Is std::vector::reserve(0); legal?

Question

Is std::vector::reserve(0); legal and what will it do?

Answer 1

There's nothing to prohibit it. The effect of reserve is:

After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of reserve().¹

Since the value of capacity() can never be less than 0 (it's unsigned), this can never have any effect; it can never cause a reallocation.

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^{1. c++ standard, [vector.capacity]}