Database-Generator | 一个依赖 IDEA DatabaseTools | Build Tool library

As a general rule, constexpr finishes execution before link time. So you cannot have a constexpr term whose value depends on things in different compilation units.

Your singleton value of "number of relations" thus cannot be calculated at compile time, nor can the index of each relation be calculated at compile time.

As a first attempt, we can get around this by asking each relation to pick a number, then verify there is no conflict. If checked at compile time, this would generate a constexpr value that depends on different compilation units; thus impossible.

We can check for "is the index we picked in bounds" at compile time. We can check "is there a collision" at runtime.

If we do this at static initialization time, and we require that a single line be inserted at main, we can detect if there is wasted space as well.

Replace the generation of DBs with a requirement that each relation claim an index. Have the indexes be checked to be in bound. At runtime, at the start of main, validate that there are no holes in your index table. This keeps relations decoupled from each other (they don't have to know about each other).

Now, this won't work if you expect to have an engine reused in multiple different projects with a different number of relations, because one will develop holes that the other doesn't want.

We could back up our requirement; remove the need to manually write constexpr indexes, and have those also generated at startup (like your code does). We simply add in runtime checks for "am I beyond bounds" and another check for "did we waste space" injected in helper at the start of main.

Now we get decoupling, no wasted indexes, and no manual index management. There is a trivial startup cost (which basically matches what you pay with the above code).