lhs.get(), nullptr)
    
13. std::less<std::shared_ptr<T>::element_type*>()(nullptr, rhs.get())
    
14. nullptr < lhs
    
15. rhs < nullptr
    
16. !(nullptr < lhs)
    
17. !(rhs < nullptr)
    
18. !(lhs < nullptr)
    
19. !(nullptr < rhs)
    
20. std::compare_three_way{}(x.get(), static_cast<std::shared_ptr<T>::element_type*>(nullptr))
    

Notes

In all cases, it is the stored pointer (the one returned by get()) that is compared, rather than the managed pointer (the one passed to the deleter when use_count goes to zero). The two pointers may differ in a shared_ptr created using the aliasing constructor.

Example

#include <iostream>
#include <memory>
 
int main()
{
    std::shared_ptr<int> p1(new int(42));
    std::shared_ptr<int> p2(new int(42));
 
    std::cout << std::boolalpha
        << "(p1 == p1)       : " 
        << (p1 == p1) << '\n'
        << "(p1 <=> p1) == 0 : " 
        << ((p1 <=> p1) == 0) << '\n' // Since C++20
 
    // p1 and p2 point to different memory locations, so p1 != p2
        << "(p1 == p2)       : " 
        << (p1 == p2) << '\n'
        << "(p1 < p2)        : " 
        << (p1 < p2) << '\n'
        << "(p1 <=> p2) < 0  : " 
        << ((p1 <=> p2) < 0) << '\n'   // Since C++20
        << "(p1 <=> p2) == 0 : " 
        << ((p1 <=> p2) == 0) << '\n'; // Since C++20
}

Possible Result

(p1 == p1)       : true
(p1 <=> p1) == 0 : true
(p1 == p2)       : false
(p1 < p2)        : true
(p1 <=> p2) < 0  : true
(p1 <=> p2) == 0 : false

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3427	C++20	operator<=>(shared_ptr, nullptr_t) was ill-formed	definition fixed