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Unique_ptr: pointer being freed was not allocated when emplaced in list

I have a simple C++ program like so:

#include <iostream>
#include <list>
#include <memory>

int main(void) {
std::list<std::unique_ptr<int>> l;
int x = 5;
// throws runtime error: pointer being freed was not allocated
l.emplace_back(&x);
}


When the program is run I obtain the following output:

a.out(59842,0x7fffb13d1380) malloc: *** error for object 0x7ffee81489ac: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
[1] 59842 abort ./a.out

a.out(59842,0x7fffb13d1380) malloc: *** error for object 0x7ffee81489ac: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
[1] 59842 abort ./a.out

This tells me that during the destruction of the list the unique_ptr object that had been created is being destroyed, and during that destruction the pointer to x is being freed when it was never malloced.

x

malloc

However I would expect that the same would occur in this situation where a unique_ptr falls out of scope and is destroyed:

unique_ptr

destroyed

#include <iostream>
#include <list>
#include <memory>

int main(void) {
int x = 5;

// does not throw runtime error when falling out of scope and being destructed
std::unique_ptr<int> p(&x);
}


However when the unique_ptr p above falls out of scope and is destroyed the program runs fine. Why is the first program giving me an error when destroying the unique_ptr but the second one is not? Why does emplacing the unique_ptr in a list cause its destruction to fail?

unique_ptr p

unique_ptr

unique_ptr

2 Answers
2

I think what happened is that the variable p in your second example got just optimized away and thus not resulted in a run-time error.

p

But the error which occurs is clear, it is trying to free a memory which was allocated on a stack. Note that smart pointers are usually used in conjunction with heap allocated memory, not a memory allocated on a stack.

I've copied your code and compiled it:

$ g++ omg.cpp && ./a.exe
Aborted (core dumped)


You may try to add another level of indirection:

int main(void) {
int x = 5;
{
std::unique_ptr<int> p(&x);
}
}


and see if it helps.

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