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One of the problems with stack allocation is on platforms like the ESP32 and most of the arduinos, they don't give you a lot of stack space. I know usually one grows up and the other grows down but I run out of stack declaring 2kB blocks sometimes so there might be some kind of artificial limit.
I see you're using both #pragma once and #ifdef include guards. Is that really necessary? GCC supports both, going back to at least version 4.8, so the #pragma doesn't even need to be wrapped in an #ifdef _MSC_VER. But maybe you know something I don't, or maybe you're using some other compiler that doesn't understand the #pragma?
Also, picking nits, since I have nothing better to offer, I see that this is a memory pool for contrained memory environments. That must be a constrained, contained memory situation, correct?
1. Identifiers starting with underscores are reserved. If you use them then your program is non-conforming for no good reason.
2. The comparison against capacity in both the static and dynamic classes result in never being able to use the last byte of the pool: The "used()>=capacity" should be "used()>capacity". To test it instantiate a pool of 10 bytes and allocate 6. The (capacity() - used()) is then 4, but a further allocation of 4 fails. A further allocation of 3, on the other hand, succeeds and (capacity() - used()) is then 1.
3. The static pool could benefit from a #warning directive when C is too large. Right now a 8MB C when instantiating it (1024 * 1024 * 8) would almost certainly overflow the stack, and 8MB is not a lot of memory.
1. I thought they were only reserved for globals. I stand corrected.
2. Good catch.
3. The static pool can and often is declared as a global, making it heap/not stack, which is where it's primarily designed to go. DynamicMemoryPool is probably a better choice if you need a locally scoped pool because it always allocates from the heap.