How to properly free the memory allocated by placement new?

Question

I've been reading somewere that when you use placement new then you have to call the destructor manually.

Consider the folowing code:

   // Allocate memory ourself
char* pMemory = new char[ sizeof(MyClass)];

// Construct the object ourself
MyClass* pMyClass = new( pMemory ) MyClass();

// The destruction of object is our duty.
pMyClass->~MyClass();

As far as I know operator delete normally calls the destructor and then deallocates the memory, right? So why don't we use delete instead?

delete pMyClass;  //what's wrong with that?

in the first case we are forced to set pMyClass to nullptr after we call destructor like this:

pMyClass->~MyClass();
pMyClass = nullptr;  // is that correct?

BUT the destructor did NOT deallocate memory, right? So would that be a memory leak?

I'm confused, can you explain that?

Answer 1

Using the new expression does two things, it calls the function operator new which allocates memory, and then it uses placement new, to create the object in that memory. The delete expression calls the object's destructor, and then calls operator delete. Yeah, the names are confusing.

//normal version                   calls these two functions
MyClass* pMemory = new MyClass;    void* pMemory = operator new(sizeof(MyClass));
                                   MyClass* pMyClass = new( pMemory ) MyClass();
//normal version                   calls these two functions
delete pMemory;                    pMyClass->~MyClass();
                                   operator delete(pMemory);

Since in your case, you used placement new manually, you also need to call the destructor manually. Since you allocated the memory manually, you need to release it manually.

However, placement new is designed to work with internal buffers as well (and other scenarios), where the buffers were not allocated with operator new, which is why you shouldn't call operator delete on them.

#include <type_traits>

struct buffer_struct {
    std::aligned_storage_t<sizeof(MyClass), alignof(MyClass)> buffer;
};
int main() {
    buffer_struct a;
    MyClass* pMyClass = new (&a.buffer) MyClass(); //created inside buffer_struct a
    //stuff
    pMyClass->~MyClass(); //can't use delete, because there's no `new`.
    return 0;
}

The purpose of the buffer_struct class is to create and destroy the storage in whatever way, while main takes care of the construction/destruction of MyClass, note how the two are (almost*) completely separate from each other.

_{*we have to be sure the storage has to be big enough}