Why `std::prev` does not fire an error with an iterator of `std::unordered_set`?

Question

My question is related to the question linked below. Bidirectional iterators in unordered_map?

Since I did not know std::unordered_set does not support bidirectional iterators, I happened to write a code similar to this one.

int main(){
    unordered_set<int> y{4};
    std::cout << *(std::prev(y.end())) << std::endl;
}

This program is COMPILED, but the last line of the code crashed the program. Puzzled by that, I encountered the linked question. However, I still don't understand why this program is compiled instead of throwing error messages while the code in the linked code(which is boost::unordered_set ) cannot be compiled. Could you clarify it?

FYI, I am using Mingw64 with g++ 4.8.2 / Windows 7 / 64 bit environment.

Answer 1

std::prev only produces defined behavior for bidirectional iterators.

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The GNU ISO C++ library (used by GCC 4.8.2) uses std::advance to implement std::prev , and std::advance itself is implemented like this:

• for random access iterators:

   __i += __n; 

• for bidirectional iterators:

   if (__n > 0) while (__n--) ++__i; else while (__n++) --__i; 

• for all other iterators:

   while (__n--) ++__i; 

So you can see that for an iterator of unordered_set , the function actually does not use the operator-- which produces the compiler error in the other question you linked.

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It is your duty to make sure that an iterator passed to std::prev is bidirectional. If that is not the case the C++ standard does not give you any guarantees what happens. GCC chooses to just silently ignore it, but it might as well crash your program.

Answer 2

std::prev可能使用std::advance ，其中，当参数（输入迭代器）不是双向的时，行为是不确定的。