Add a common mistake case for header-only classes in components build doc

Add a common mistake case for header-only classes in components build doc

R=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2686133003
Cr-Commit-Position: refs/heads/master@{#451390}
Committed: https://chromium.googlesource.com/chromium/src/+/e0ee33c5a7febbfffbe75d12d21a7eb159d88d73

[brettw, 2017-02-09 21:58:44]

LGTM with change

https://codereview.chromium.org/2686133003/diff/1/docs/component_build.md
File docs/component_build.md (right):

https://codereview.chromium.org/2686133003/diff/1/docs/component_build.md#new...
docs/component_build.md:257: ### Exporting a symbol that is not linked in your
component
I think the confusion you ran into is specific to header implementations. I
think (hope?) other instances of not defining the implementation in your module
will be pretty obvious.

So I'd make this more direct:

### Exporting functions and classes implemented in headers

When you implement a symbol in a header the compiler will put that in every
necessary translation unit and the linker will pick one. If the symbol is never
referenced by code in the shared library it's supposed exported from, it will
never be instantiated and never exported. The result will be undefined external
symbol errors when linking. Exported symbols should be declared in a header but
always implemented in a .cc file.

[majidvp, 2017-02-17 20:35:20]

I used your wording.

[majidvp, 2017-02-17 20:35:23]

The CQ bit was checked by majidvp@chromium.org

[majidvp, 2017-02-17 20:35:23]

The patchset sent to the CQ was uploaded after l-g-t-m from brettw@chromium.org
Link to the patchset: https://codereview.chromium.org/2686133003/#ps20001
(title: "Use brettw wording")

[commit-bot: I haz the power, 2017-02-17 20:36:12]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-02-17 22:41:26]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1487363723232930,
"parent_rev": "2e044871bfa429df521ccbb61f80f244b103dc3f", "commit_rev":
"e0ee33c5a7febbfffbe75d12d21a7eb159d88d73"}

[commit-bot: I haz the power, 2017-02-17 22:42:02]

Description was changed from

==========
Add a common mistake case for header-only classes in components build doc

R=brettw@chromium.org
==========

to

==========
Add a common mistake case for header-only classes in components build doc

R=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2686133003
Cr-Commit-Position: refs/heads/master@{#451390}
Committed:
https://chromium.googlesource.com/chromium/src/+/e0ee33c5a7febbfffbe75d12d21a...
==========

[commit-bot: I haz the power, 2017-02-17 22:42:03]

Committed patchset #2 (id:20001) as
https://chromium.googlesource.com/chromium/src/+/e0ee33c5a7febbfffbe75d12d21a...