std::unique_ptr::reset

members of the primary template, unique_ptr<T>
void reset( pointer ptr = pointer() );	(1)
members of the specialization unique_ptr<T[]>
void reset( pointer ptr = pointer() );	(2)	(until C++17)
	(3)
template< class U > void reset( U ) = delete;		(until C++17)
template< class U > void reset( U );		(since C++17)
		(4)
void reset( std::nullptr_t p );			(until C++17)
void reset( std::nullptr_t p = nullptr );			(since C++17)

Replaces the managed object.

1) Given current_ptr, the pointer that was managed by *this, performs the following actions, in this order:

Saves a copy of the current pointer old_ptr = current_ptr
Overwrites the current pointer with the argument current_ptr = ptr
If the old pointer was non-empty, deletes the previously managed object if(old_ptr != nullptr) get_deleter()(old_ptr).

2) Behaves the same as the reset member of the primary template.

3) In the specialization for dynamic arrays, std::unique_ptr<T[]>, this template member is provided to prevent using reset() with a pointer to derived (which would result in undefined behavior with arrays).

4) In the specialization for dynamic arrays, std::unique_ptr<T[]>, this overload is necessary to allow reset to nullptr (which would otherwise be prohibited by the template overload). Equivalent to reset(pointer())

(until C++17)

3) Behaves the same as the reset member of the primary template, except that it will only participate in overload resolution if either

U is the same type as pointer, or
pointer is the same type as element_type* and U is a pointer type V* such that V(*)[] is convertible to element_type(*)[].

4) Equivalent to reset(pointer())

(since C++17)

Parameters

ptr

pointer to a new object to manage

Return value

(none).

Exceptions

noexcept specification:

noexcept

Notes

To replace the managed object while supplying a new deleter as well, move assignment operator may be used.

A test for self-reset, i.e. whether ptr points to an object already managed by *this, is not performed, except where provided as a compiler extension or as a debugging assert. Note that code such as p.reset(p.release()) does not involve self-reset, only code like p.reset(p.get()) does.

Example

#include <iostream>
#include <memory>
 
struct Foo {
    Foo() { std::cout << "Foo...\n"; }
    ~Foo() { std::cout << "~Foo...\n"; }
};
 
struct D {
    void operator() (Foo* p) {
        std::cout << "Calling delete for Foo object... \n";
        delete p;
    }
};
 
int main()
{
    std::cout << "Creating new Foo...\n";
    std::unique_ptr<Foo, D> up(new Foo(), D());  // up owns the Foo pointer (deleter D)
 
    std::cout << "Replace owned Foo with a new Foo...\n";
    up.reset(new Foo());  // calls deleter for the old one
 
    std::cout << "Release and delete the owned Foo...\n";
    up.reset(nullptr);      
}

Output:

Creating new Foo...
Foo...
Replace owned Foo with a new Foo...
Foo...
Calling delete for Foo object...
~Foo...
Release and delete the owned Foo...
Calling delete for Foo object...
~Foo...