问询inplace_merge处理未排序范围的行为及是否属未定义行为

Great question—this gets to a core rule of C++ standard library algorithms that’s easy to overlook if you’re not familiar with how the standard defines requirements and behavior.

Let’s break this down clearly:

• The C++ standard explicitly states that std::inplace_merge requires its input ranges to be sorted (each of the two subranges [first, middle) and [middle, last) must be sorted using the same comparator). This is a mandatory precondition for the function to work as specified.

When you violate a precondition of a standard library algorithm, the result is Undefined Behavior (UB). Here’s what that means in practical terms:

• The C++ standard makes no guarantees about what the program will do. Your test where you ended up with an unsorted array is just one possible outcome—but a compliant compiler could just as easily:
  • Crash with a segmentation fault
  • Produce completely random or garbage output
  • Optimize away the entire inplace_merge call (and potentially surrounding code)
  • Behave differently depending on optimization levels, compiler version, or target platform

Why doesn’t the documentation explicitly call this out? Because it’s a universal rule for all standard library components: if you don’t meet the stated preconditions, UB is the result. The docs don’t repeat this for every single algorithm (like std::sort requiring random-access iterators, or std::upper_bound requiring a sorted range) because it’s assumed you understand this core principle.

So to directly answer your question: Yes, a standards-compliant compiler is absolutely allowed to cause a segmentation fault (or any other unexpected behavior) when std::inplace_merge is called on unsorted ranges. The behavior is completely undefined, and you should never rely on any observed outcome in this scenario.

内容的提问来源于stack exchange，提问作者Erel Segal-Halevi