Use templates to raise the level of abstraction of code

Reason

Generality. Reuse. Efficiency. Encourages consistent definition of user types.

Example, bad

Conceptually, the following requirements are wrong because what we want of T is more than just the very low-level concepts of "can be incremented" or "can be added":

template<typename T>
    requires Incrementable<T>
T sum1(vector<T>& v, T s)
{
    for (auto x : v) s += x;
    return s;
}

template<typename T>
    requires Simple_number<T>
T sum2(vector<T>& v, T s)
{
    for (auto x : v) s = s + x;
    return s;
}

Assuming that Incrementable does not support + and Simple_number does not support +=, we have overconstrained implementers of sum1 and sum2. And, in this case, missed an opportunity for a generalization.

Example

template<typename T>
    requires Arithmetic<T>
T sum(vector<T>& v, T s)
{
    for (auto x : v) s += x;
    return s;
}

Assuming that Arithmetic requires both + and +=, we have constrained the user of sum to provide a complete arithmetic type. That is not a minimal requirement, but it gives the implementer of algorithms much needed freedom and ensures that any Arithmetic type can be used for a wide variety of algorithms.

For additional generality and reusability, we could also use a more general Container or Range concept instead of committing to only one container, vector.

Note

If we define a template to require exactly the operations required for a single implementation of a single algorithm (e.g., requiring just += rather than also = and +) and only those, we have overconstrained maintainers. We aim to minimize requirements on template arguments, but the absolutely minimal requirements of an implementation is rarely a meaningful concept.

Note

Templates can be used to express essentially everything (they are Turing complete), but the aim of generic programming (as expressed using templates) is to efficiently generalize operations/algorithms over a set of types with similar semantic properties.

Enforcement

• Flag algorithms with "overly simple" requirements, such as direct use of specific operators without a concept.
• Do not flag the definition of the "overly simple" concepts themselves; they might simply be building blocks for more useful concepts.