C++11 using unique_ptr with custom deleter

Question

I'm trying to learn C++11 unique_ptr usage by doing a simple linked list program. For the life of me I can't figure out why I'm getting a compile error when using a custom deleter.

#include <cstdio>
#include <limits>
#include <memory>
#include <cstdlib>
#include <iostream>

using namespace std;

struct node {
    int value;
    struct node* next;
};

typedef struct node Node;

std::unique_ptr<Node> createList()
{

    std::unique_ptr<Node> head(new Node);
    Node* temp=head.get();
    temp->value=0;
    for(int i=1;i<8;i++) {
        if(temp->next==nullptr) {
            temp->next=new Node();
            temp=temp->next;
            temp->value=i;
            temp->next=nullptr;
        }
    //temp=temp->next;
    }
    return head;
}

int main()
{
    auto del1 = [](Node* p) { while(p) {std::cout << "Deleting value is : " << p->value;struct node* n=p->next;delete p; p=n;} return; };

    std::unique_ptr< Node, decltype(del1) > head(std::move(createList()),del1);
}

And here is the compilation error

sh-4.3$ g++ -std=c++11 -o main *.cpp                                                                                                                   
main.cpp: In function 'int main()':                                                                                                                    
main.cpp:38:82: error: no matching function for call to 'std::unique_ptr<node, main()::<lambda(Node*)> >::unique_ptr(std::remove_reference<std::unique_
ptr<node> >::type, main()::<lambda(Node*)>&)'                                                                                                          
         std::unique_ptr< Node, decltype(del1) > head(std::move(createList()),del1);                                                                   
                                                                                  ^                                                                    
In file included from /usr/include/c++/5.3.1/memory:81:0,                                                                                              
                 from main.cpp:3:                                                                                                                      
/usr/include/c++/5.3.1/bits/unique_ptr.h:228:2: note: candidate: template<class _Up, class> std::unique_ptr<_Tp, _Dp>::unique_ptr(std::auto_ptr<_Up>&&)
  unique_ptr(auto_ptr<_Up>&& __u) noexcept;                                                                                                            
  ^                                                                                                                                                    
/usr/include/c++/5.3.1/bits/unique_ptr.h:228:2: note:   template argument deduction/substitution failed:                                               
main.cpp:38:82: note:   'std::remove_reference<std::unique_ptr<node> >::type {aka std::unique_ptr<node>}' is not derived from 'std::auto_ptr<_Up>'     
         std::unique_ptr< Node, decltype(del1) > head(std::move(createList()),del1);                                                                   
                                                                                  ^                                                                    
In file included from /usr/include/c++/5.3.1/memory:81:0,                                                                                              
                 from main.cpp:3:                                                                                                                      
/usr/include/c++/5.3.1/bits/unique_ptr.h:220:2: note: candidate: template<class _Up, class _Ep, class> std::unique_ptr<_Tp, _Dp>::unique_ptr(std::uniqu
e_ptr<_Up, _Ep>&&)                                                                                                                                     
  unique_ptr(unique_ptr<_Up, _Ep>&& __u) noexcept                                                                                                      
  ^                                                                                                                                                    
/usr/include/c++/5.3.1/bits/unique_ptr.h:220:2: note:   template argument deduction/substitution failed:                                               
main.cpp:38:82: note:   candidate expects 1 argument, 2 provided                                                                                       
         std::unique_ptr< Node, decltype(del1) > head(std::move(createList()),del1);    

Any ideas?

Answer 1

An alternative design would be

struct Node {
    Node(int value) : value(value) {}

    int value;
    std::unique_ptr<Node> next;
};

std::unique_ptr<Node> createList()
{
    std::unique_ptr<Node> head = std::make_unique<Node>(0);

    Node* node = head.get();
    for(int i = 1; i < 8; i++) {
        node->next = std::make_unique<Node>(i);
        node = node->next.get();
    }
    return head;
}

Answer 2

createList() returns a std::unique_ptr<Node>. The constructor you're trying to use takes a Node* as the first argument:

unique_ptr( pointer p, /* see below */ d1 ); (3)    
unique_ptr( pointer p, /* see below */ d2 ); (4)    

Hence the error.

If you want to keep the custom deleter local to main(), you'll have to just pull the pointer out of createList:

std::unique_ptr<Node, decltype(del1)> head(
    createList().release(), // NB release(), not get()!
    del1);

or change createList() itself to return a std::unique_ptr<Node, decltype(del1)>.

Answer 3

The two-argument constructor for unique_ptr takes a raw pointer and a deleter, not a smart pointer and a deleter.

To make your example safe, createList should probably return a unique_ptr that would delete all the nodes in the first place.

Answer 4

You should return the correct type from createList:

#include <cstdio>
#include <limits>
#include <memory>
#include <cstdlib>
#include <iostream>

using namespace std;

struct node {
    int value;
    struct node* next;
};

typedef struct node Node;

auto createList()
{
    auto del1 = [](Node* p) { while(p) {std::cout << "Deleting value is : " << p->value;struct node* n=p->next;delete p; p=n;} return; };
    std::unique_ptr< Node, decltype(del1) > head(new Node,del1);

    Node* temp=head.get();
    temp->value=0;
    for(int i=1;i<8;i++) {
        if(temp->next==nullptr) {
            temp->next=new Node();
            temp=temp->next;
            temp->value=i;
            temp->next=nullptr;
        }
    //temp=temp->next;
    }
    return head;
}

int main()
{
    auto node = createList();
 }

Otherwise, in the code shown in the question, you should take ownership of the internal data and move them, being the pointers of different types:

int main()
{
    auto del1 = [](Node* p) { while(p) {std::cout << "Deleting value is : " << p->value;struct node* n=p->next;delete p; p=n;} return; };

    std::unique_ptr< Node, decltype(del1) > head(createList().release(),del1);
}

Note the call to .release().
See here for further details.