A two phase to-string API? First compute the total size of the single allocation then populate the bytes?

I'm trying to see if there is any prior art in the to-string space that accomplishes a two phase approach. In theory, you could design an API that first asks the data "How many bytes would it take to make you into a string?". From there it could allocate memory with that capacity. Then it could provide that allocation to the same data to populate the bytes.

I want it to be very light weight and easy to add to a type. The `AbslHashValue(...)` API really nails the ergonomics of an extension point in C++, imo. But when it comes to to-string, it gets pretty hairy pretty fast.

`AbslHashValue(...)` benefits from the fact that the resulting hash has a fixed bit width. You just combine/combine_contiguous recursively and you're done.

Some hypothetical `MyToString(...)` would need to likely be split into two functions. `MyToStringSize(...)` and `MyToStringValue(...)` which already makes it more obnoxious to add support for in your type.

But it gets worse. What if inclusion of the type names is important? I can imagine wanting a lever at the top level that says do or do not include them. So for the case where you do include them, how do you succinctly compute the length of namespace + scope-resolution-operator + type name.

And what about templates? Do you also include the angle brackets? Do you recursively include type names between them? And what about potential line noise like allocator types? I can see wanting to include them and not wanting to include them.

Further, what about hashtables? If you store the keys and values in separate ranges for a more data oriented design, how do you model the fact that each K-V pair goes together? You don't want to copy them because that might be expensive. So do you supply a proxy object where it has two pointers? Now that means you have to build an entire TYPE inside your type just to support to-string. Not very ergonomic.

Anyway, wanted to discuss this to see if anyone has ideas in this space. It seems to me that to-string as an operation should be unambiguously single allocation. But unless I'm mistaken, `absl::StrCat(...)` and other such APIs only "limit" the number of allocations and cannot put the upper bound at exactly 1.

https://redd.it/1tgqkd2
@r_cpp