auto that did not copy

#include <iostream>
#include <vector>

int main() {
    std::vector<bool> v = {true, false};
    auto first = v[0]; // not a bool
    bool copy = v[0];  // a real bool
    v[0] = false;
    std::cout << "first=" << first << " copy=" << copy << '\n';
}

vector<bool> is specialized — v[0] returns a proxy object that refers to the bit, not a bool. auto deduces the proxy type, so first still points at the bit and follows the later change; bool copy takes a real copy.

Observed output: first=0 copy=1.

Fix: spell the type (bool first = v[0];) or avoid vector<bool> where value semantics matter.

Lesson: auto deduces what the expression returns, and for vector<bool> that is a proxy.

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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