Master Software Design Patterns and Architecture in C++/

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Policy Idiom

Policy Idiom

Learn about the C++ idiom policy.

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Policy and traits are generally used together. We’ll start with policy, and then move on to discuss traits in the next lesson.

Policy

A policy is a generic function or class whose behavior can be configured. Typically, there are default values for the policy parameters, such as std::vector and std::unordered_map.

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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;

Let’s look at the code above. Each container has a default allocated for its elements, depending on the value of T (line 5) or std::pair<const Key, T> (line 8). Additionally, ...