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AFAIK std::vector does use memcpy (std::copy as well) when objects are trivially copyable.

That is correct yet the compilers definition of what is trivially copyable might be more strict than what you expect. For example, objects that are trivially relocatable can also be memcpy'd for reserve/realloc, but the compiler will not be able to figure that on its own.

std::vector itself falls in this category: trivially relocatable, definitely not trivially copyable. So a vector of vectors will not necessarily be able to use memcpy but rather fall back to copy/move assignment. This is not very significant in performance for this type (vector move being cheap) but a language gotcha nonetheless (as the move constructor will be called n times in every capacity change)

Trivially copyable is a word of power (well, two words I guess), it's meaning is well defined and you can statically assert for it.

What unfortunately is not defined is (trivially) relocatable as that's not a property that can be safely be inferred so it is not (yet) part of the standard. Some libraries still have this concept and require some sort of opt in.

Since C++11, you can use template traits to determine if a type is trivially copyable, and even add static_asserts to your code to ensure future changes dont break expectations.

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