Compiler error while using shared_ptr with a point

问题:

I am new to using smart pointers in C++ and my current issue is that I am converting C code to C++ (C++11/14/17) and I am having some problems understanding using shared_ptr with a pointer to pointer. I have derived a toy example which I believe illustrates the problem

Following is the header file

#include <memory>
using std::shared_ptr;

struct DataNode
{
  shared_ptr<DataNode> next;
} ;


 struct ProxyNode
 {
   shared_ptr<DataNode> pointers[5];
 } ;


 struct _test_
 {
   ProxyNode** flane_pointers;
  };

And the actual code test.cpp

#include <stdint.h>
#include "test.h"


shared_ptr<DataNode> newNode(uint64_t key);

shared_ptr<ProxyNode> newProxyNode(shared_ptr<DataNode> node);

int main(void)
{
  // Need help converting this to a C++ style calling
  ProxyNode** flane_pointers = (ProxyNode**)malloc(sizeof(ProxyNode*) * 100000);
  // Here is my attempt (incomplete)
  ProxyNode** flane_pointers = new shared_ptr<ProxyNode> ?
  shared_ptr<DataNode> node = newNode(1000);
  flane_pointers[1] = newProxyNode(node)
} 

shared_ptr<ProxyNode> newProxyNode(shared_ptr<DataNode> node) 
{

  shared_ptr<ProxyNode> proxy(new ProxyNode());
 return proxy;
}

shared_ptr<DataNode> newNode(uint64_t key) 
{
 shared_ptr<DataNode> node(new DataNode());
 return node;
}

I am getting these compiler errors -

 test.cpp: In function ‘int main()’:
 test.cpp:12:42: error: cannot convert ‘std::shared_ptr<ProxyNode>’ to  ‘ProxyNode*’ in assignment
  flane_pointers[1] = newProxyNode(node)

Compiled with

  g++ -c -g test.h test.cpp

g++ version is 7.3.0 (on Ubuntu 18)

I need help converting the C style malloc allocation with a C++ style calling for a pointer to a pointer and then how to fix the compiler errors. My apologies if it looks like I am missing something obvious.

回答1:

If this ProxyNode** flane_pointers; is supposed to be a matrix of ProxyNode then the most simple approach would be to create vector of vectors containing ProxyNode like this:

struct DataNode
{
    std::shared_ptr<DataNode> next;
};


struct ProxyNode
{
    // here instead of C style array you can use std::array
    std::shared_ptr<DataNode> pointers[5];
};

TEST(xxx, yyy) {
    std::vector<std::vector<ProxyNode> > matrixOfProxyNodes;
    // insert empty vector of ProxyNode
    matrixOfProxyNodes.push_back(std::vector<ProxyNode>());
    // insert ProxyNode to the first vector of ProxyNodes
    matrixOfProxyNodes[0].push_back(ProxyNode());
    // insert shared_ptr to DataNode in position 0, 0 of matrix of ProxyNodes
    matrixOfProxyNodes[0][0].pointers[0] = std::make_shared<DataNode>();
}

If you really need the pointer to pointer (but I really don't see the purpose for that) you can do:

    // create shared pointer to shared pointer to ProxyNode
    // and initialize it to nullptr
    std::shared_ptr<std::shared_ptr<ProxyNode> > ptr2ptr2ProxyNode(nullptr);
    // dynamically create new shared_ptr to ProxyNode
    ptr2ptr2ProxyNode.reset(new std::shared_ptr<ProxyNode>(nullptr));
    // dynamically create ProxyNode
    ptr2ptr2ProxyNode->reset(new ProxyNode());
    (*ptr2ptr2ProxyNode)->pointers[0] = std::make_shared<DataNode>();

Note that std::shared_ptr does not behave exactly as raw pointer, for example you shouldn't allocate the memory for shared_ptr with operator new[]. Here is explained why.

Also I see that you probably want to implement some kind of list or other chain. std::shared_ptr can suffer from cyclic dependency, so be careful and use std::weak_ptr when needed.