Hi! I'm getting started with capnp, using C++ bindings (with C++17 compiled
by Clang 12.) I'm having trouble storing remote object references, i.e.
xx::Client objects, in STL collections. For example:
std::map<string,Foobar::Client> foobars;
...
Foobar::Client c = promise.wait(waitScope).getFoobar();
foobars.insert({name, c}); // Compile error
The error is "*the parameter for this explicitly-defaulted copy constructor
is const, but a member or base requires it to be non-const*."
I eventually worked out that the error is because *Client's copy
constructor takes a `Client&` parameter, i.e. requires a mutable reference,
which breaks the std::pair class* because it needs to be able to copy a
const reference. And if you can't put a client into a std::pair, you can't
put it into a std::map or std::unordered_map.
I assume there must be a reason for leaving out the standard `const` in the
copy constructor's parameter; but I can't work out what it would be. I know
Client objects are wrappers for a pointer to a ref-counted value, so
copy-constructing one bumps the value's ref-count, but that value is a
separate object so it shouldn't matter whether the Client reference is
const or not. (FYI, I've implemented my own ref-counted objects in C++ so
I'm pretty familiar with the details...)
Is there a good workaround for storing RPC client objects in STL maps?
Thanks!
--Jens
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