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0 pointscolonwqbang1y ago0 comments

C++ build systems are typically based on file timestamps. Modifying a header file triggers recompilation of all translation units including that header.

There are workarounds like pimpl (aka. C style encapsulation). But this requires extra boilerplate and indirection. C++ modules might fix it at some point, but after 35 years of not having them in C++ most real life codebases aren't set up that way and may never be.

0 comments

gumby1y ago

I don't think of pimpl as a tool for speeding up compilation, but for black box encapsulation.

If the compile time (when adding a method) is really an issue you can chop up and reconfigure your include files. A pain, but perhaps saves you time in the long run.

Of course (waves hands) modules will magically improve things...someday.

colonwqbangOP1y ago

I think it can be both things.

Haven't you ever seen someone do

    struct Thing;
    struct OtherThing;

in lieu of just including "thing.h"? I see it frequently in real life code bases and I can't see a reason for it other than compilation time optimisation.

gumby1y ago

Sure, I do that all the time too. But you can't call a method (or look inside Thing, or pass it as an argument, only a pointer to it) without including the definition.

Hmm, there might be some interesting linker hacks to patch things up post compilation. But then you'd want some way to do the forward declaration for cases where Thing could have been passed in registers...

DerekL1y ago

This is sometimes required to break dependency cycles. Also, you can use this to rearrange declarations in the same file.

germandiago1y ago

Pimpl is both. I use it as black box also, but it does both things.

j / k navigate · click thread line to collapse

0 pointscolonwqbang1y ago0 comments

C++ build systems are typically based on file timestamps. Modifying a header file triggers recompilation of all translation units including that header.

0 comments

gumby1y ago

I don't think of pimpl as a tool for speeding up compilation, but for black box encapsulation.

If the compile time (when adding a method) is really an issue you can chop up and reconfigure your include files. A pain, but perhaps saves you time in the long run.

Of course (waves hands) modules will magically improve things...someday.

colonwqbangOP1y ago

I think it can be both things.

Haven't you ever seen someone do

    struct Thing;
    struct OtherThing;

in lieu of just including "thing.h"? I see it frequently in real life code bases and I can't see a reason for it other than compilation time optimisation.

gumby1y ago

Sure, I do that all the time too. But you can't call a method (or look inside Thing, or pass it as an argument, only a pointer to it) without including the definition.

DerekL1y ago

This is sometimes required to break dependency cycles. Also, you can use this to rearrange declarations in the same file.

germandiago1y ago

Pimpl is both. I use it as black box also, but it does both things.

j / k navigate · click thread line to collapse