限制迭代器是否有效 | 限制迭代器是否有效

本文介绍了限制迭代器是否有效的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我在实际生产代码中发现了以下内容.我怀疑它实际上具有未定义的行为，但是，我找不到有关cppreference的相关信息.您能否确认这是UB或有效代码，以及为什么这是UB/有效(最好带有标准引号)?

I found the following in actual production code.My suspicion is that it actually has undefined behavior into it, however, I couldn't find the related info on cppreference. Can you confirm this is UB or valid code and why this is UB/valid (preferably with a quote of the standard)?

#include <vector>

int main(int, char **)
{
    auto v = std::vector<int>({1,2,3,4,5});
    auto begin = v.begin();
    auto outOfRange = begin + 10;
    auto end = v.end();
    auto clamped = std::min(outOfRange, end);
    return (clamped == end) ? 0 : 42;
}

编译器资源管理器上的代码

如您所见，begin + 10将创建一个超出std::vector范围的迭代器.但是，由于使用std::min进行了限制，因此未使用该迭代器.

As you can see begin + 10 will create an iterator that's out of range of the std::vector.However, that iterator ain't being used, as it is clamped using std::min.

推荐答案

根据标准§5/5.7 :

如果为gcc打开迭代器调试，则可以验证这一点

You can verify this if you turn on iterator debugging for gcc

# g++ main.cpp -D_GLIBCXX_DEBUG -o main
# ./main
C:/mingw-w64/i686-8.1.0-win32-dwarf-rt_v6-rev0/mingw32/lib/gcc/i686-w64-mingw32/8.1.0/include/c++/debug/safe_iterator.h:374:
Error: attempt to advance a dereferenceable (start-of-sequence) iterator 10
steps, which falls outside its valid range.

Objects involved in the operation:
    iterator @ 0x0061FE3C {
      type = __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<int*, std::__cxx1998::vector<int, std::allocator<int> > >, std::__debug::vector<int, std::allocator<int> > > (mutable iterator);
      state = dereferenceable (start-of-sequence);
      references sequence with type 'std::__debug::vector<int, std::allocator<int> >' @ 0x0061FE50
    }

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