Use templates to express containers and ranges

Reason

Containers need an element type, and expressing that as a template argument is general, reusable, and type safe. It also avoids brittle or inefficient workarounds. Convention: That's the way the STL does it.

Example

template<typename T>
    // requires Regular<T>
class Vector {
    // ...
    T* elem;   // points to sz Ts
    int sz;
};

Vector<double> v(10);
v[7] = 9.9;

Example, bad

class Container {
    // ...
    void* elem;   // points to size elements of some type
    int sz;
};

Container c(10, sizeof(double));
((double*) c.elem)[7] = 9.9;

This doesn't directly express the intent of the programmer and hides the structure of the program from the type system and optimizer.

Hiding the void* behind macros simply obscures the problems and introduces new opportunities for confusion.

Exceptions: If you need an ABI-stable interface, you might have to provide a base implementation and express the (type-safe) template in terms of that. See Stable base.

Enforcement

• Flag uses of void*s and casts outside low-level implementation code