Avoid adjacent parameters that can be invoked by the same arguments in either order with different meaning

Reason

Adjacent arguments of the same type are easily swapped by mistake.

Example, bad

Consider:

void copy_n(T* p, T* q, int n);  // copy from [p:p + n) to [q:q + n)

This is a nasty variant of a K&R C-style interface. It is easy to reverse the "to" and "from" arguments.

Use const for the "from" argument:

void copy_n(const T* p, T* q, int n);  // copy from [p:p + n) to [q:q + n)

Exception

If the order of the parameters is not important, there is no problem:

int max(int a, int b);

Alternative

Don't pass arrays as pointers, pass an object representing a range (e.g., a span):

void copy_n(span<const T> p, span<T> q);  // copy from p to q

Alternative

Define a struct as the parameter type and name the fields for those parameters accordingly:

struct SystemParams {
    string config_file;
    string output_path;
    seconds timeout;
};
void initialize(SystemParams p);

This tends to make invocations of this clear to future readers, as the parameters are often filled in by name at the call site.

Note

Only the interface's designer can adequately address the source of violations of this guideline.

Enforcement strategy

(Simple) Warn if two consecutive parameters share the same type.

We are still looking for a less-simple enforcement.