How does =delete on destructor prevent stack allocation?

Question

In this SO question is stated that this construct prevents stack allocation of instance.

class FS_Only {
    ~FS_Only() = delete;  // disallow stack allocation
};

My question is, how does it prevents allocation? I understand, that is not possible to delete this instance, either explicitly or implicitly. But I think, that would lead to memory leak or run time error, respectively.

Are compilers smart enough to sort this out and raise compiler error? Also why would one need this?

Answer 1

The destructor of a variable with automatic storage duration (i.e. a local variable) would need to run when the variable's lifetime ends. If there is no accessible destructor the compiler refuses to compile the code that allocates such a variable.

Basically the distinction between "stack allocation" (an inaccurate choice of term by the way) and free store allocation is that with local variables constructor/destructor calls always come in pairs, whereas with free store allocation you can construct an object without ever destructing it. Therefore by preventing access to the destructor your code makes it impossible to allocate a local variable of the type (if the constructor runs the destructor must also run, but there is no destructor so the program is rejected).

Answer 2

I understand, that is not possible to delete this instance, either explicitly or implicitly.

More than that, it's not possible to destroy any instance; whether by deleting it or otherwise.

Declaring an automatic variable (or "stack allocation", if you like) doesn't just cause the instance to be created when the program reaches the point of declaration; it also causes it to be destroyed when the program leaves that block. With a deleted destructor, that can't be done, so the declaration is not allowed.

This also prevents you from declaring a static or thread-local variable, since that also generates code to destroy the variable when the program or thread ends.

So the only way to create one of these is with new, and once you've done that you can never destroy it. However, this doesn't altogether prevent stack allocation:

char memory[sizeof(FS_Only)] alignas(FS_Only);
FS_Only * not_fs = new (memory) FS_Only;

Also why would one need this?

In my view, you wouldn't. A mandatory memory leak is a horrible way to ensure that an object is never destroyed at the wrong time. Instead, use techniques such as RAII to manage any objects that need a dynamic lifetime, ensuring that they always have a well defined owner (or shared owners) responsible for deleting them after use. The smart pointers in the C++11 standard library are a good starting point.