CODEWITHSUNDEEP
c++shared-ptrdestructordelete-operatorreference-counting
harveyspecter
harveyspecter

Reputation: 31

Why does deleting a valid heap-allocated pointer give "free(): invalid pointer" error?

I am new to C++ and am trying to do a very basic implementation of a shared_ptr (WIP). I am trying to delete a heap-allocated pointer in the destructor right after finding the underlying value by dereferencing. While the dereferencing happens fine, deletion of '''ref_count''' variable is causing problems. Can someone please help? '''

#include<iostream>

template<typename T>
class shared_ptr{
 private:
  T* native_ptr_ = nullptr;
  int* ref_count_ = nullptr;

  inline void increment_count() {
    *ref_count_++;
  }

  inline void decrement_count() {
    *ref_count_--;
  }

 public:
  shared_ptr() {
    std::cout << "shared_ptr: empty constructor" << std::endl;
    native_ptr_ = nullptr;
    ref_count_ = nullptr;
  }

  shared_ptr(T* ptr) {
    std::cout << "shared_ptr: constructor" << std::endl;
    if (ptr) {
      native_ptr_ = ptr;
      ref_count_ = new int(1);
    }
  }

  ~shared_ptr() {
    std::cout << "shared_ptr: destructor" << std::endl;
    if (ref_count_) {
      decrement_count();
      if (ref_count_ && use_count() == 0) {
        std::cout << *ref_count_ << " " << *native_ptr_ << std::endl;
        delete ref_count_;
        delete native_ptr_;
        ref_count_ = nullptr;
        native_ptr_ = nullptr;
      }
    }
  }

  int use_count() const {
    if(ref_count_) {
        return *ref_count_;
    } else {
        return 0;
    }
  }
};

int main() {
  // case1
  int* temp = new int(0);
  shared_ptr<int> a1(temp);
  return 0;
}

'''

'''

shared_ptr: constructor
shared_ptr: destructor
0 0
free(): invalid pointer
Aborted (core dumped)

'''

Upvotes: 0

Views: 536

Answers (1)

Stephen Newell
Stephen Newell

Reputation: 7863

Operator precedence is causing you problems. You're actually decrementing the pointer, then deferencing the new address. If you crank up your warnings, you'll get something like this:

p.cpp:35:7:   required from ‘shared_ptr<T>::~shared_ptr() [with T = int]’
p.cpp:58:26:   required from here
p.cpp:14:5: warning: value computed is not used [-Wunused-value]
   14 |     *ref_count_--;
      |     ^

You can add parenthesis to de-reference the pointer before decrementing.

Upvotes: 6

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