why is the error: Pointer being freed was not allocated

Question

#include <iostream>

using namespace std;
int main(int argc, char *argv[]) {
    int* a = new int;
    *a = 5;

    int* b = new int;
    *b = 6;

    int* temp = a;
    delete(a);
    a = b;
    b = temp;

    cout << *temp << endl;
    delete (temp); // error



    return 0;
}

error : malloc: *** error for object 0x7ffdff400630: pointer being freed was not allocated. But if I do not delete(a) , it works.

Answer 1

When you use

int* temp = a;

both a and temp point to the same memory that was allocated in the line

int* a = new int;

When you use

delete(a);

that memory was deallocated. At that time, both temp and a are dangling pointers.

By using

a = b;
b = temp;

you changed a to point to a valid memory but now both b and temp are dangling pointers.

Use of temp in

cout << *temp << endl;
delete (temp); // error

is not correct. Both of them are cause for undefined behavior. With undefined behavior, it is pointless to try to make sense of what happens -- anything can happen.

But if I do not delete(a) , it works.

That makes sense. Without that call, all the pointers continue to be valid till the end of the function.

You should add code to delete both the pointers before the end of the function though. But then you have to keep track of which pointers are valid to be used in the calls to delete .

You can print their values to figure that out.

std::cout << "a: " << (void*)a << std::endl;
std::cout << "b: " << (void*)b << std::endl;
std::cout << "temp: " << (void*)temp << std::endl;

Answer 2

In

int* temp = a;

You make copy of the pointer, So a and temp points to same thing, Then in

delete a;

You delete a so temp becomes a dangling pointer, de-referencing it causes undefined behavior.

Ideally you should do

temp = null; 

just after the delete or for the better use shared pointer that ships with modern CPP.

#include <iostream>
#include <memory>

auto main() -> int
{
    auto ptr = std::make_shared<int>(5);

    std::cout << "ptr value is " << *ptr << std::endl;

    auto ptr1 = ptr; // Reference counted copy

    std::cout << "ptr value is " << *ptr1 << std::endl;
    std::cout << "Total reference count :" << ptr1.use_count() << std::endl;
    ptr.reset(); // Resetting  ptr, thus reference count drops to 1
    std::cout << "Total reference count :" << ptr1.use_count() << std::endl;;

    return 0;
}