range-v3：奇怪的行为

Question

I am trying to play with range-v3 and I encountered a problems : it does not extract values from a vector as I would have wanted.

See the code below:

1. When running, it outputs (0, 0), instead of what I would have thought, ie (1, 0)

2. If I uncomment the line auto pairs = ... , then the result is changed, and the output becomes (33144464,0), although the variable pairs is unused (and the assert will fail)

#include <iostream>
#include <vector>
#include <range/v3/all.hpp>

auto foo()
{
    auto values = std::vector<int> { 1, 0 };
    // auto pairs = std::vector< std::pair<int, int> > { { 1, 0 }, { 0, 1 }, { 0, 0 } };

    return ranges::view::for_each(values, [=](int nb)
    {
        std::cout << " nb=" << nb << std::endl;
        assert( (nb == 0) || (nb == 1) );
        return ranges::yield(nb);
    });
}


int main()
{
    ranges::for_each(foo(), [](auto v) {
        std::cout << v << "\n";
    });
}

This code was compiled with g++ (g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0) and clang++ (clang version 8.0.0 (tags/RELEASE_800/final)) with the following commands:

g++ foo.cpp -std=c++14 -Irange-v3/include -Wall -Wpedantic
clang++ foo.cpp -std=c++14 -Irange-v3/include -Wall -Wpedantic

I am using a fresh clone for ranges-v3, and I can reproduce this on ubuntu and OSX (with AppleClang).

Answer 1

There is a experimental feature coming in clang called -Wlifetime that can be used on godbolt . It gives the following warnings pointing to the return from foo .

[x86-64 clang (experimental -Wlifetime) #1] warning: returning a dangling Pointer [-Wlifetime]
[x86-64 clang (experimental -Wlifetime) #1] note: pointee 'values' left the scope here