Is it OK to discard placement new return value when initializing objects

Question

This question originates from the comment section in this thread, and has also got an answer there. However, I think it is too important to be left in the comment section only. So I made this Q&A for it.

Placement new can be used to initialize objects at allocated storage, e.g.,

using vec_t = std::vector<int>;
auto p = (vec_t*)operator new(sizeof(vec_t));
new(p) vec_t{1, 2, 3}; // initialize a vec_t at p

According to cppref,

Placement new

If placement_params are provided, they are passed to the allocation function as additional arguments. Such allocation functions are known as "placement new", after the standard allocation function void* operator new(std::size_t, void*), which simply returns its second argument unchanged. This is used to construct objects in allocated storage [...]

That means new(p) vec_t{1, 2, 3} simply returns p, and p = new(p) vec_t{1, 2, 3} looks redundant. Is it really OK to ignore the return value?

Answer 1

Ignoring the return value is not OK both pedantically and practically.

From a pedantic point of view

For p = new(p) T{...}, p qualifies as a pointer to an object created by a new-expression, which does not hold for new(p) T{...}, despite the fact that the value is the same. In the latter case, it only qualifies as pointer to an allocated storage.

The non-allocating global allocation function returns its argument with no side effect implied, but a new-expression (placement or not) always returns a pointer to the object it creates, even if it happens to use that allocation function.

Per cppref's description about the delete-expression (emphasis mine):

For the first (non-array) form, expression must be a pointer to a object type or a class type contextually implicitly convertible to such pointer, and its value must be either null or pointer to a non-array object created by a new-expression, or a pointer to a base subobject of a non-array object created by a new-expression. If expression is anything else, including if it is a pointer obtained by the array form of new-expression, the behavior is undefined.

Failing to p = new(p) T{...} therefore makes delete p undefined behavior.

From a practical point of view

Technically, without p = new(p) T{...}, p does not point to the newly-initialized T, despite the fact that the value (memory address) is the same. The compiler may therefore assume that p still refers to the T that was there before the placement new. Consider the code

p = new(p) T{...} // (1)
...
new(p) T{...} // (2)

Even after (2), the compiler may assume that p still refers to the old value initialized at (1), and make incorrect optimizations thereby. For example, if T had a const member, the compiler might cache its value at (1) and still use it even after (2).

p = new(p) T{...} effectively prohibits this assumption. Another way is to use std::launder(), but it is easier and cleaner to just assign the return value of placement new back to p.

Something you may do to avoid the pitfall

template <typename T, typename... Us>
void init(T*& p, Us&&... us) {
  p = new(p) T(std::forward<Us>(us)...);
}

template <typename T, typename... Us>
void list_init(T*& p, Us&&... us) {
  p = new(p) T{std::forward<Us>(us)...};
}

These function templates always set the pointer internally. With std::is_aggregate available since C++17, the solution can be improved by automatically choosing between () and {} syntax based on whether T is an aggregate type.