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https://issues.apache.org/jira/browse/PROTON-1288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15446616#comment-15446616
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Alan Conway commented on PROTON-1288:
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That also addresses the "null vs. default value" issue in a consistent way
which is good, I'm almost sold. Here's my conflict:
- Performance-wise it would be nice to return a scalar rather than a value for
properties that are scalars. Actually in practice the underlying C has
pn_data_t's and we can return a ref but if it's assigned to a new value that's
a needless allocatin.
- Type-safety wise it would be nice to return restricted scalars, like a
proton::string_scalar which can only be std::string or null - similar to how we
handle message_id.
- but API wise it would be nice to have consistent names like foo_value()
rather than foo_scalar(), bar_value() etc. :(
We could combine both like this:
proton::string_scalar message::reply_to_value();
Now the following works since scalars can be assigned to a value:
value rt = m.reply_to_value();
if (m.reply_to_value().empty()) ...
if (m.reply_to_value()type() == ) ...
But so does this, and is more type-safe efficient:
string_scalar rt = m.reply_to_value();
I think that might be the best of both worlds. We could provide a uniform
setter that takes a value:
void message::reply_to(const value& v);
That willl accept scalar_string or a value containing a string. That allows you
to efficiently set a missing value since an proton::value() is just null
pointer. We could add a const string_scalar& setter if we really cared but I
suspect we never will.
> c++ provide access to message maps as proton::value
> ---------------------------------------------------
>
> Key: PROTON-1288
> URL: https://issues.apache.org/jira/browse/PROTON-1288
> Project: Qpid Proton
> Issue Type: Bug
> Components: cpp-binding
> Affects Versions: 0.14.0
> Reporter: Alan Conway
> Assignee: Alan Conway
> Fix For: 0.15.0
>
>
> We need to provide access to the message property maps as a proton::value.
> Given
> {code}
> std::map<string, scalar> my_map;
> proton::message m;
> {code}
> Here are some options:
> 1. Add new message accessor: proton::value& properties_value()
> proton::get(m.properties_value(), my_map)
> 2. Add value() accessor to cached_map
> proton::get(m.properties().value(), my_map)
> 3. Make cached_map : public proton::value()
> proton::get(m.properties(), my_map)
> 3 is neatest, 1 provides a clearer separation between the case where you want
> a convenient cached map vs. you want to decode to your own C++ map, work on
> it, and possibly re-encode it later. I lean towards 3. but open to persuasion.
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