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April 30, 2025 11:55
Testing revealed difficulties regarding extending json_encode with
user-defined types. In the following chain of calls:
struct S { ... };
json_encode(std::string, std::vector<std::array<S, 6>>); // 1
json_encode(std::ostream, std::vector<std::array<S, 6>>); // 2
json_encode(std::ostream, std::array<S, 6>*, ...); // 3
json_encode(std::ostream, std::array<S, 6>); // 4
json_encode(std::ostream, S*); // 5
json_encode(std::ostream, S); // 6, implemented in the user code
the compiler might only be able to find the user function if it was
called with unqualified names and argument-dependent lookup took
action. Also, without forward declarations provided by this patch the
compiler could not reach step 4 since json_encode for array has been
declared after json_encode for pointers. This suggests possible problems
with user and other STL containers. See
https://stackoverflow.com/questions/28187170/c-enforce-second-pass-name-lookup-in-template-function
https://akrzemi1.wordpress.com/2015/11/19/overload-resolution/
for the discussion.
One possible solution is to declare
template <typename T> void json_encode(std::ostream&, const T&);
and use template specializations instead of overloaded functions, but it
needs special casing to exclude arithmetic types implemented by a
different template --- and possibly it may clash in a similar way with
other user defined types.
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Provide
json_encodeforchar*std::arrayImplement
json_encode_object--- a helper function to construct JSON objects out of variables and literals.There is a subtlety when it comes to extending
json_encodefor user-defined objects that I ran into while working on BUTTON high voltage control implementation, see 8361521. It needs further research. Please keep the commit message for the future.