C++
libraries often add new semantics to the language, and that is not so obviously a benefit. Recently I explored Kiwi project (constraint solver). It is easy to use if you solve specific cases, e.g.:
Variable x, y, z;
solver.add(x <= y);
solver.add(z >= x - y);
But if you need it in meta-way (and I always do, I rarely write non-meta code), you have to unwind all the sources[-in-headers] to find out what Expression is, how does one construct one, write dynamic expression builder, etc. Sources do consist mostly of one-line helper methods/getters/constructors that are nested and intercalled 10-15 times per operation. LPSolve was much simpler in API, since you don't mess with variables and add constraints directly as matrix values.
double row[size+1] = { };
/* ...fill row... */
add_constraint(solver, row);
/* ...fill row... */
add_constraint(solver, row);
In the end of the day, both libraries involve identical usage pattern, but for C++ I have to create a convenience unwrapper. This 'convenience' swamp is SO popular in C++. Once you say "++", you have to deal with it all the time, unless you write something really dumb and straightforward. Not to mention most libraries have huge header-only parts due to template obsession. Qt, yeah. It takes forever to compile 20 modules on reasonably priced laptop even with fine-tuned prefix header and -j9.
From the employment point of view, C++ is a nightmare. No one really knows it, and everyone thinks he does. Experienced developers can't tell what explicit constructor is, are confused with move semantics, blink hopelessly on "what is RVO". You actually employ python programmers for sky prices, while all they know is easiest part of C++/stl magic, which is effectively beginner-grade python with low-level bugs and UBs.
>wrap C++ libraries in a C API
Together with "ecosystem" such as std::cool_ptr<std::map<std::bar,std::baz>>. That's a concern.
When I want nice bridgeable object model above C, I use Objective-C or simple embedded language with proper features. Personally, I'm really tired of C++ everywhere.
