The loop variable every lambda shares

#include <functional>
#include <iostream>
#include <vector>

int main() {
    std::vector<std::function<void()>> fns;
    int i; // declared OUTSIDE the loop: still alive when the lambdas run
    for (i = 0; i < 3; ++i) {
        fns.push_back([&] { std::cout << i << ' '; });
    }
    // i is 3 by now, and every lambda captured the SAME i by reference.
    for (auto& f : fns) f();
    std::cout << '\n';
}

Every lambda captured i by reference, and there is only one i. By the time the lambdas run, the loop has finished and i is 3 — so all three print 3.

Observed output: 3 3 3.

Fix: capture by value ([i]) or capture a copy ([i = i]).

Lesson: a reference capture shares the variable, it does not snapshot it.

Harder cousin (observed, not shipped as the puzzle): when i is declared inside the for header, the lambdas outlive it — a dangling reference, and the output is outright garbage that changes run to run. Observed once on the box: 1840247600 1840247632 1840247664. That is undefined behavior, not a wrong number: the lesson is that the classic form is a logic bug and the for-header form is worse.

Facts verified 2026-11-04. The code on this page was compiled and run on the box with GCC 16.2.0 at -std=c++23 -Wall -Wextra; the outputs quoted are the observed outputs. MSVC was not tested.

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