Using std::vector<int>::back as projection for std::ranges::sort [duplicate]

Question

I was sorting a vector of vectors based on the last element of the individual vectors. But couldn't get it to work. Copilot gave me the following example:

#include <algorithm> #include <iostream> #include <vector> #include <ranges> int main() { std::vector<std::vector<int>> v = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; std::ranges::sort(v, {}, &std::vector<int>::back); for (const auto& vec : v) { for (const auto& elem : vec) { std::cout << elem << " "; } std::cout << "\n"; } }

But it doesn't work with Godbolt g++ x86-64 trunk using -std=c++2b. It complains about "unresolved overloaded function type" for the third parameter (&std::vector::back).

My question, is this supposed to work?

Here is a link to godbolt.org, https://godbolt.org/z/qrbGcc48f.

There are two overloads of back, and the compiler doesn't know which one to pick. — Some programmer dude
– Some programmer dude, Commented Dec 5, 2023 at 5:34
This also shows the dangers with using current AI models when it comes to code. They are good at copy-pasting code together in a way that looks acceptable, but can contain subtle errors. In this case the compiler caught the error, but that's not always guaranteed. You need to spend time refining your prompts, add information about things that it does wrong, and after a few iterations you might get something that works. — Some programmer dude
– Some programmer dude, Commented Dec 5, 2023 at 5:35
@Someprogrammerdude Is there a way of differentiating the two overloads? — user515430
– user515430, Commented Dec 5, 2023 at 5:42
The behavior of a program is unspecified (possibly ill-formed) if it explicitly or implicitly attempts to form a pointer to a member function in the C++ library. — Richard
– Richard, Commented Dec 5, 2023 at 5:42
Also see Alternative to taking the address of a standard library function / possibly ill-formed behaviour and How to avoid non-standard behavior in taking the address of a standard library function — Richard
– Richard, Commented Dec 5, 2023 at 5:48

user17732522 · Accepted Answer · 2023-12-05 05:41:47Z

Taking the address of standard library functions, including member functions, generally has unspecified behavior (and not compiling is a possible behavior).

How the overloads of the function are specified in the standard or a reference is also irrelevant for this. An implementation is not required to implement overload sets as specified as long as direct calls to the function behave as specified.

Exceptions apply only to specific functions that the standard designates as "addressable".

So you always need to indirect via your own function, easiest with a lambda:

std::ranges::sort(v, {}, [](auto& el){ return el.back(); });

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