When instances of classes derived from the same class need to be created based on certain conditions, they are created on the heap. The puzzle is about how to do it on the stack.
Stack Factory Puzzle
By Alex Marmer
Stack Factory Puzzle -- Alex Marmer
Comments (4)
0
0
I would use a variant but I would have to #include it first (which isn't possible the Begin-End block). Here's my answer nevertheless:
Run the code here: https://coliru.stacked-crooked.com/a/99ae3df37995553f
Yields the expected output. The leading "int" variant template argument is necessary for default variant initialization.
std::variant<int,X0,X1,X2> x;
if (i == 0) {
x.emplace<X0>();
pX = &std::get<X0>(x);
} else if (i == 1) {
x.emplace<X1>();
pX = &std::get<X1>(x);
} else {
x.emplace<X2>();
pX = &std::get<X2>(x);
}
Run the code here: https://coliru.stacked-crooked.com/a/99ae3df37995553f
Yields the expected output. The leading "int" variant template argument is necessary for default variant initialization.
0
0
My solution involves a char array on the stack in conjunction with 'placement new' and a unique_ptr with a lambda deleter function.
https://coliru.stacked-crooked.com/a/602622223a42c3bb
https://coliru.stacked-crooked.com/a/602622223a42c3bb
// Begin
constexpr auto size = std::max(
alignof(X0) - 1 + sizeof(X0),
std::max(
alignof(X1) - 1 + sizeof(X1),
alignof(X2) - 1 + sizeof(X2)
)
);
char memory[size];
if (i == 0) {
void* ptr = memory;
auto space = size;
pX = new (std::align(alignof(X0), sizeof(X0), ptr, space)) X0;
}
else if (i == 1) {
void* ptr = memory;
auto space = size;
pX = new (std::align(alignof(X1), sizeof(X1), ptr, space)) X1;
}
else {
void* ptr = memory;
auto space = size;
pX = new (std::align(alignof(X2), sizeof(X2), ptr, space)) X2;
}
std::unique_ptr<X, void (*)(X*)> x{
pX,
[](X* ptr) { ptr->~X(); }
};
// End
0
0
for default variant initialization:
https://en.cppreference.com/w/cpp/utility/variant/monostate
https://en.cppreference.com/w/cpp/utility/variant/monostate
0
0
https://github.com/amarmer/Stack-Factory-Puzzle/issues/3#issuecomment-420779367
but std::variant::emplace uses placement new too (at least in STL of gcc8.2.1)
but std::variant::emplace uses placement new too (at least in STL of gcc8.2.1)
Add a Comment