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Ensure latched messages are updated on every split #2261

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Ensure latched messages are updated on every split #2261

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018b60a
Ensure latched messages are updated on every split
Jul 29, 2022
4cd4763
Add note regarding map::insert function behavior
Aug 1, 2022
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6 changes: 5 additions & 1 deletion tools/rosbag/src/recorder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -347,7 +347,11 @@ void Recorder::doQueue(const ros::MessageEvent<topic_tools::ShapeShifter const>&
ros::M_string::const_iterator it2 = out.connection_header->find("callerid");
if (it2 != out.connection_header->end())
{
latched_msgs_.insert({{subscriber->getTopic(), it2->second}, out});
auto const result = latched_msgs_.insert({{subscriber->getTopic(), it2->second}, out});
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Suggested change
auto const result = latched_msgs_.insert({{subscriber->getTopic(), it2->second}, out});
latched_msgs_[{subscriber->getTopic(), it2->second}] = out;

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The [] operator does exactly what's expected here, in case the key doesn't exist in the map, it will be created and when it does exist, it will be updated.

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Thanks for the suggestion - the [] operator is indeed cleaner since it removes the need for the extra manual check (in lines 351-354).

I'll try to update it shortly (I don't have the test environment handy anymore).

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@AndreasNagel I tried your suggestion and unfortunately it fails to build:
error: no matching function for call to ‘rosbag::OutgoingMessage::OutgoingMessage()’

I believe this is due to the behavior of the [] operator when a key is not found in the map. From the docs:

If k does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value.

When a key is missing, the [] operator tries to create an OutgoingMessage object with an empty constructor that does not exist: OutgoingMessage(). (The object seems to only have one constructor that requires 4 arguments.)

I didn't find an easy way to go around this without creating an empty constructor for OutgoingMessage - I'd prefer to avoid introducing a change like that.

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Good point, I missed that OutgoingMessage doesn't have a default constructor. I agree, there's no point in creating one just to use the [] operator. There's insert_or_assign in c++17, but nothing with equivalent functionality in c++14, so what you have there is probably as good as it gets. Still hoping, that this change will get merged one day.

if (not result.second) // The map::insert function does not update values of existing keys
{
result.first->second = out;
}
}
}
}
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