Visual C++ Debug Assertion Failed? - c++

Question

In the code below is implemented a smart pointer that executes fine, but at the end I get the following message:
Here is the code:
smart_ptr.h:
#ifndef SMART_PTR_H
#define SMART_PTR_H
/*
Class: Auto_ptr
It implements a generic
smart pointer that doesn't need
to be deleted explicitly, i.e.
it provides garbage collection.
*/
template<class T>
class Auto_ptr{
public:
// constructors
explicit Auto_ptr(T* p = nullptr): value(p) { }; // constructor
Auto_ptr(Auto_ptr& p); // copy constructor
Auto_ptr& operator= (const Auto_ptr& p); // copy assignment
~Auto_ptr() { std::cout << "pointer deleted\n"; delete value; } // destructor
// access operators
const T& operator* () const { return *value; } // dereference operator
const T* operator->() const { return value; } // indirect class member access (arrow) operator
// non-modifying members
T* get() { return value; } // getter method
void reset(T* v); // reassing new value(default value: nullptr)
T* release(); // transfers the object to another pointer; without destroying it
private:
// data member
T* value;
};
//--------------------------------------------------------------------------------------------------------
// class Auto_ptr member implementations
// Constructors
// copy constructor
template<class T>
Auto_ptr<T>::Auto_ptr(Auto_ptr& p) {
value = p.release();
}
// copy assignment
template<class T>
Auto_ptr<T>& Auto_ptr<T>::operator= (const Auto_ptr& p ) {
if (this == &p) return *this;
if (value) delete value;
value = p.value;
return *this;
}
/*
Function: release()
Use: T ptr = auto_ptr_obj.release();
It transfers the pointer value to the
caller, setting the data member value
to nullptr.
*/
template <class T>
T* Auto_ptr<T>::release() {
T* temp = value;
value = nullptr;
return temp;
}
/*
Function: reset()
Use: auto_ptr_obj.release(new_pointer);
It deletes the object pointer to by
pointer value and assings new_pointer;
*/
template <class T>
void Auto_ptr<T>::reset(T* v) {
delete value;
value = v;
}
#endif
main.cpp:
#include <iostream>
#include "smart_ptr.h"
#include "assert.h"
//=====================================================================
void test1 () {
std::cout <<"\nTest constructor and get() member.\n";
Auto_ptr<int> p(new int);
*p.get() = 5;
std::cout <<"p points to: "<< *p << "\n";
//assert(*p, 5);
std::cout <<"TEST 1 DONE\n";
}
void test2 () {
std::cout <<"\nTest reset() and release() members.\n";
Auto_ptr<int> p(new int);
*p.get() = 5;
std::cout <<"p points to: "<< *p << "\n";
p.reset(new int(10));
std::cout <<"reset() p points to: "<< *p << "\n";
//assert(*p, 10);
int *temp = p.release();
std::cout <<"caller of release(), temp points to: "<< *temp << "\n";
//assert(*temp, 10);
// nullptr dereferece error
// std::cout <<"p after being release()d points to: "<< *p << "\n";
std::cout <<"TEST 2 DONE\n";
}
void test3 () {
std::cout <<"\nTest copy constructor and copy assignment.\n";
Auto_ptr<int> p1(new int(10));
Auto_ptr<int> p2(p1);
std::cout <<"copy constructed p2 points to: "<< *p2 << "\n";
//assert(*p2, 10);
Auto_ptr<int> p3(new int(20));
p1 = p3;
std::cout <<"copy assigned p1 points to: "<< *p1 << "\n";
//assert(*p1, 20);
std::cout <<"TEST 3 DONE\n";
}
//=====================================================================
int main () {
test1 ();
test2 ();
test3 ();
getchar();
}
The interesting note is that in the Live example the error in not reproducible.
What could be the cause of this message ?

Answer 1

This is generally the result of a "double delete" - in this case I suspect it is your copy assignment.
You assign value = p.value; and both copies hold the same value and both attempt to delete it.
You essentially need to move the pointer from one object to the other, as is the case with the copy constructor. You will also need to remove the const as well.