Ошибка	Последствие
Определение не-`inline` функции в заголовке	`multiple definition` на этапе линковки
`return std::move(local);`	Подавление NRVO — копирование/перемещение вместо элизии
Передача большого объекта по значению вместо `const&`	Лишняя копия на горячем пути
Использование `T*` там, где нужна непустая ссылка	Теряется гарантия «не `nullptr`»
Считать `T&&` в шаблоне всегда rvalue-ссылкой	Это forwarding reference — связывается и с lvalue
Изменение дефолтного аргумента в заголовке	Тихий ABI-break — клиенты с разными дефолтами
Две перегрузки, отличающиеся только типом возврата	Ошибка компиляции — return type не в сигнатуре
Скрытие наследованных перегрузок без `using Base::f`	`d.f(1)` не компилируется, хотя `Base::f(int)` существует
`noexcept` на функции, которая может бросить	`std::terminate` при первом же исключении
Глубокая рекурсия без хвостовой оптимизации	Stack overflow в release; разный результат в debug/release

beginner

Functions

Declarations, definitions, overloads, parameters, return and stack, functors and lambdas — functions as first-class citizens in C++.

Functions

Practice: 32 questions · 1 task →

Sections

Contents

Detailed explanation

Functions — the unit of structure and the contract between TUs

Functions in C++ are not "named blocks of code" — they are the basic unit of contract between translation units. How a function is declared, how it accepts arguments, how it returns, and what guarantees it makes (noexcept, [[nodiscard]]) decides the library's interface, the optimizer's behavior, and the set of errors you can catch at build time.

Many subtle C++ bugs — dangling references, surprise copies, ODR violations, ambiguous overloads — are not bugs in the function body. They are bugs in the signature. So it pays to think of a function as a structure with five independent dimensions: where it is declared and defined, how it takes input, how it returns, what guarantees it makes, and how it can be passed around as a value.

C++ has layered extra entities on top of functions over time — functors (classes with operator()), lambdas (anonymous functions with captured context), std::function (a type-erased wrapper). They all obey the same signature and overload rules but offer different trade-offs between flexibility and cost. The full map lives in the layers below.

Topic map

Declarations, definitions, and ODR — declaration vs definition, ODR, inline, default arguments, what may live in a header.
Parameters — value, ref, const&, move — four ways to pass, move semantics, forwarding reference, ref-qualifiers on member functions.
Overloading and overload resolution — what's part of the signature, candidate ranking, ambiguity, name hiding, SFINAE/concepts.
Return values, noexcept, and the stack — RVO/NRVO, structured bindings, noexcept, [[nodiscard]], recursion, stack frame.
Functors — classes with operator(), std::function, std::bind, the cost of type erasure.
Lambda expressions — captures, generic lambdas, mutable, constexpr lambdas, IIFE, recursive lambdas.

Common traps

Mistake	Consequence
Defining a non-`inline` function in a header	`multiple definition` at link time
`return std::move(local);`	Inhibits NRVO — copy/move instead of elision
Passing a large object by value where `const&` would do	One needless copy on a hot path
Using `T*` where a non-null reference belongs	Loses the "not `nullptr`" guarantee
Treating `T&&` in a template as always rvalue	It is a forwarding reference — binds to lvalues too
Changing a default argument in a header	Silent ABI break — clients see different defaults
Two overloads that differ only in return type	Compile error — return type is not part of the signature
Hiding inherited overloads without `using Base::f`	`d.f(1)` doesn't compile even though `Base::f(int)` exists
`noexcept` on a function that can throw	`std::terminate` on the first thrown exception
Deep recursion with no tail-call guarantee	Stack overflow in release; diverging debug/release behavior

Interview relevance

Functions on an interview are a C++ semantics test, not a syntax test. A candidate who confuses declaration with definition, or thinks inline is about optimization, usually does not pass a middle-level loop.

Typical checks:

The difference between declaration and definition, ODR, why inline belongs on a header function.
When to use const&, when to pass T by value, when to use T&&.
What a forwarding reference is and how it differs from an rvalue reference.
What happens with std::vector::push_back if the element's move constructor is not noexcept.
How f(1.0f) resolves when f(int) and f(double) both exist.
What RVO/NRVO are and why return std::move(local) is an anti-pattern.
How std::function differs from a raw function pointer in performance.

Common wrong answer: "inline makes the function faster by forcing the compiler to inline the body." That is exactly the opening to explain that modern optimizers ignore inline as a hint, and the keyword's real purpose is to allow multiple definitions across TUs.

Why it matters

Functions are the basic unit of structure in C++, and many subtle bugs about overloads, copies, ownership, and the stack start here.

Ошибка	Последствие
Определение не-`inline` функции в заголовке	`multiple definition` на этапе линковки
`return std::move(local);`	Подавление NRVO — копирование/перемещение вместо элизии
Передача большого объекта по значению вместо `const&`	Лишняя копия на горячем пути
Использование `T*` там, где нужна непустая ссылка	Теряется гарантия «не `nullptr`»
Считать `T&&` в шаблоне всегда rvalue-ссылкой	Это forwarding reference — связывается и с lvalue
Изменение дефолтного аргумента в заголовке	Тихий ABI-break — клиенты с разными дефолтами
Две перегрузки, отличающиеся только типом возврата	Ошибка компиляции — return type не в сигнатуре
Скрытие наследованных перегрузок без `using Base::f`	`d.f(1)` не компилируется, хотя `Base::f(int)` существует
`noexcept` на функции, которая может бросить	`std::terminate` при первом же исключении
Глубокая рекурсия без хвостовой оптимизации	Stack overflow в release; разный результат в debug/release

beginner

Functions

Declarations, definitions, overloads, parameters, return and stack, functors and lambdas — functions as first-class citizens in C++.

Functions

Practice: 32 questions · 1 task →

Sections

Contents

Detailed explanation

Functions — the unit of structure and the contract between TUs

Topic map

Declarations, definitions, and ODR — declaration vs definition, ODR, inline, default arguments, what may live in a header.
Parameters — value, ref, const&, move — four ways to pass, move semantics, forwarding reference, ref-qualifiers on member functions.
Overloading and overload resolution — what's part of the signature, candidate ranking, ambiguity, name hiding, SFINAE/concepts.
Return values, noexcept, and the stack — RVO/NRVO, structured bindings, noexcept, [[nodiscard]], recursion, stack frame.
Functors — classes with operator(), std::function, std::bind, the cost of type erasure.
Lambda expressions — captures, generic lambdas, mutable, constexpr lambdas, IIFE, recursive lambdas.

Common traps

Mistake	Consequence
Defining a non-`inline` function in a header	`multiple definition` at link time
`return std::move(local);`	Inhibits NRVO — copy/move instead of elision
Passing a large object by value where `const&` would do	One needless copy on a hot path
Using `T*` where a non-null reference belongs	Loses the "not `nullptr`" guarantee
Treating `T&&` in a template as always rvalue	It is a forwarding reference — binds to lvalues too
Changing a default argument in a header	Silent ABI break — clients see different defaults
Two overloads that differ only in return type	Compile error — return type is not part of the signature
Hiding inherited overloads without `using Base::f`	`d.f(1)` doesn't compile even though `Base::f(int)` exists
`noexcept` on a function that can throw	`std::terminate` on the first thrown exception
Deep recursion with no tail-call guarantee	Stack overflow in release; diverging debug/release behavior

Interview relevance

Typical checks:

The difference between declaration and definition, ODR, why inline belongs on a header function.
When to use const&, when to pass T by value, when to use T&&.
What a forwarding reference is and how it differs from an rvalue reference.
What happens with std::vector::push_back if the element's move constructor is not noexcept.
How f(1.0f) resolves when f(int) and f(double) both exist.
What RVO/NRVO are and why return std::move(local) is an anti-pattern.
How std::function differs from a raw function pointer in performance.

Why it matters

Functions are the basic unit of structure in C++, and many subtle bugs about overloads, copies, ownership, and the stack start here.

Функции

Функции — единица структуры и контракт между TU

Карта темы

Частые ошибки и ловушки

Значение для собеседований

Functions

Functions — the unit of structure and the contract between TUs

Topic map

Common traps

Interview relevance

Функции

Функции — единица структуры и контракт между TU

Карта темы

Частые ошибки и ловушки

Значение для собеседований

Functions

Functions — the unit of structure and the contract between TUs

Topic map

Common traps

Interview relevance