Pass small amounts of data between threads by value, rather than by reference or pointer

Reason

A small amount of data is cheaper to copy and access than to share it using some locking mechanism. Copying naturally gives unique ownership (simplifies code) and eliminates the possibility of data races.

Note

Defining "small amount" precisely is impossible.

Example

string modify1(string);
void modify2(string&);

void fct(string& s)
{
    auto res = async(modify1, s);
    async(modify2, s);
}

The call of modify1 involves copying two string values; the call of modify2 does not. On the other hand, the implementation of modify1 is exactly as we would have written it for single-threaded code, whereas the implementation of modify2 will need some form of locking to avoid data races. If the string is short (say 10 characters), the call of modify1 can be surprisingly fast; essentially all the cost is in the thread switch. If the string is long (say 1,000,000 characters), copying it twice is probably not a good idea.

Note that this argument has nothing to do with async as such. It applies equally to considerations about whether to use message passing or shared memory.

Enforcement

???