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docs: clear notes and add examples
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‎reference_to_md/cpp/keyword/friend.md‎

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|
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---
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[Keywords](../keywords.html "cpp/keyword")
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| [alignas](alignas.html "cpp/keyword/alignas")(C++11)
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---
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[alignof](alignof.html "cpp/keyword/alignof")(C++11)
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[and](and.html "cpp/keyword/and")
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[and_eq](and_eq.html "cpp/keyword/and eq")
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[asm](asm.html "cpp/keyword/asm")
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[auto](auto.html "cpp/keyword/auto")(*)
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[bitand](bitand.html "cpp/keyword/bitand")
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[bitor](bitor.html "cpp/keyword/bitor")
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[bool](bool.html "cpp/keyword/bool")
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[break](break.html "cpp/keyword/break")
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[case](case.html "cpp/keyword/case")
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[catch](catch.html "cpp/keyword/catch")
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[char](char.html "cpp/keyword/char")
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[char8_t](char8_t.html "cpp/keyword/char8 t")(C++20)
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[char16_t](char16_t.html "cpp/keyword/char16 t")(C++11)
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[char32_t](char32_t.html "cpp/keyword/char32 t")(C++11)
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[class](class.html "cpp/keyword/class")
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[compl](compl.html "cpp/keyword/compl")
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[concept](concept.html "cpp/keyword/concept")(C++20)
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[const](const.html "cpp/keyword/const")
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[consteval](consteval.html "cpp/keyword/consteval")(C++20)
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[constexpr](constexpr.html "cpp/keyword/constexpr")(C++11)
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[constinit](constinit.html "cpp/keyword/constinit")(C++20)
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[const_cast](const_cast.html "cpp/keyword/const cast")
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[continue](continue.html "cpp/keyword/continue")
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[contract_assert](contract_assert.html "cpp/keyword/contract assert")(C++26)` `
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[co_await](co_await.html "cpp/keyword/co await")(C++20)
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[co_return](co_return.html "cpp/keyword/co return")(C++20)
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[co_yield](co_yield.html "cpp/keyword/co yield")(C++20)
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[decltype](../keywords/decltype.html "cpp/keyword/decltype")(C++11)
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[default](default.html "cpp/keyword/default")
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| [delete](delete.html "cpp/keyword/delete")
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---
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[do](do.html "cpp/keyword/do")
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[double](double.html "cpp/keyword/double")
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[dynamic_cast](dynamic_cast.html "cpp/keyword/dynamic cast")
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[else](else.html "cpp/keyword/else")
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[enum](enum.html "cpp/keyword/enum")
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[explicit](explicit.html "cpp/keyword/explicit")
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[export](export.html "cpp/keyword/export")
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[extern](extern.html "cpp/keyword/extern")
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[false](false.html "cpp/keyword/false")
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[float](float.html "cpp/keyword/float")
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[for](for.html "cpp/keyword/for")
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**friend**
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[goto](goto.html "cpp/keyword/goto")
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[if](../keywords/if.html "cpp/keyword/if")
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[inline](inline.html "cpp/keyword/inline")
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[int](int.html "cpp/keyword/int")
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[long](long.html "cpp/keyword/long")
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[mutable](mutable.html "cpp/keyword/mutable")
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[namespace](namespace.html "cpp/keyword/namespace")
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[new](new.html "cpp/keyword/new")
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[noexcept](noexcept.html "cpp/keyword/noexcept")(C++11)` `
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[not](not.html "cpp/keyword/not")
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[not_eq](not_eq.html "cpp/keyword/not eq")
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[nullptr](nullptr.html "cpp/keyword/nullptr")(C++11)
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[operator](operator.html "cpp/keyword/operator")
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[or](or.html "cpp/keyword/or")
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[or_eq](or_eq.html "cpp/keyword/or eq")
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[private](private.html "cpp/keyword/private")
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[protected](protected.html "cpp/keyword/protected")
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[public](public.html "cpp/keyword/public")
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| [register](register.html "cpp/keyword/register")
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---
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[reinterpret_cast](reinterpret_cast.html "cpp/keyword/reinterpret cast")
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[requires](requires.html "cpp/keyword/requires")(C++20)
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[return](return.html "cpp/keyword/return")
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[short](short.html "cpp/keyword/short")
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[signed](signed.html "cpp/keyword/signed")
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[sizeof](sizeof.html "cpp/keyword/sizeof")(*)
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[static](../keywords/static.html "cpp/keyword/static")
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[static_assert](static_assert.html "cpp/keyword/static assert")(C++11)
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[static_cast](static_cast.html "cpp/keyword/static cast")
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[struct](struct.html "cpp/keyword/struct")
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[switch](switch.html "cpp/keyword/switch")
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[template](template.html "cpp/keyword/template")
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[this](this.html "cpp/keyword/this")
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[thread_local](thread_local.html "cpp/keyword/thread local")(C++11)
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[throw](throw.html "cpp/keyword/throw")
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[true](true.html "cpp/keyword/true")
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[try](try.html "cpp/keyword/try")
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[typedef](typedef.html "cpp/keyword/typedef")
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[typeid](typeid.html "cpp/keyword/typeid")
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[typename](typename.html "cpp/keyword/typename")
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[union](union.html "cpp/keyword/union")
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[unsigned](unsigned.html "cpp/keyword/unsigned")
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[using](using.html "cpp/keyword/using")
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[virtual](virtual.html "cpp/keyword/virtual")
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[void](void.html "cpp/keyword/void")
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[volatile](volatile.html "cpp/keyword/volatile")
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[wchar_t](wchar_t.html "cpp/keyword/wchar t")
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[while](while.html "cpp/keyword/while")
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[xor](xor.html "cpp/keyword/xor")
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[xor_eq](xor_eq.html "cpp/keyword/xor eq")
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### Usage
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* [`friend` specifier](../language/friend.html "cpp/language/friend")
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* [`friend` specifier](../language/friend.md)

‎reference_to_md/cpp/language/examples/friend/README.md‎

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# [friend declaration](declaration.cpp)
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# [syntax](syntax.cpp)
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# TODO:
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# [example](example.cpp)
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* non-member friends
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* use cases
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* stream insertion and extraction operators

‎reference_to_md/cpp/language/examples/friend/declaration.cpp‎

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#include <iostream>
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#include <string>
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// ============================================
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// Friend: grants access to private & protected members
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// Uses: | class body
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// ============================================
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// 1. friend function
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class BankAccount
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{
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private:
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std::string owner;
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double balance;
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protected:
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std::string account_number;
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public:
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BankAccount(const std::string& owner_, double balance_)
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: owner(owner_), balance(balance_), account_number("ACC-12345") {}
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// 1.1 friend function declaration
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// → grants access to private & protected members
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friend void print_account_details(const BankAccount& acc);
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// 1.2 friend function definition AVAILABLE ALSO | class body
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friend void add_interest(BankAccount& acc, double rate)
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{
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acc.balance += acc.balance * rate; // balance private member
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std::cout << "Interest added. New balance: $" << acc.balance << std::endl;
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}
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};
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// friend function definition -- from -- PREVIOUSLY being declared | class body
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void print_account_details(const BankAccount& acc)
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{
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std::cout << "Account Details:" << std::endl;
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std::cout << " Owner: " << acc.owner << std::endl; // private member
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std::cout << " Balance: $" << acc.balance << std::endl; // private member
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std::cout << " Account #: " << acc.account_number << std::endl; // protected member
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}
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// 2. friend class accessing private & protected members
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// Forward declaration for friend class example
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class Mechanic;
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class Car
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{
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private:
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std::string model;
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int engine_temp;
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bool engine_running;
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protected:
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std::string vin;
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public:
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Car(const std::string& model_)
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: model(model_), engine_temp(90), engine_running(false), vin("VIN123456") {}
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// 2.1 friend class declaration | class body
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// → grants Mechanic access -- to -- Car's private & protected members
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friend class Mechanic;
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void display() const
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{
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std::cout << "Car: " << model << std::endl;
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}
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};
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// 2.1.1 friend class can access Car's private & protected members
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class Mechanic
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{
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std::string name;
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public:
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Mechanic(const std::string& name_) : name(name_) {}
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// access Car's private members
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void diagnose(const Car& car) const
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{
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std::cout << "\nMechanic " << name << " diagnosing:" << std::endl;
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std::cout << " Model: " << car.model << " (private)" << std::endl;
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std::cout << " Engine temp: " << car.engine_temp << "°C (private)" << std::endl;
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std::cout << " Running: " << (car.engine_running ? "Yes" : "No") << " (private)" << std::endl;
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std::cout << " VIN: " << car.vin << " (protected)" << std::endl;
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}
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// can modify Car's private members
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void start_engine(Car& car) const
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{
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std::cout << "\nMechanic " << name << " starting engine..." << std::endl;
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car.engine_running = true; // Access private member
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car.engine_temp = 120; // Modify private member
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std::cout << " Engine started!" << std::endl;
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}
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void stop_engine(Car& car) const
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{
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car.engine_running = false;
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car.engine_temp = 90;
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}
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};
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int main()
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{
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std::cout << "=== Friend: Access Grant Examples ===" << std::endl;
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std::cout << std::endl;
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// Example 1: Friend function
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std::cout << "1. Friend Function Example:" << std::endl;
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BankAccount acc("Alice", 1000.0);
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print_account_details(acc); // Can access private & protected members
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add_interest(acc, 0.05);
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std::cout << std::endl;
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// Example 2: Friend class
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std::cout << "2. Friend Class Example:" << std::endl;
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Car car("Toyota Corolla");
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car.display();
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Mechanic mechanic("Bob");
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mechanic.diagnose(car); // Can access ALL private & protected members
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mechanic.start_engine(car); // Can modify private members
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mechanic.diagnose(car);
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std::cout << std::endl;
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std::cout << "=== Summary ===" << std::endl;
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std::cout << "✓ Friend function: grants access to private & protected members" << std::endl;
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std::cout << "✓ Friend class: grants ALL members access to private & protected" << std::endl;
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std::cout << "✓ Friend declarations appear | class body" << std::endl;
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return 0;
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}

‎reference_to_md/cpp/language/examples/friend/example.cpp‎

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#include <iostream>
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#include <sstream>
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class MyClass
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{
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int i; // friends have access to non-public, non-static
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static inline int id{6}; // and static (possibly inline) members
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friend std::ostream& operator<<(std::ostream& out, const MyClass&);
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friend std::istream& operator>>(std::istream& in, MyClass&);
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friend void change_id(int);
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public:
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MyClass(int i = 0) : i(i) {}
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};
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std::ostream& operator<<(std::ostream& out, const MyClass& mc)
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{
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return out << "MyClass::id = " << MyClass::id << "; i = " << mc.i;
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}
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std::istream& operator>>(std::istream& in, MyClass& mc)
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{
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return in >> mc.i;
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}
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void change_id(int id) { MyClass::id = id; }
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int main()
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{
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MyClass mc(7);
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std::cout << mc << '\n';
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// mc.i = 333*2; // error: i is a private member
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std::istringstream("100") >> mc;
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std::cout << mc << '\n';
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// MyClass::id = 222*3; // error: id is a private member
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change_id(9);
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std::cout << mc << '\n';
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}

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