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Ahmedelkomy
Asked: 2026-09-11T21:57:14-05:00 In:

Why does std::views::filter on temporary containers trigger undefined behavior and dangling references in C++20?

I wrote the following modern C++20 pipeline to filter records returned by a database query helper function:

#include <iostream>
#include <vector>
#include <ranges>

std::vector<int> getTemperatures() {
    return {18, 25, 32, 14, 29, 36};
}

int main() {
    // Pipeline on temporary container returned by value
    auto warm_days = getTemperatures() | std::views::filter([](int t) { return t > 20; });

    for (int t : warm_days) {
        std::cout << t << " "; // Undefined Behavior / Garbage output!
    }
}

When compiling with GCC 13 or Clang 17 with -std=c++20, the loop prints garbage values or crashes with a segmentation fault. Why does this trigger undefined behavior when standard STL algorithms work fine?

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1 Answer

  1. Abhishek
    Begginer

    Direct Technical Solution: In C++20, range view adaptors (like std::views::filter, std::views::transform, and std::views::take) are strictly non-owning view wrappers. They do not duplicate or take ownership of the underlying container; they only store iterators pointing directly into the underlying sequence.

    In your code, getTemperatures() returns a temporary std::vector<int> by value. At the semicolon ending the initialization expression auto warm_days = getTemperatures() | ...;, the temporary vector reaches the end of its full-expression lifetime and is immediately destructed. Consequently, the iterators stored inside warm_days become dangling pointers into deallocated stack/heap memory, causing undefined behavior upon iteration.

    Modern C++20 Fix (Lifetime Preservation)

    #include <iostream>
#include <vector>
#include <ranges>

std::vector<int> getTemperatures() {
    return {18, 25, 32, 14, 29, 36};
}

int main() {
    // Solution 1: Bind the temporary to a named local variable
    // This extends the container lifetime across the entire scope.
    const auto temps = getTemperatures();
    auto warm_days = temps | std::views::filter([](int t) { return t > 20; });

    std::cout << "Warm days: ";
    for (int t : warm_days) {
        std::cout << t << " ";
    }
    std::cout << "
";

    return 0;
}
    • 0