How to remove unique_ptr by pointer from a container?

Question

Creating an object and giving ownership to a container using a unique_ptr is no problem. How would one remove an element by raw pointer?

std::set<std::unique_ptr<MyClass>> mySet;

MyClass *myClass = new MyClass();
mySet.insert(std::unique_ptr<MyClass>(myClass));

// remove myClass from mySet?

Answer 1

You will need to find the iterator corresponding to the myClass element and then pass that iterator to mySet.erase(). The iterator may be found using the std::find_if algorithm with a custom Predicate functor that understands how to dereference unique_ptr and compare it to the raw pointer myClass.

You can not use the overloaded size_t set::erase ( const key_type& x ); since the raw pointer (even if wrapped in a temporary unique_ptr) will not be found in mySet.

Answer 2

Not as pretty as I would've liked. But the following does the job:

#include <memory>
#include <set>
#include <iostream>

struct do_nothing
{
    void operator()(const void*) const {}
};

struct MyClass
{
    MyClass() {std::cout << "MyClass()\n";}
    MyClass(const MyClass&) {std::cout << "MyClass(const MyClass&)\n";}
    ~MyClass() {std::cout << "~MyClass()\n";}
};

int main()
{
    std::set<std::unique_ptr<MyClass>> mySet;

    MyClass *myClass = new MyClass();
    mySet.insert(std::unique_ptr<MyClass>(myClass));

    // remove myClass from mySet?
    std::set<std::unique_ptr<MyClass>>::iterator i =
        lower_bound(mySet.begin(), mySet.end(),
                    std::unique_ptr<MyClass, do_nothing>(myClass));
    if (i != mySet.end() && *i == std::unique_ptr<MyClass, do_nothing>(myClass))
        mySet.erase(i);
}