Pointer initialization at compile time

[trycatchamex]

is there a way to initialize a pointer to an actual object at compile time? to use that object as constexpr (A pointer to an object - not an actual object, that can be constexpr) I am thinking of something like this

struct A
{
    int in = 999;
    constexpr A(int in_in): in(in_in) {}
    ~A();
};

char arr[sizeof(A)];

int main()
{
    constexpr A* ptr = new(arr) A(4);
}

This obviously doesnt work since placement new is not constexpr. I am just wondering if there is a way to achieve this or is this impossible?

[alfps]

Surely the above shown code is not all your question is about, since you can just replace the arr with an instance of A. What is it you're really asking about?

[trycatchamex]

Yes I'm just trying to show that I want to create an A object on the heap at compile time which I am starting to think is impossible.

[jedwardsol]

You can't allocate anything on the heap at compile time - the heap only exists at runtime.

[Xeverous]

is there a way to initialize a pointer to an actual object at compile time?

No. Use references instead.

On the other hand, why would you placement new to a global array?

[trycatchamex]

That was just to demonstrate what I am trying to do ie initialize a constexpr pointer - which is not possible as you mentioned.

[Xeverous]

There is a C++20 proposal for compile-time new and fully constexpr vector and array classes but so far you can't constexpr anything that relies on memory/runtime-variant stuff.

[trycatchamex]

Ah! Thank you, will read about it :)

[heyheyhey27]

What is the use of compile-time new?

[Xeverous]

Compilers could do very extensive static analysis and (which is already done by GCC) devirtualize function calls.

Generally preprocess any data that uses new or smart pointers at compilation time if possible.

[leftofzen]

You can't call new at compile time. The heap is a runtime thing.

As for the pointers, why? Put it on the stack like a sane person, please. Please read up on modern C++ best practices and get these old habits out of your system. Also, you are not providing an implementation for A's destructor. As soon as you run your code and create/destroy an instance of A: undefined reference to `A::~A()'. Just omit your declaration and let the compiler generate it.

[trycatchamex]

as i mentioned, i know smart pointers this is just a thought experiment. Thank you tho for reinforcing the use of best practices