1-Day C++ basics Crash Revision Sheet
Built for the day before your interview: zero assembly jargon, plain-English intuition, the exact foundational rules & traps interviewers test, short memorizable C++ code templates, and ready-to-speak answers across all 40 topics.
Pointers vs References Deep Dive
A pointer is an independent variable storing a memory address, while a reference is an alias bound forever to an existing variable. Choosing between them determines whether your API permits missing values (`nullptr`) or guarantees valid data.
- 1.
Pointer Mechanics: A pointer (
T*) is a distinct variable holding a memory address. It can point to nothing (nullptr), be reassigned anytime to point to different objects, and supports pointer arithmetic (ptr + 1). - 2.
Reference Mechanics: A reference (
T&) is an alias (a permanent nickname) for an existing object. It must be initialized at declaration, can never benullptr, and cannot be reseated after creation. - 3.
Passing Conventions: Pass small primitive types by value (
int), large read-only objects byconst T&(avoids expensive copies), mutable output parameters byT&, and optional or nullable objects byT*(orstd::optional<T>). - 4.
The Re-seating Trap: Writing
ref = y;does not makerefpoint toy. It copiesy's value into whateverrefwas originally bound to. References can never change what they bind to. - 5.
Dangling References: Never return a reference or pointer to a local stack variable. Once the function exits, its stack frame is discarded, leaving a dangling reference that causes Undefined Behavior when accessed.
#include <iostream>
void demonstrate() {
int a = 10, b = 20;
int* ptr = &a; // Pointer holds address of a
*ptr = 15; // Modifies a through pointer
ptr = &b; // Rebound! ptr now points to b
int& ref = a; // Reference is an alias for a
ref = 50; // Modifies a directly (a is now 50)
ref = b; // TRAP: Does NOT rebind ref to b! Copies b into a
const int& cRef = a; // Read-only alias: cRef = 100 is a compile error
}Use const T& by default for non-primitive inputs because it guarantees the object exists without copying it. Use a non-const reference T& for out-parameters that must be modified. Only use a pointer when the argument is optional and can be nullptr.
In well-defined C++, a reference can never be null and cannot be reassigned to another variable. Writing to an existing reference simply copies data into the underlying object. If a reference ever points to null, it was created via undefined behavior like dereferencing a null pointer.
At the language level, a reference has no distinct identity or address; &ref yields the address of the referent. Under the hood, compilers typically implement references as read-only pointers, but when optimized, they often occupy zero storage and compile directly into the referent's memory location.
const Correctness & Type Qualifiers (const, mutable, volatile, constexpr)
`const` enforces compile-time immutability, preventing accidental data mutation and enabling compiler optimizations. Type qualifiers tell the compiler where and when data is allowed to change, from compile-time `constexpr` to hardware-driven `volatile`.
- 1.
The Asterisk Rule: Read pointer declarations from right to left around the
*. Ifconstis before the*(const int*orint const*), the pointed-to data is read-only. Ifconstis after the*(int* const), the pointer address is locked and cannot point elsewhere. - 2.
const Member Functions: Placing
constafter a method signature (e.g.,int get() const) marks thethispointer asconst ClassName*. This method promises not to modify any member variables, allowing it to be safely invoked onconstobjects andconst¶meters. - 3.
The mutable Escape Hatch: A
mutablemember variable can be modified even inside aconstmember function. Its primary real-world uses are internal caching/memoization and locking astd::mutexduring read-only member access. - 4.
constexpr vs const:
constmeans read-only (value might be decided at runtime like user input), whereasconstexprguarantees computation at compile time. Aconstexprvariable or function can be used for array sizes and template parameters. - 5.
The volatile Misconception: In C++,
volatiletells the compiler never to optimize away or reorder reads/writes (useful for hardware registers and signal handlers). It is NOT thread-safe and provides no atomic guarantees or memory fences; always usestd::atomicfor concurrency.
#include <mutex>
class Cache {
int data = 42;
mutable std::mutex mtx; // mutable allows locking inside const methods
mutable int accessCount = 0;
public:
int get() const {
std::lock_guard<std::mutex> lock(mtx);
++accessCount; // Allowed because accessCount is mutable
return data;
}
};
constexpr int square(int x) { return x * x; } // Evaluated at compile time
void pointerRules() {
int x = 10, y = 20;
const int* p1 = &x; // Data is const: *p1 = 5 fails; p1 = &y OK
int* const p2 = &x; // Pointer is const: *p2 = 5 OK; p2 = &y fails
}In const int* p, the integer data is read-only, but the pointer can point to another address. In int* const p, the pointer address is frozen to one memory location, but you can modify the integer data. In const int* const p, neither the pointed-to data nor the pointer address can be modified.
You declare a member variable as mutable when it does not affect the object's observable logical state. The two standard use cases are thread synchronization primitives like std::mutex that must be locked inside const methods, and internal caching or access counters.
No, volatile does not make code thread-safe in C++. It only prevents compiler read/write optimizations for memory-mapped hardware or signal handlers; it does not generate CPU memory barriers or make operations atomic. For thread safety, always use std::atomic or mutexes.
Stack vs Heap, Placement new & Memory Layout
Process memory is strictly partitioned between fast automatic stack frames and dynamic heap buffers. Understanding how memory is laid out and how placement new constructs objects in pre-allocated bytes is critical for high-performance C++.
- 1.
Process Memory Layout (Low to High):
.text(compiled machine code) →.rodata(string literals, constants) →.data(initialized globals) →.bss(uninitialized/zeroed globals) → Heap (grows upward toward higher addresses) → Stack (grows downward from high addresses). - 2.
Stack vs Heap Allocation: The stack is blazing fast because allocation simply moves the stack pointer; variables are freed automatically when they go out of scope. The heap requires finding free memory blocks, incurring allocation overhead, fragmentation risk, and manual lifetime management.
- 3.
The delete vs delete[] Cookie Bug: When you allocate an array with
new T[N], the heap allocator stores an extra hidden prefix (a cookie) storing the element countN. If you calldelete ptrinstead ofdelete[] ptr, the destructor only runs once or interprets the cookie as data, leading to instant undefined behavior. - 4.
Placement new: Placement
new (buffer) T(args)constructs an object inside already-allocated memory without requesting new bytes from the OS. It is howstd::vectorcreates elements inside its reserved buffer without default-initializing the entire capacity. - 5.
Manual Destruction with Placement new: Because placement
newdid not allocate heap memory, you must NEVER calldelete ptron it. Instead, you must explicitly call the destructorptr->~T();and then independently free or reuse the underlying buffer.
#include <new> // Required for placement new
#include <string>
#include <iostream>
void placementNewDemo() {
// 1. Allocate raw uninitialized memory on stack
alignas(std::string) char buffer[sizeof(std::string)];
// 2. Construct object in-place inside the raw buffer
std::string* str = new (buffer) std::string("Fast In-Place Object");
std::cout << *str << '\n';
// 3. Manually call destructor (mandatory for placement new!)
str->~basic_string();
// 4. Do NOT call delete str; the stack buffer cleans up automatically!
}malloc only allocates raw uninitialized bytes and returns void*, while new allocates memory, computes the exact size, and executes the constructor to produce a strongly-typed object. Similarly, free merely releases bytes, whereas delete calls the object's destructor before releasing memory.
When allocating with new[], the runtime stores the array length in a hidden metadata cookie right before the first element. Calling single delete skips looking for this cookie, calls the destructor for only the first element, and passes an invalid pointer offset to the memory deallocator.
Placement new constructs an object in an already existing, pre-allocated memory address without performing a heap allocation. Containers like std::vector use it in emplace_back to construct objects directly into reserved capacity memory without default-constructing unused slots.
static vs global vs extern, Storage Classes & Compilation Pipeline
`static` alters variable lifetimes, linkage visibility, and class ownership depending on where it appears. Understanding `extern` and the compilation pipeline ensures you avoid multiple definition errors and static initialization order bugs.
- 1.
The 4 Meanings of static: (1) Local static: created on first execution, persists until program exit, thread-safe init since C++11; (2) Global/file static: internal linkage, visible only inside that
.cppfile; (3) Class static variable: single shared variable across all instances, defined outside the class; (4) Class static method: called without an instance, has nothispointer. - 2.
extern and Linkage: By default, global variables have external linkage (accessible across files). An
extern int g_counter;declaration informs other translation units that the variable exists elsewhere without reserving new storage, avoiding One Definition Rule (ODR) violations. - 3.
The 4 Compilation Stages: Source code (
.cpp) → (1) Preprocessor: expands#include,#define, strips comments (.i) → (2) Compiler: translates to assembly (.s) → (3) Assembler: creates machine code object files (.o/.obj) → (4) Linker: resolves cross-file symbols and addresses into an executable. - 4.
Static Initialization Order Fiasco: The initialization order of global static objects across different translation units is undefined. If global object
AinfileA.cppaccesses global objectBinfileB.cppduring its constructor, it may access uninitialized memory and crash. - 5.
Meyers' Singleton Fix: To eliminate the static initialization order fiasco, wrap the static object inside a function:
static MyClass& get() { static MyClass instance; return instance; }. It initializes safely on first call in a thread-safe manner.
#include <iostream>
class Counter {
public:
static int totalCount; // Declared here, defined outside class
static void printTotal() {
std::cout << "Total: " << totalCount << '\n'; // No 'this' pointer
}
};
int Counter::totalCount = 0; // Definition in one .cpp file
// Meyers' Singleton: Solves Static Initialization Order Fiasco
class Database {
public:
static Database& instance() {
static Database db; // Thread-safe lazy initialization (C++11)
return db;
}
private:
Database() = default;
};Inside a function, it creates a persistent local variable initialized once. At file scope, it gives internal linkage so the symbol is private to that file. Inside a class declaration, a static variable is shared by all instances, and a static method can be invoked without an object instance since it has no this pointer.
It occurs when global or static variables across different .cpp files depend on each other during startup, because C++ provides no guaranteed initialization order between separate translation units. You prevent it using Meyers' Singleton: wrap the static object inside a function so it initializes safely on first access.
extern tells the compiler that a variable or function is defined in another translation unit, allowing multiple files to reference the same symbol. In header files, you declare extern int count; so every file including it knows the symbol exists, while the actual definition int count = 0; lives in exactly one .cpp file.
Memory Bugs, Alignment & Struct Padding
CPUs access memory in aligned 4 or 8-byte words, causing compilers to inject hidden padding bytes between struct members. Struct layout optimization and avoiding classic memory corruption bugs are essential systems programming skills.
- 1.
Why Struct Padding Exists: CPUs fetch data significantly faster when addresses are multiples of the data type's natural size (e.g., 4-byte
inton 4-byte boundaries, 8-bytedoubleon 8-byte boundaries). Compilers add unused padding bytes between members to ensure this alignment. - 2.
The Reordering Rule (Largest to Smallest): Placing small types like
charnext to large types likedoublewastes memory on padding. By sorting members from largest to smallest (double→int→short→char), you pack fields tightly and minimize padding overhead. - 3.
The 5 Classic Memory Bugs: (1) Memory Leak: heap memory allocated but never freed; (2) Dangling Pointer: using memory after calling
delete; (3) Double Free: callingdeletetwice on the same pointer; (4) Buffer Overflow: writing beyond allocated bounds; (5) Uninitialized Memory: reading garbage values. - 4.
alignof and alignas:
alignof(T)queries the required byte boundary alignment of typeT.alignas(N)forces the compiler to align a variable or struct to anN-byte boundary (e.g.,alignas(64)aligns to a CPU cache line to prevent false sharing). - 5.
Dangling Pointer Prevention: Whenever you delete a raw pointer, immediately reset it:
delete ptr; ptr = nullptr;. Callingdelete nullptr;is a safe no-op in C++, preventing accidental double-free crashes.
#include <iostream>
struct BadLayout { // Total size: 24 bytes (due to padding)
char a; // 1 byte + 7 bytes padding
double b; // 8 bytes
int c; // 4 bytes + 4 bytes tail padding
};
struct GoodLayout { // Total size: 16 bytes (saved 8 bytes!)
double b; // 8 bytes
int c; // 4 bytes
char a; // 1 byte + 3 bytes tail padding
};
void checkAlignment() {
std::cout << "Bad size: " << sizeof(BadLayout) << '\n'; // 24
std::cout << "Good size: " << sizeof(GoodLayout) << '\n'; // 16
std::cout << "Double alignment: " << alignof(double) << '\n'; // 8
}Compilers insert padding bytes between struct members so each member starts at a memory address that is a multiple of its natural alignment requirement. Tail padding is also added at the end so arrays of the struct remain properly aligned for every element.
Order struct members in descending order of size: place 8-byte types (double, pointers) first, followed by 4-byte types (int, float), 2-byte types (short), and finally 1-byte types (char, bool). This packs fields tightly and eliminates internal alignment gaps.
A dangling pointer holds the memory address of an object that has already been deallocated; dereferencing it causes undefined behavior or crashes. You protect against it by resetting pointers to nullptr immediately after delete, scoping lifetimes tightly, and preferring smart pointers like std::unique_ptr.
Smart Pointers: unique_ptr, shared_ptr, weak_ptr & Control Block
Smart pointers automate heap resource management using RAII, eliminating memory leaks and dangling pointers. They express clear ownership semantics: exclusive ownership with `unique_ptr`, shared reference-counted ownership with `shared_ptr`, and cycle-breaking with `weak_ptr`.
- 1.
unique_ptr (Exclusive Ownership): Owns a resource exclusively with zero runtime overhead compared to a raw pointer. It cannot be copied (copy constructor is deleted), only moved (
std::move), and automatically deallocates its resource when it goes out of scope. - 2.
shared_ptr & The Control Block: Manages shared ownership by storing a pointer to the managed object and a dynamically allocated Control Block containing a thread-safe strong reference count, a weak count, and a custom deleter. When the strong count hits zero, the managed object is destroyed.
- 3.
Why std::make_shared is Superior: Creating a
shared_ptrviastd::make_shared<T>()performs 1 single heap allocation for both the object and its Control Block together (cache-friendly and fast), whereasstd::shared_ptr<T>(new T)performs 2 separate heap allocations. - 4.
Circular References with shared_ptr: If two objects hold
shared_ptrreferences to each other (e.g., Parent ↔ Child or Graph nodes), their reference counts can never reach zero, causing a permanent memory leak. - 5.
weak_ptr to the Rescue:
std::weak_ptrholds a non-owning observer reference to ashared_ptrresource without incrementing the strong reference count. To access the object safely, you must callwp.lock(), which returns a validshared_ptrornullptrif already destroyed.
#include <memory>
#include <iostream>
struct Node {
int value;
std::shared_ptr<Node> next;
std::weak_ptr<Node> prev; // weak_ptr prevents circular reference leak!
Node(int v) : value(v) {}
};
void smartPointerDemo() {
// 1. Exclusive ownership: zero overhead
auto uPtr = std::make_unique<int>(100);
// auto uPtrCopy = uPtr; // COMPILE ERROR: unique_ptr cannot be copied
auto uPtrMoved = std::move(uPtr); // Ownership transferred
// 2. Shared ownership with weak observer
auto sPtr = std::make_shared<int>(200);
std::weak_ptr<int> wPtr = sPtr; // Does not increment strong count
if (auto locked = wPtr.lock()) { // Safely verify and access resource
std::cout << "Value: " << *locked << '\n';
}
}std::unique_ptr has zero memory or runtime overhead compared to a raw pointer and communicates clear, exclusive ownership. std::shared_ptr introduces thread-safe atomic reference counting and heap-allocated control blocks, so it should only be used when true shared ownership is necessary.
std::make_shared combines the managed object and the control block into a single contiguous heap allocation, saving memory and improving cache locality. It is also exception-safe, preventing memory leaks if another argument throws before the constructor completes.
Circular references occur when two objects hold std::shared_ptr handles to each other, keeping their reference counts permanently at or above 1 so neither is ever destroyed. Replacing one direction with std::weak_ptr observes the object without incrementing the strong count, allowing normal destruction.
Constructors, Destructors & Initialization Order
Object construction and destruction follow strict, immutable order rules governed by class declaration layout, not initializer list order. Mastering constructor mechanics and `explicit` prevents subtle bugs and unintended type conversions.
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Deep vs Shallow Copy & The Rule of 3 (Copy-and-Swap)
A default shallow copy duplicates raw pointer values rather than heap data, creating twin owners that crash the program with double-free errors. The Rule of 3 and the Copy-and-Swap idiom guarantee crash-proof, exception-safe resource management.
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Lvalues, Rvalues, Move Semantics & The Rule of 5 / 0
Move semantics transfers ownership of expensive resources in O(1) time without copying memory bytes. `std::move` casts an lvalue to an rvalue, allowing constructors and operators to steal pointers and leave the source in a valid, destructible state.
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Copy Elision (RVO / NRVO) & Object Lifetime Traps
Return Value Optimization (RVO) completely bypasses both copy and move constructors by constructing returned values directly in the caller's target memory. Misunderstanding lifetime extension and premature `std::move` calls can severely degrade performance.
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Exception Handling, Stack Unwinding & noexcept Guarantees
When an exception is thrown, stack unwinding automatically destroys local objects in reverse order, making RAII the gold standard for leak-free C++. Understanding exception safety levels and the `noexcept` contract protects against instant crashes.
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The 4 Pillars of OOP in Production C++
Object-Oriented Programming structures code into maintainable components by binding data to behavior and defining explicit interface contracts. In C++, understanding encapsulation, abstraction, inheritance, and polymorphism is essential for building robust systems.
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How virtual Works: vtable & vptr Memory Layout
Runtime polymorphism in C++ relies on compiler-generated virtual tables and hidden instance pointers. Mastering vtables and vptrs demystifies the memory footprint of polymorphic types and prevents subtle constructor lifetime bugs.
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Virtual Destructors, Object Slicing & override / final
Deleting derived objects through base pointers without a virtual destructor causes undefined behavior and resource leaks. Combining virtual destructors with override, final, and reference semantics eliminates object slicing and catches inheritance bugs early.
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Multiple Inheritance, Diamond Problem & C++ Casts
The Diamond Problem arises when a derived class inherits two copies of a shared ancestor, creating member ambiguity. Virtual inheritance solves this duplication, while explicit C++ casts replace risky C-style casting with type-safe conversion boundaries.
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Function Overloading, Name Mangling, Operator Overloading & friend
Function and operator overloading let user-defined types integrate naturally with C++ syntax. Name mangling differentiates overloaded functions under the hood, while friend declarations allow controlled private access without compromising class encapsulation.
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Function & Class Templates + Template Specialization
Templates act as compile-time blueprints that allow writing high-performance, type-independent algorithms and data structures. The compiler generates concrete machine code for each type used, while specialization enables custom logic for specific types or pointer patterns.
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Universal References (T&&), Reference Collapsing & std::forward
Universal references and reference collapsing allow templates to bind transparently to both lvalues and rvalues. Perfect forwarding with std::forward preserves an argument's exact value category when passing it along to other functions.
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Variadic Templates, if constexpr & C++20 Concepts
Modern C++ elevates template programming through variadic parameter packs, compile-time branching, and concepts. These features replace complex metaprogramming hacks with clean, expressive code and readable compiler diagnostics.
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Lambdas & Closures Under the Hood
Lambdas in C++ are anonymous function objects that the compiler lowers into standard structs with operator(). Understanding capture modes, lifetime risks, and the overhead of std::function ensures efficient, memory-safe modern C++ code.
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Sequence Containers (vector, deque, list, forward_list, array)
Sequence containers store elements linearly; choosing between them comes down to CPU cache locality in contiguous buffers versus pointer stability and fast front insertions.
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Ordered Associative Containers (map, multimap, set, multiset)
Ordered associative containers store sorted elements in a balanced Red-Black Tree, guaranteeing O(log N) lookup, insertion, and deletion for every operation.
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Unordered Associative Containers (unordered_map, unordered_set)
Unordered containers use hash tables with separate chaining to provide average O(1) lookups and insertions, but degrade to O(N) when hash collisions spike.
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Container Adaptors (stack, queue, priority_queue) & flat_map
Container adaptors provide restricted LIFO, FIFO, and heap interfaces on top of underlying sequence containers, while C++23 flat containers wrap sorted contiguous vectors for maximum cache speed.
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Iterator Categories, Invalidation Rules & Essential STL Algorithms
Iterator invalidation is the most common memory bug in STL code; knowing whether modifying a container invalidates pointers and iterators prevents undefined behavior and crashes.
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Strings, Small String Optimization (SSO), string_view & span
Modern C++ eliminates heap allocations for short strings via Small String Optimization and avoids deep string copies using non-owning views like std::string_view and std::span.
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Doubly Linked List — Full Implementation & 6 Scenario Problems
Implementing a doubly linked list with dummy sentinel nodes eliminates edge-case null checks, making insertion, unlinking, and LRU cache splicing clean and bug-free.
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Implement Your Own std::vector<T> (MyVector)
Building a generic vector requires managing three raw member pointers/sizes, handling 2x geometric growth, and transferring elements safely during reallocation.
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Implement Your Own std::string (MyString)
Implementing a custom string class tests deep copy memory allocation, null-terminator management, and crash-proof Rule-of-5 move semantics.
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Implement Your Own unique_ptr<T> and shared_ptr<T>
Smart pointers wrap raw heap pointers in RAII structs to automate memory deallocation; unique_ptr enforces exclusive ownership by deleting copies, while shared_ptr shares ownership via reference counting.
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Implement Your Own Hash Map (MyHashMap<K, V>)
A live-coded hash map uses an array of bucket lists to resolve hash collisions with separate chaining and rehashes elements when the load factor exceeds a set threshold.
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Implement a Fixed-Block Memory Pool / Custom Arena Allocator
Memory pools pre-allocate a large chunk of memory to hand out fixed-size blocks in O(1) time using an embedded free list, eliminating heap fragmentation and OS malloc locking overhead.
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Threads, Lifecycle & Data Races (std::thread & std::jthread)
A thread is an independent path of execution that shares heap memory with other threads but gets its own private stack, requiring strict lifetime management to avoid crashes.
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Mutexes, RAII Locks & Deadlock Prevention
A mutex guarantees mutual exclusion so only one thread accesses shared data at a time, and RAII lock wrappers guarantee the mutex is released even when exceptions occur.
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Condition Variables & Classic Sync Coding Puzzles
Condition variables allow threads to sleep efficiently until signaled by another thread, avoiding wasteful busy-waiting loops.
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Atomics, CAS Loops, Memory Ordering & Thread-Safe Singleton
std::atomic enables lock-free synchronization through hardware-backed indivisible operations, eliminating mutex overhead for simple values.
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std::async, future, promise & Implementing a Thread Pool
Futures and promises allow easy value retrieval and exception propagation across threads, while thread pools reuse worker threads to execute tasks without spawning overhead.
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Concurrent Data Structures: Thread-Safe Queue & Doubly Linked List / LRU
Making data structures concurrent requires designing atomic composite operations rather than merely wrapping individual container methods in mutexes.
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Design Patterns Quick-Strike for C++ Interviews
C++ design patterns combine classic architectural concepts with modern C++ features like smart pointers, RAII, and move semantics to build leak-free systems.
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65-Minute Round 1 Playbook: Project Defense & Edge-Case Checklist
A structured interview framework for defending architecture decisions, handling senior follow-ups, and running the 7-point C++ edge-case checklist before submitting.
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