On Thu, 2025-02-06 at 11:04 +0000, Frederick Virchanza Gotham via Std-
Proposals wrote:
> I've never written a compiler before -- I've tinkered a bit editing
> parts of g++ and the linker -- but anyway I imagine it works
> something
> like as follows:
>
> I imagine that the compiler maintains a list of all the types (and
> unspecialised templateable types) that have been declared so far in a
> translation unit. Let's call this container "gTranUnitTypes".
> When the compiler encounters something that might be a type, e.g.
> "std::vector<int>", it looks through gTranUnitTypes to see if it's
> already been declared. If it doesn't find it, you get a compiler
> error.
>
> Well, what if a compiler were to be edited as follows:
>
> If it fails to find the type in gTranUnitTypes, it then looks for it
> in another built-in container called "gStdLibraryFwdDeclarations".
> The
> latter container contains a forward declaration of every class type
> in
> the C++ Standard library, approximately a thousand of them.
> gStdLibraryFwdDeclarations is a sorted map where the key is the hash
> of the type's name, so you can do a binary search through it.
> gStdLibraryFwdDeclarations would take up way less than a megabyte in
> memory.
>
> This would mean that we could write a translation unit as follows:
>
> [ beginning of translation unit ]
> std::vector<int> my_global_vec;
> [ end of translation unit ]
>
> and it would compile just fine because the compiler can find a
> forward
> declaration of "std::vector" inside gStdLibraryFwdDeclarations.
>


This already exists:

$ g++ novector.cc
novector.cc:2:6: error: ‘vector’ in namespace ‘std’ does not name a
template type
2 | std::vector<int> a;
| ^~~~~~
novector.cc:1:1: note: ‘std::vector’ is defined in header ‘<vector>’;
this is probably fixable by adding ‘#include <vector>’
+++ |+#include <vector>
1 |


The compiler doesn't forward-declare std::vector here (and it wouldn't
work anyway), but it could if it wanted to.

But I think it's better addressed by a new header, or modules.



> So of course the next question is: ? ? ? ? ? How Will This Affect
> Compile Times ? ? ? ? ?
>
> Well it will only affect compile times for translation units that
> fail
> to find the type in gTranUnitTypes -- that is to say: It will only
> affect compile times for translation units that are going to fail to
> compile anyway. And you can speed up compile times for other
> translation units by getting rid of "#include <vector>".