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A startling pattern has emerged across multiple top-tier large language models (LLMs): They consistently hallucinate about a fundamental behavior of C++'s std::vector. When asked about element destruction order during vector teardown, models including Gemini 3 Pro, ChatGPT 5.2, and Claude 4.5 Opus confidently—and incorrectly—insist elements are destructed last-in-first-out (LIFO).

This misconception was exposed through practical testing. As reported by a developer, a straightforward program demonstrates actual destruction order:

#include <vector>
#include <iostream>

struct A {
    int i{};
    A(int i_) : i(i_) {}
    ~A() { std::cout << "Destruct " << i << std::endl; }
};

int main() {
    std::vector<A> vec;
    vec.reserve(5);
    vec.emplace_back(1);
    vec.emplace_back(2);
}

Output:

Destruct 1
Destruct 2

The result clearly shows first-in-first-out (FIFO) destruction, contradicting every LLM's response. When pressed, some models backtracked, vaguely suggesting the behavior is "implementation-dependent" or referencing outdated Stack Overflow discussions—despite the C++ standard explicitly mandating element destruction in creation order.

Why Do LLMs Get This Wrong?

Analysis suggests LLMs confuse std::vector destruction with class member destruction rules:
- Class members are destroyed in reverse declaration order (LIFO), a detail LLMs overgeneralize.
- std::vector stores elements contiguously and destroys them sequentially from first to last during its own destruction—a critical distinction.

This isn't merely academic. Developers relying on LLMs for nuanced C++ guidance risk introducing subtle bugs, especially in resource-heavy scenarios:

"Assuming LIFO destruction could cause resource leaks or double-free errors if later elements hold dependencies on earlier ones," explains a veteran C++ engineer. "The standard's FIFO guarantee exists precisely to enable predictable cleanup."

The Path Forward

This incident underscores two realities:
1. LLMs struggle with precise technical specifications, especially when reasoning contradicts common patterns (like class member destruction).
2. Empirical verification remains essential—even for "basic" questions. Tools like Godbolt are indispensable for validating compiler behavior.

The author predicts LLMs will correct this hallucination within six months as this analysis enters training data. Until then, developers should treat LLM-generated C++ advice with healthy skepticism—and keep their compilers closer than their chatbots.

Source: am17an.bearblog.dev