destructor on placement-new - c++

Question

Is there a pattern to automatically call a destructor of a placement-new initialized object on the stack when it exits scope? I want to skip the need to memorize to call the destructor explicitly. Or, is there a different method than the placement-new to construct a stack based object with a variable size data[] tail? I use g++.
/* g++ f.cpp -o f.exe */
/* 8< --- f.cpp ---- */
#include <stdio.h>
#include <stdlib.h>
#include <string>
class aclass {
public:
aclass(int size) : size_(size) {};
~aclass() { /* do something */ };
int size_;
char data[0];
};
void f(int size)
{
char v[sizeof(aclass) + size];
aclass *p = new(static_cast<void*>(&v)) aclass(size);
p->~aclass();
}
int main(int argc, char **argv)
{
f(10);
f(100);
return 0;
}

Answer 1

You can create a wrapper class like this:
template <typename T>
class Foo {
private:
T *m_ptr;
public:
Foo(void *area, int size) {
m_ptr = new(area) T(size);
}
Foo(const Foo &) = delete;
~Foo() {
m_ptr->~T();
}
void operator=(const Foo &) = delete;
T *operator->() {
return m_ptr;
}
};
Usage:
void f(int size) {
char v[sizeof(aclass)+size];
Foo<aclass> p(v, size);
p->doSomething(); // call a function from aclass
}
Note that you're using a GCC extension, as size is not a compile-time constant.
If it was a compile-time constant, then you could put v into Foo (and size would be a template parameter), so f would be simpler.

Answer 2

There is already a standard wrapper template class which does this - std::unique_ptr.
Note the caveats in the comments
#include <cstdio>
#include <cstdlib>
#include <string>
#include <memory>
class aclass {
public:
aclass(int size);
~aclass();
int size_;
char data[0]; // this is illegal in standard c++
};
// deleter which only calls destructor, does not deallocate
struct inplace_deleter
{
template<class T>void operator()(T* p) const noexcept
{
p->~T();
}
};
void f(int size)
{
// VLAs are not standard c++. This is a compiler extension on gcc.
char v[sizeof(aclass) + size];
using ptr_type = std::unique_ptr<aclass, inplace_deleter>;
auto p = ptr_type(new(&v) aclass(size));
// auto deleted
}
int main()
{
f(10);
f(100);
return 0;
}
https://godbolt.org/z/qEwld-