c++ - Is it dangerous to use placement new on an old object without explicitly calling the destructor first?

Question

I would like to recycle memory for an object rather than deallocating and reconstructing it. Is the following usage of "placement new" safe, assuming that Foo in practice does not contain pointers (but might contain functions)?

Also, is the final delete call safe, and will it correctly call the destructor on the second "new" object, and correctly free the memory afterwards?

#include <new>
struct Foo {
    int hello;
    int world;
};

int main() {
    Foo* foo = new Foo;
    // Do something with foo
    // Done with foo, writing a new version of foo on top of the old one.
    new(foo) Foo();

    delete(foo);
}

The simple example above compiles and runs without errors, but I cannot tell by running it whether it might blow up for some reason in a more complex environment.

Answer 1

It's safe because the object you're overwriting has a trivial destructor. From n3337, chapter 3.8 (Object lifetime):

4 A program may end the lifetime of any object by reusing the storage which the object occupies or by explicitly calling the destructor for an object of a class type with a non-trivial destructor. For an object of a class type with a non-trivial destructor, the program is not required to call the destructor explicitly before the storage which the object occupies is reused or released; however, if there is no explicit call to the destructor or if a delete-expression (5.3.5) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has unde?ned behavior.

The delete call is safe, too. You're calling it on a pointer that you got from new and there's a live object at that place.

And as you hinted in the question, it could invoke undefined behaviour if the destructor is non-trivial and has side effects - you need to call it explicitly in that case. Whether or not the class contains pointers is not directly important - reusing the storage is safe even in that case, but of course, you could introduce memory leaks and other bugs that way.