What makes C++ FFI painful?

[Hécate]

Hi devs,

Pardon me for the naïve question. I know that C++ FFI is *hard* in 
Haskell. From the perspective of an end-user I have heard that `text`'s 
new UTF-8 validation caused problems when released, as it relied on C++ 
code, but I never had any problems with it personally. From the GHC  
perspective, what makes C++ a difficult language to interface with?

Cheers,
Hécate

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