Prefer `T*` over `T&` when "no argument" is a valid option

Reason

A pointer (T*) can be a nullptr and a reference (T&) cannot, there is no valid "null reference". Sometimes having nullptr as an alternative to indicated "no object" is useful, but if it is not, a reference is notationally simpler and might yield better code.

Example

string zstring_to_string(zstring p) // zstring is a char*; that is a C-style string
{
    if (!p) return string{};    // p might be nullptr; remember to check
    return string{p};
}

void print(const vector<int>& r)
{
    // r refers to a vector<int>; no check needed
}

Note

It is possible, but not valid C++ to construct a reference that is essentially a nullptr (e.g., T* p = nullptr; T& r = *p;). That error is very uncommon.

Note

If you prefer the pointer notation (-> and/or * vs. .), not_null<T*> provides the same guarantee as T&.

Enforcement

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