The whole book test_《C++20Get the details》_notes

Multiple-Choice Questions

1. Which of the following are advantages of C++20 Concepts?
   a) Improved error messages during template instantiation
   b) Automatic memory management for templates
   c) Enable function overloading based on semantic constraints
   d) Elimination of header files

2. Which syntax forms are valid for defining a function template using Concepts?
   a) template T add(T a, T b)
   b) auto add(std::integral auto a, std::integral auto b)
   c) void add(T a, T b) requires std::integral
   d) std::integral add(std::integral a, std::integral b)

3. What are the key benefits of C++20 Modules?
   a) Faster compilation times due to reduced header redundancy
   b) Isolation of preprocessor macros across modules
   c) Automatic parallel execution of code
   d) Elimination of the One Definition Rule (ODR)

4. Which statements about std::span are correct?
   a) It owns the elements it references.
   b) It can automatically deduce the size of a contiguous sequence.
   c) It is a replacement for std::vector.
   d) It supports both static and dynamic extents.

5. What is true about the consteval specifier in C++20?
   a) It ensures a function is evaluated at runtime.
   b) It implicitly makes a function constexpr.
   c) It can only be applied to virtual functions.
   d) It guarantees compile-time evaluation for every invocation.

6. Which features are part of the C++20 Ranges library?
   a) Lazy evaluation of operations
   b) Direct container manipulation without iterators
   c) Thread-safe container operations
   d) Pipeline syntax using the | operator

7. What are valid uses of the [[nodiscard]] attribute?
   a) To enforce error checking for function return values
   b) To optimize compiler-generated code
   c) To mark deprecated functions
   d) To suppress compiler warnings for unused variables

8. Which statements about std::jthread are correct?
   a) It automatically joins its thread upon destruction.
   b) It supports cooperative interruption via std::stop_token.
   c) It is a drop-in replacement for std::thread with identical behavior.
   d) It requires manual resource management.

9. What is true about the Three-Way Comparison Operator (<=>)?
   a) It can be defaulted to generate all six comparison operators.
   b) It returns a boolean value.
   c) It requires manual implementation for derived classes.
   d) It supports lexicographical comparison of class members.

10. Which operations are valid for std::atomic_ref?
    a) Atomic updates on non-atomic objects
    b) Thread-safe access to subobjects of the referenced object
    c) Guaranteed lock-free implementation
    d) Cooperative interruption of threads

11. What is true about constinit?
    a) It ensures constant initialization for static storage duration variables.
    b) It implies constexpr.
    c) It can be applied to dynamically allocated memory.
    d) It solves the static initialization order fiasco.

12. Which statements about Coroutines are correct?
    a) They require manual stack management.
    b) co_await suspends execution until a result is available.
    c) co_yield returns a value and resumes execution later.
    d) They are exclusively for multi-threaded applications.

13. What are valid uses of requires clauses?
    a) To constrain template parameters using logical combinations of concepts
    b) To define runtime preconditions for functions
    c) To specify exception guarantees
    d) To check the validity of expressions at compile time

14. Which features are part of the C++20 Calendar and Time Zone library?
    a) Time zone-aware time points
    b) Mathematical constants like π
    c) Literals for days (d) and years (y)
    d) Formatting utilities for dates

15. What is true about the std::format library?
    a) It uses Python-style format strings.
    b) It is type-safe compared to printf.
    c) It requires manual memory allocation for output.
    d) It supports user-defined types via specialization.

16. Which statements about Lambda improvements in C++20 are correct?
    a) Lambdas can have template parameters.
    b) Stateless lambdas can be default-constructed.
    c) Lambdas cannot capture this implicitly.
    d) consteval lambdas are evaluated at runtime.

17. What is true about std::semaphore in C++20?
    a) It supports both counting and binary semaphores.
    b) It is part of the header.
    c) It replaces std::mutex for all use cases.
    d) Its acquire() method decrements the counter.

18. Which statements about Designated Initialization are correct?
    a) It allows initializing class members in any order.
    b) It works only with aggregate types.
    c) It is compatible with private class members.
    d) Missing designators result in zero-initialization.

19. What is true about the std::ranges algorithms?
    a) They operate directly on containers without iterators.
    b) They are always evaluated eagerly.
    c) They support pipeline composition using |.
    d) They replace all existing functions.

20. Which statements about std::stop_source and std::stop_token are correct?
    a) They enable cooperative thread cancellation.
    b) A stop_token can be used to poll for cancellation requests.
    c) They are part of the header.
    d) std::jthread automatically manages stop_source lifetime.

21. What is true about the no_unique_address attribute?
    a) It allows overlapping storage for empty class members.
    b) It guarantees unique memory addresses for all members.
    c) It is used to optimize space in class layouts.
    d) It applies only to virtual base classes.

22. Which statements about std::latch and std::barrier are correct?
    a) std::latch can be reused multiple times.
    b) std::barrier supports phase-based synchronization.
    c) Both are defined in the header.
    d) std::latch decrements a counter on each arrive_and_wait().

23. What is true about the std::source_location class?
    a) It provides compile-time information about the source code.
    b) It replaces the __FILE__ and __LINE__ macros.
    c) It requires runtime computation to capture location data.
    d) It is part of the header.

24. Which statements about the Spaceship Operator (<=>) are correct?
    a) It returns a value of type std::strong_ordering.
    b) It can be used to generate equality operators (==, !=).
    c) It requires explicit implementation for all comparison categories.
    d) It simplifies the definition of comparison operators for classes.

25. What is true about the std::is_constant_evaluated() function?
    a) It returns true during compile-time evaluation.
    b) It is used to optimize runtime performance.
    c) It can differentiate between constexpr and consteval contexts.
    d) It is part of the header.

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Answers and Explanations

1. a, c
   Concepts improve error messages and enable overloading based on constraints (Section 4.1.2). Modules eliminate headers, not Concepts.

2. a, b, c
   Valid syntax includes constrained templates, abbreviated function templates, and trailing requires clauses (Section 4.1.4). Option d is invalid.

3. a, b
   Modules reduce header redundancy and isolate macros (Section 4.2.2). ODR still applies.

4. b, d
   std::span deduces size and supports static/dynamic extents (Section 5.2.1). It does not own elements.

5. b, d
   consteval ensures compile-time evaluation and is a subset of constexpr (Section 4.5.1).

6. a, b, d
   Ranges support lazy evaluation, direct container ops, and pipeline syntax (Section 5.1).

7. a
   [[nodiscard]] enforces checking return values (Section 4.8.1). It does not optimize code or mark deprecation.

8. a, b
   std::jthread auto-joins and supports interruption (Section 6.6). It is not identical to std::thread.

9. a, d
   <=> can be defaulted for lexicographical comparisons (Section 4.3.4). It returns an ordering type, not a boolean.

10. a
    std::atomic_ref applies atomic ops to non-atomic objects (Section 6.2.1). Subobjects are not thread-safe.

11. a, d
    constinit ensures compile-time initialization and avoids static order issues (Section 4.5.4). It does not imply constexpr.

12. b, c
    Coroutines use co_await/co_yield for suspension/resumption (Section 6.1). They are not limited to multi-threading.

13. a, d
    requires clauses constrain templates and validate expressions (Section 4.1.9). They are not for runtime checks.

14. a, c
    The library includes time zones and literals (Section 5.6). Formatting is separate.

15. a, b, d
    std::format uses Python syntax, is type-safe, and supports custom types (Section 5.5).

16. a, b
    Lambdas support templates and stateless default construction (Section 4.7). this capture detection is improved.

17. a, d
    Semaphores are counting and use acquire() (Section 6.3). They are in .

18. b, d
    Designated initialization works with aggregates and zero-initializes missing fields (Section 4.4).

19. a, c
    Ranges operate on containers and support pipelines (Section 5.1). Evaluation can be lazy.

20. a, b, d
    stop_source/token enable cancellation and are managed by jthread (Section 6.5).

21. a, c
    no_unique_address optimizes space for empty members (Section 4.8.3).

22. b, d
    barrier supports phases; latch decrements a counter (Section 6.4). They are in /.

23. a, b
    std::source_location replaces macros and is compile-time (Section 5.7.4). It is in .

24. a, b, d
    <=> returns ordering categories and simplifies operator definitions (Section 4.3). Categories are automatic.

25. a, d
    is_constant_evaluated() detects compile-time context (Section 5.7.2). It is in .

C++20 Programming Challenges

1. Constrained Matrix Multiplication with Concepts
   Problem: Implement a matrix multiplication function using concepts to ensure input matrices satisfy mathematical requirements (contiguous containers, compatible dimensions). Test with std::vector> and invalid types.

Key Points:

• Use std::ranges::contiguous_range and custom dimension checks.
• Leverage requires clauses for template constraints.

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2. Module-Based Math Library
   Problem: Create a C++20 module math that exports add, multiply, and power functions. Import the module in another translation unit and verify symbol visibility.

Key Points:

• Module interface vs. implementation partitions.
• Export control and name visibility rules.

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3. Lazy Prime Number Generator with Ranges
   Problem: Generate an infinite sequence of prime numbers using range views (filter + transform). Extract the first 20 primes and print them.

Key Points:

• Compose range adaptors with | operator.
• Lazy evaluation for infinite sequences.

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4. Custom Coroutine-Based Event Loop
   Problem: Implement an event loop using coroutines that processes tasks cooperatively. Schedule async I/O simulations with co_await.

Key Points:

• Coroutine state machine mechanics.
• co_await for suspension/resumption.

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5. Spaceship Operator for Complex Numbers
   Problem: Define a Complex class with compiler-generated operator<=> and test ordering in sorted containers.

Key Points:

• defaulted three-way comparison.
• Strong/weak/partial ordering scenarios.

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6. Type-Safe Heterogeneous Collection with std::span
   Problem: Use std::span to create a function that processes elements from different containers (std::array, std::vector) without copying.

Key Points:

• std::span’s bounds checking.
• Contiguous memory requirements.

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7. Compile-Time JSON Parser with constexpr
   Problem: Build a constexpr JSON parser that validates syntax and extracts values at compile time. Test with static assertions.

Key Points:

• consteval for strict compile-time execution.
• Recursive constexpr parsing.

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8. Thread-Safe FIFO Queue with Semaphores
   Problem: Implement a FIFO queue using C++20 std::counting_semaphore to enforce thread-safe producer-consumer access.

Key Points:

• Semaphore acquire/release mechanics.
• RAII wrappers for exception safety.

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9. Atomic Shared State with std::atomic
   Problem: Create a thread-safe global counter using std::atomic> and verify atomic updates.

Key Points:

• Atomic smart pointer operations.
• Memory ordering guarantees.

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10. Timezone-Aware Meeting Scheduler
    Problem: Use to schedule meetings across time zones (e.g., “New York 9:00 AM” to “London” local time).

Key Points:

• std::chrono::time_zone conversions.
• Daylight saving time handling.

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11. CRTP with Non-Type Template Parameters
    Problem: Implement Curiously Recurring Template Pattern (CRTP) using NTTPs to create compile-time polymorphic counters.

Key Points:

• NTTP type restrictions (structural types).
• Compile-time polymorphism.

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12. Fold Expressions with Variadic Concepts
    Problem: Write a concat function using fold expressions and concepts to ensure all arguments are string-like (std::string, std::string_view).

Key Points:

• Variadic templates with constrained auto.
• Concept-based overloading.

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13. Stateful Lambda Pipeline with std::views
    Problem: Create a range pipeline that tracks state between iterations (e.g., moving average of last 3 elements).

Key Points:

• Stateful lambda captures.
• Range view composition challenges.

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14. Custom Formatter for Scientific Notation
    Problem: Extend std::formatter to display double in engineering notation (exponent multiples of 3). Test with std::format.

Key Points:

• Format string parsing API.
• Locale-independent formatting.

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15. Barrier-Based Parallel Matrix Transposition
    Problem: Transpose a large matrix using std::barrier to synchronize worker threads operating on matrix blocks.

Key Points:

• Barrier phase synchronization.
• Thread partitioning and data races.

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Solutions & Explanations

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