Why does C++ forbid forward declarations of non-template std:: types?

user_700825 0
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The C++ standard does not allow code to forward declare classes in namespace std, even when they are not templates.

I see no good reason to do this, so I wonder what is the motivation?

It could be a huge compile time improvement. For example std::mutex is a simple class, but the <mutex> header drags in a ton of stuff (at least on my implementation).

If somebody wonders why I am making a distinction between templates and non-templates, then it is because template instantiations can differ significantly based on what template arguments are provided, so I guess that might be trickier to implement/support.

c++20 forward-declaration c++
1 Answers

for example std::mutex is a simple class

How do you know? The standard describes it as a class, but nothing is stopping an implementer from doing

namespace std {
    using mutex = _Internal_detail::_PthreadMutex;
}

Granted, I omitted some (likely very verbose and expert friendly) checks for when this alias should be set. But this is a valid implementation technique!

Now if you add your forward declaration, you will get an error since an alias and a class are different things. That makes such forward declarations inherently non-portable, and as such, they are deemed to produce undefined behavior.

user_8176430
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