use of std::experimental::optional with g++ v8.2.0

Question

What was once known as std::experimental::optional is now known as std::optional in C++17.

But some libraries -- such as libpqxx -- haven't yet been updated to drop the experimental namespace.

So now with the new version of Ubuntu 18.10 which comes with g++ v8.2.0, attempting to compile projects that use libpqxx results in errors like this:

/usr/include/pqxx/internal/statement_parameters.hxx:213:13: error: ‘experimental’ in namespace ‘std’ does not name a type const std::experimental::optional<Arg> &arg) ^~~~~~~~~~~~ /usr/include/pqxx/internal/statement_parameters.hxx:213:35: error: expected unqualified-id before ‘<’ token const std::experimental::optional<Arg> &arg) ^ /usr/include/pqxx/internal/statement_parameters.hxx:213:35: error: expected ‘)’ before ‘<’ token const std::experimental::optional<Arg> &arg) ^ )

Is there some sort of flag I can pass in to g++ so it defines that old experimental namespace?

These are the relevant version numbers in Ubuntu 18.10:

> dpkg -l | egrep "libpqxx|g\+\+" ii g++ 4:8.2.0-1ubuntu1 amd64 GNU C++ compiler ii g++-8 8.2.0-7ubuntu1 amd64 GNU C++ compiler ii libpqxx-6.2 6.2.4-4 amd64 C++ library to connect to PostgreSQL ii libpqxx-dev 6.2.4-4 amd64 C++ library to connect to PostgreSQL (development files)

experimental::optional still exists in libstdc++, and is still in the <experimental/optional> header. Is that include just missing? — Barry
– Barry, Commented Oct 22, 2018 at 15:41
Looking at the source code it seems that libpqxx does try to use std::optional when possible, but apparently its build system misdetects this feature. The release note says you can fix the error by defining PQXX_HIDE_EXP_OPTIONAL. — cpplearner
– cpplearner, Commented Oct 22, 2018 at 16:53

Stéphane · Accepted Answer · 2018-10-22 21:06:46Z

As @cpplearner pointed out in a comment above, libpqxx has a macro (PQXX_HIDE_EXP_OPTIONAL) you can define prior to including the pqxx header files. More of a temporary workaround than a solution, but in my case it allowed me to get past the error and working on the code I needed.

This is the definition I added to my cmake file:

ADD_DEFINITIONS ( -DPQXX_HIDE_EXP_OPTIONAL )

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