国产av日韩一区二区三区精品,成人性爱视频在线观看,国产,欧美,日韩,一区,www.成色av久久成人,2222eeee成人天堂

Home Backend Development C++ How does the \'ellipsis\' (...) notation work in variadic templates and what are its different roles in packing and unpacking arguments?

How does the \'ellipsis\' (...) notation work in variadic templates and what are its different roles in packing and unpacking arguments?

Oct 31, 2024 am 04:26 AM

How does the

Variadic Templates: Unveiling the Mysteries of the Ellipsis

Variadic templates, introduced in C 11, expand the capabilities of templates by allowing functions and classes to accept and process a variable number of arguments. However, certain aspects of their syntax can evoke curiosity. One such enigma is the enigmatic "ellipsis" (...) notation, which triggers questions about its purpose and usage.

In the context of variadic templates, the ellipsis (...) serves as a critical tool for either packing or unpacking arguments and parameters. Its role is determined by its relative position within the template definition or call.

Unpacking: Expanding Parameters into Arguments

When placed to the right of an expression, the ellipsis acts as an unpacker. It enables the expansion of template parameters into a sequence of actual arguments. For instance, in the following function template:

<code class="cpp">template<class T, class... Args>
unique_ptr<T> make_unique(Args&&... args) {
    return unique_ptr<T>(new T(std::forward<Args>(args)...));
}</code>

The ellipsis in std::forward(args)... unpacks the Args template parameter into the args function argument list, enabling the construction of a new unique_ptr with the provided arguments.

Packing: Combining Arguments into a Parameter

Conversely, when placed to the left of a parameter, the ellipsis functions as a packer. It packs a sequence of arguments into a single parameter. This packing occurs during both template argument deduction and function call invocation.

For illustration, consider the following function call:

<code class="cpp">make_unique<int>(1, 2, 3);</code>

The ellipsis in make_unique(... ) packs the arguments 1, 2, and 3 into a single parameter pack, matching the Args` template parameter.

The example template function f demonstrates the packing and unpacking capability:

<code class="cpp">template <typename ...T>
void f(T ...args) {
    g(args...);  // Unpacking: Pattern = args
    h(x(args)...);  // Unpacking: Pattern = x(args)
    m(y(args...)...);  // Packing: Pattern = args (arg to y())
}</code>

When calling f with T as {int, char, short}, each function call expands as:

<code class="cpp">g(arg0, arg1, arg2);           
h(x(arg0), x(arg1), x(arg2));
m(y(arg0, arg1, arg2));</code>

Placement Inconsistencies

You may notice an apparent inconsistency in the placement of the ellipsis between the template argument list and the function parameter list. In the template argument, it appears in the middle of the parameter pack, while in the function parameter, it appears at the end. This difference arises from the fact that in the template argument, the ellipsis is a syntactic sugar representing the unpacking of a parameter pack, while in the function parameter, it is a part of the syntax that defines the type of the packed argument.

In summary, the ellipsis (...) in variadic templates plays a pivotal role in expanding parameters into arguments and combining arguments into parameters, enabling the creation of flexible and extensible code that can handle a varying number of arguments.

The above is the detailed content of How does the \'ellipsis\' (...) notation work in variadic templates and what are its different roles in packing and unpacking arguments?. For more information, please follow other related articles on the PHP Chinese website!

Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undress AI Tool

Undress AI Tool

Undress images for free

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

C   Polymorphism: Enhancing Code Reusability and Flexibility C Polymorphism: Enhancing Code Reusability and Flexibility Jun 10, 2025 am 12:04 AM

Polymorphism in C is implemented through virtual functions and abstract classes, enhancing the reusability and flexibility of the code. 1) Virtual functions allow derived classes to override base class methods, 2) Abstract classes define interfaces, and force derived classes to implement certain methods. This mechanism makes the code more flexible and scalable, but attention should be paid to its possible increase in runtime overhead and code complexity.

C   Polymorphism : is function overloading a kind of polymorphism? C Polymorphism : is function overloading a kind of polymorphism? Jun 20, 2025 am 12:05 AM

Yes, function overloading is a polymorphic form in C, specifically compile-time polymorphism. 1. Function overload allows multiple functions with the same name but different parameter lists. 2. The compiler decides which function to call at compile time based on the provided parameters. 3. Unlike runtime polymorphism, function overloading has no extra overhead at runtime, and is simple to implement but less flexible.

C   Destructors code samples C Destructors code samples Jun 13, 2025 am 12:04 AM

The destructor in C is used to free the resources occupied by the object. 1) They are automatically called at the end of the object's life cycle, such as leaving scope or using delete. 2) Resource management, exception security and performance optimization should be considered during design. 3) Avoid throwing exceptions in the destructor and use RAII mode to ensure resource release. 4) Define a virtual destructor in the base class to ensure that the derived class objects are properly destroyed. 5) Performance optimization can be achieved through object pools or smart pointers. 6) Keep the destructor thread safe and concise, and focus on resource release.

What Are the Different Kinds of Polymorphism in C  ? Explained What Are the Different Kinds of Polymorphism in C ? Explained Jun 20, 2025 am 12:08 AM

C has two main polymorphic types: compile-time polymorphism and run-time polymorphism. 1. Compilation-time polymorphism is implemented through function overloading and templates, providing high efficiency but may lead to code bloating. 2. Runtime polymorphism is implemented through virtual functions and inheritance, providing flexibility but performance overhead.

How to Implement Polymorphism in C  : A Step-by-Step Tutorial How to Implement Polymorphism in C : A Step-by-Step Tutorial Jun 14, 2025 am 12:02 AM

Implementing polymorphism in C can be achieved through the following steps: 1) use inheritance and virtual functions, 2) define a base class containing virtual functions, 3) rewrite these virtual functions by derived classes, and 4) call these functions using base class pointers or references. Polymorphism allows different types of objects to be treated as objects of the same basis type, thereby improving code flexibility and maintainability.

C  : Is Polymorphism really useful? C : Is Polymorphism really useful? Jun 20, 2025 am 12:01 AM

Yes, polymorphisms in C are very useful. 1) It provides flexibility to allow easy addition of new types; 2) promotes code reuse and reduces duplication; 3) simplifies maintenance, making the code easier to expand and adapt to changes. Despite performance and memory management challenges, its advantages are particularly significant in complex systems.

C   Destructors: Common Errors C Destructors: Common Errors Jun 20, 2025 am 12:12 AM

C destructorscanleadtoseveralcommonerrors.Toavoidthem:1)Preventdoubledeletionbysettingpointerstonullptrorusingsmartpointers.2)Handleexceptionsindestructorsbycatchingandloggingthem.3)Usevirtualdestructorsinbaseclassesforproperpolymorphicdestruction.4

Polymorphism in C  : A Comprehensive Guide with Examples Polymorphism in C : A Comprehensive Guide with Examples Jun 21, 2025 am 12:11 AM

Polymorphisms in C are divided into runtime polymorphisms and compile-time polymorphisms. 1. Runtime polymorphism is implemented through virtual functions, allowing the correct method to be called dynamically at runtime. 2. Compilation-time polymorphism is implemented through function overloading and templates, providing higher performance and flexibility.

See all articles