2nd/10_Wiki/Topic_Programming/Topic_CPP/CPP_Functions_Lambda.md

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id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
cpp-functions-lambda	C++ Lambda Functions	Programming_Language	draft	conceptual		anonymous function capture clause capture by value vs reference C++ 람다 함수		B	0.87	2026-07-04	2026-07-04			cpp programming-language w3schools functions lambda	https://www.w3schools.com/cpp/cpp_functions_lambda.asp

CPP Lambda Functions

🎯 한 줄 통찰 (One-line insight)

The capture clause's TWO forms — [x] (by value, a frozen COPY at the moment of creation) versus [&x] (by reference, always the LATEST value) — produce genuinely different output for the identical-looking code: the same show() lambda prints 10 in one version and 20 in the other, purely because x was changed to 20 AFTER the lambda was defined — meaning the capture-clause choice determines whether a lambda is a snapshot of the past or a live window into the present. [S1]

🧠 핵심 개념 (Core concepts)

• Lambda function — [capture](parameters) { code }; — an anonymous, throwaway function written inline, with no separate name/declaration required. [S1]
• auto + lambda — a lambda is typically stored via auto varName = [...]() {...}; then called like a normal function (varName();). [S1]
• Capture by value ([x]) — the lambda gets a COPY of x frozen at creation time; later changes to the outer x don't affect the lambda's copy. [S1]
• Capture by reference ([&x]) — the lambda uses the ORIGINAL x; it always sees the latest value, even changes made AFTER the lambda was defined. [S1]
• Passing a lambda as an argument — requires #include <functional> and a function<returnType(paramTypes)> parameter type in the RECEIVING function. [S1]
• Regular function vs. lambda — use a regular function when reused often, needs a clear name, or logic is long; use a lambda for one-off, short, throwaway logic or when passing behavior into another function. [S1]

📖 세부 내용 (Details)

• Basic no-parameter lambda: auto message = []() { cout << "Hello World!\n"; }; message();. [S1]
• Lambda with parameters: auto add = [](int a, int b) { return a + b; }; cout << add(3, 4); // 7. [S1]
• Passing a lambda to another function: void myFunction(function<void()> func) { func(); func(); } auto message = []() { cout << "Hello World!\n"; }; myFunction(message); — requires #include <functional>. [S1]
• Capture by value freezes the value at creation: int x = 10; auto show = [x]() { cout << x; }; show(); // 10, even if x changes afterward. [S1]
• Capture by reference sees the LATEST value: int x = 10; auto show = [&x]() { cout << x; }; x = 20; show(); // 20 — reflects the change made after the lambda was created. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

• 캡처 방식에 따라 동일한 코드가 다른 결과를 냄: [x]로 값 캡처하면 람다 생성 시점의 x 값이 고정되지만, [&x]로 참조 캡처하면 이후 x가 바뀐 값을 그대로 반영한다는 점이 동일한 구조의 두 예제(10 vs 20 출력)로 직접 대비되어 증명됨. [S1]

🛠️ 적용 사례 (Applied in summary)

반복문 안에서 매 순회마다 다른 값을 캡처하는 람다를 즉석에서 정의해 사용하는 패턴(for (int i = 1; i <= 3; i++) { auto show = [i]() {...}; show(); })이 원문에서 직접 실전 활용 사례로 제시됨. [S1]

💻 코드 패턴 (Code patterns)

Capture by value vs. capture by reference producing different output from identical-looking code (C++):

int x = 10;
auto showCopy = [x]() { cout << x; };   // captures a frozen copy
auto showRef = [&x]() { cout << x; };   // captures the live variable
x = 20;
showCopy(); // 10 (unaffected by the later change)
showRef();  // 20 (reflects the latest value)

✅ 검증 상태 및 신뢰도

• 상태: draft
• 검증 단계: conceptual
• 출처 신뢰도: B (W3Schools — widely used educational reference, not a primary standards body)
• 신뢰 점수: 0.87
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: C++ Tutorial
• 관련 개념: CPP Functions Recursion, CPP References, CPP OOP
• 참조 맥락: 함수 섹션 마지막 — 객체지향 프로그래밍 기초(OOP Basics) 섹션으로 이어짐.

📚 출처 (Sources)

• [S1] W3Schools — C++ Lambda Functions — https://www.w3schools.com/cpp/cpp_functions_lambda.asp

📝 변경 이력 (Change history)

• 2026-07-04: Initial draft synthesized from the W3Schools "C++ Lambda Functions" page (Astra wiki-curation, P-Reinforce v3.1 format).