Idioms and design patterns are both common solutions to recurring problems in software development, but they differ in scope, granularity, and formality:
- Scope:
- Idioms: Idioms are small, language-specific coding techniques or patterns that address specific programming challenges within a particular programming language. They often involve leveraging language features or conventions to achieve a desired outcome efficiently and effectively.
- Design Patterns: Design patterns are higher-level, language-agnostic architectural solutions to common design problems in software engineering. They provide general reusable templates for solving design issues and promoting best practices in software design.
- Granularity:
- Idioms: Idioms tend to be more granular and focused on specific coding constructs or techniques within a single programming language. They often involve manipulating language features or syntax to achieve particular goals.
- Design Patterns: Design patterns are more comprehensive and deal with broader design concepts and relationships between components within a software system. They provide templates for organizing and structuring code at a higher level of abstraction.
- Formality:
- Idioms: Idioms are typically informal and are commonly passed down through experience, code reviews, or programming literature within a specific programming community. They may not always have formal names or documentation.
- Design Patterns: Design patterns are more formalized and well-documented solutions to common design problems. They often have recognized names, descriptions, and implementation guidelines outlined in literature such as the Gang of Four (GoF) book “Design Patterns: Elements of Reusable Object-Oriented Software.”
In this post we will try to explore these common C++ idioms:
1- RAII
2-Pimpl
3- Curiously Recurring Template Pattern (CRTP)
4- Copy-and-swap
5- Type Erasure
6- Non-Virtual Interface (NVI)
7- SFINAE (Substitution Failure Is Not An Error)
The first part will focus on the RAII and PImpl idioms:
RAII(Resource Acquisition Is Initialization)
In C++, one of the most commonly used idioms is the RAII (Resource Acquisition Is Initialization) idiom. RAII is a powerful and widely adopted technique for managing resources such as memory, file handles, network connections, and locks in a deterministic and exception-safe manner. It ties the lifetime of resources to the lifetime of objects, ensuring that resources are properly acquired and released.
Here’s how RAII works and where it’s commonly used:
1-Managing Memory: RAII is extensively used in C++ for managing dynamic memory allocation using pointers, especially with std::unique_ptr and std::shared_ptr from the C++ Standard Library. These smart pointer classes ensure that memory is automatically deallocated when the object containing the pointer goes out of scope.
{
std::shared_ptr<int> ptr(new int); // Resource acquired
// Use ptr
} // Resource released automatically when ptr goes out of scope2-File Handling: RAII is employed for managing file resources, ensuring that files are closed properly after use, even in the presence of exceptions or early returns.
{
std::ifstream file("example.txt"); // Resource acquired
// Read from file
} // Resource released automatically when file goes out of scope
3-Locking Mechanisms: RAII is used for managing locks to ensure that critical sections of code are properly synchronized and that locks are released when they are no longer needed.
{
std::ifstream file("example.txt"); // Resource acquired
// Read from file
} // Resource released automatically when file goes out of scope
4-Resource Management in Custom Classes: Developers often implement RAII in their own classes to manage custom resources such as database connections, network sockets, and GPU resources.
RAII promotes resource safety, reduces the likelihood of resource leaks and dangling pointers, and simplifies resource management by tying it to object lifetimes. It’s considered one of the cornerstones of modern C++ programming and is widely used throughout the language and its standard library.
Pimpl
The Pimpl idiom, which stands for “Pointer to Implementation,” is a C++ design technique used to hide the implementation details of a class from its users. It helps improve encapsulation, reduce compile-time dependencies, and minimize recompilation when implementation details change.
Here’s how the Pimpl idiom works:
- Separation of Interface and Implementation: With the Pimpl idiom, the public interface of a class is defined in its header file, while the private implementation details are encapsulated in a separate implementation file. This separation allows clients of the class to interact only with its public interface, hiding the complexity of the implementation.
- Pointer to Implementation: Instead of directly including the implementation details in the class declaration, the class contains a pointer to an opaque (i.e., forward-declared) implementation class. This pointer is typically declared as a private member of the class.
- Forward Declaration: Since the implementation details are hidden from the class’s users, only the declaration of the implementation class is needed in the header file. This is achieved using a forward declaration (
class Impl;), avoiding the need to include the implementation details in the header. - Reduced Compilation Dependencies: By separating the interface and implementation and using forward declarations, changes to the implementation details (e.g., adding, removing, or modifying private members) do not require recompilation of the code that uses the class’s interface. This can significantly reduce build times, especially in large codebases.
- Dynamic Memory Allocation: Typically, the implementation class is allocated dynamically on the heap using
new, and the pointer to it is managed by the public class. This allows for dynamic polymorphism and enables the use of the “bridge” and “strategy” design patterns.
Here’s a simplified example demonstrating the Pimpl idiom:
// MyClass.h (Header file)
#pragma once
class MyClass {
public:
MyClass();
~MyClass();
void doSomething();
private:
class Impl; // Forward declaration
Impl* pImpl; // Pointer to implementation
};// MyClass.cpp (Implementation file)
#include "MyClass.h"
class MyClass::Impl {
public:
void doInternalWork() {
// Implementation details...
}
};
MyClass::MyClass() : pImpl(new Impl()) {}
MyClass::~MyClass() {
delete pImpl;
}
void MyClass::doSomething() {
pImpl->doInternalWork();
}
By employing the Pimpl idiom, the implementation details of MyClass are hidden from users of the class, and changes to the implementation can be made without affecting client code or requiring recompilation of dependent files. This improves modularity, maintainability, and build times in C++ projects.