Idioms for Polymorphism and Templates

This post is unique because I have written about all the topics mentioned in this post already. Therefore, I only provide a few words about the topic and a link to the post mentioned.

I decided on this particular post for two reasons. First, all the idioms for polymorphism and templates in C++ are very important and often used. Second, I don’t write posts about topics I have already a few months ago written about.

Polymorphism

Polymorphism is the property that different types support the same interface. In C++, we distinguish between dynamic polymorphism and static polymorphism.

Dynamic Polymorphism

Dynamic Polymorphism takes place at run time, based on object orientation, and enables us to separate between the interface and the implementation of a class hierarchy. To get late binding, dynamic dispatch, or dispatch at run time, you need virtuality and an indirection such as a pointer or a reference.

Dynamic versus Static Polymorphism

Curiously Recurring Template Pattern (CRTP)

Here is the crucial idea of CRTP: A class Derived derives from a class template Base, and Base has Derived as a template argument.

template <typename T>
class Base {
    ...
};

class Derived : public Base<Derived> {
    ...
};

More about Dynamic versus Static Polymorphism

Static polymorphism is based on CRTP.

Modernes C++ Mentoring

"Fundamentals for C++ Professionals" (open)

"Design Patterns and Architectural Patterns with C++" (open)

"C++20: Get the Details" (open)

"Concurrency with Modern C++" (open)

"Generic Programming (Templates) with C++": October 2024

"Embedded Programming with Modern C++": October 2024

"Clean Code: Best Practices for Modern C++": March 2025

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Static Polymorphism

Static polymorphism happens at compile time and has no runtime performance cost.

More about Dynamic versus Static Polymorphism

The Overload Pattern

Typically, you use the overload pattern for a std::variant. std::variant is a type-safe union. with one value from one of its types. std::visit allows you to apply a visitor to it. Exactly here comes the Overload Pattern in C++20 very handy in play.

template<typename ... Ts> 
struct Overload : Ts ... { 
    using Ts::operator() ... ; 
};

Templates

Templates extend C++ with many new idioms.

Mixins

Mixins are a popular idea in the design of classes to mix in new code. You can implement mixins in C++ by using CRTP. A prominent example is the class std::enable_shared_from_this.

Mixins

Expression Templates

Expression templates are typically used in linear algebra and are “structures representing a computation at compile-time, which structures are evaluated only as needed to produce efficient code for the entire computation” (https://en.wikipedia.org/wiki/Expression_templates). In other words, expression templates are only evaluated when needed.

Avoiding Temporaries with Expression Templates

Policy

A policy is a generic function or class whose behavior can be configured. Typically, there are default values for the policy parameters. std::vector and std::unordered_map exemplify this design idea in the Standard Template Library.

template<class T, class Allocator std::allocator<T>>        
class vector; 

template<class Key,
    class T,
    class Hash = std::hash<Key>,                               
    class KeyEqual = std::equal_to<Key>,                       
    class allocator = std::allocator<std::pair<const Key, T>>  
class unordered_map;

Policy

Traits

Traits are class templates that provide characteristics of a generic type. They can extract one or more characteristics of a class template.

Software Design with Traits and Tag Dispatching

Tag Dispatching

Tag dispatching is a way to simulate function overloading based on concepts, often based on traits.

Type Erasure

Type Erasure enables using various concrete types through a single generic interface. In C, you base it on void pointers; in C++, on object orientation or templates.

Type Erasure

What’s Next?

I’m happy to present in my next post a guest post from Alex Eisenhut. Alex will write about his passion: good software architecture.

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