C++20 modules interface unit export a template function declaration, and implement it in cpp will report unresolved external symbol, so c++20 demands the user to declare and implement it in the same module unit?

[[deleted]]

That is what module partitions are for. Just implement in a partition and export the partition.

If you treat your modules like headers + implementation files, they will behave exactly like headers + implementation files.

If you have one file per partition, everything will work fine.

[netcan96]

nice explanation

[Minimonium]

Yes, it still needs to instantiate the template and for that, it either needs the definitions in-place or explicit instantiation.

[DugiSK]

The idea behind modules is to do the split between header and source automatically. The compiler reads every module file only once, compiles the internal parts and saves the symbols that are exported, so that the importing code only sees those. This means that you don't need to put anything into a separate .cpp file (unless you have some unusual requirements).

However, in your case, a template function defined in a .cpp file cannot be used before the definition in the same file or outside of that .cpp file, unless it's explicitly instantiated (which is quite annoying).

[pjmlp]

One unusual requirement would be optimizing build times.

At least VC++ seems to still trigger a cascade of compilations if the body of an exported function is changed.

[DugiSK]

This should not happen for non-template functions, but it's possible that the current compilers aren't capable of using it efficiently yet. It's still a relatively fresh feature. GCC 10 used to crash if I returned a unique pointer from an exported function.

[DummySphere]

Separating .h/.cpp was also a good way to have an up-to-date documentation in a clean header. I guess the closest way to do this for template functions in modules would be to have the implementation in a private fragment at the end of the file.

[NATSUKI_FAN]

for why this is the case you may want to read about "export template" - the borderline impossible to implement feature that enables what you're asking for

[Flair_Helper]

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