泛型编程中的Concept, Model和Policy

A crude explanation

Concept

A set of requirements on a type, e.g. a RandomAccessible concept requires the type implements:operator[] (int) in O(1) time.

Model

When a type meet a concept, it's a model of that concept.

Policy

A unit of behaviour, which can combine with other units of behaviour to build complex classes. For illustration:

template<class T, class StoragePolicy>
class array : public StoragePolicy {
    public:
        T& operator[](int i) { return data[i]; };
};

// policy 1
template <class T, int N>
class fixed_storage {
    T data[N];
};

// policy 2
template<class T>
class dynamic_storage{
    T* data;
    public:
        void push_back(const T& val) {
            // code for dynamic array insertationk
        }
};

usage of policy:

int main(){
    array<int, fixed_storage<int, 10>> fixed_array;
    array<int, dynamic_storage<int>> dynamic_array;
}

Reference

• In the generic programming/TMP world what exactly is a model / a policy and a “concept”?

More

• Boost - Generic Programming Techniques
• Wiki: Policy-based design
• Wiki: Concept (generic programming)
• Andrei Alex's book: 《Modern C++ Design : Generic Programming and Design Patterns Applied》