Avoid error handling based on global state (e.g. `errno`)

Reason

Global state is hard to manage and it is easy to forget to check it. When did you last test the return value of printf()?

See also: Simulating RAII

Example, bad

int last_err;

void f(int n)
{
    // ...
    p = static_cast<X*>(malloc(n * sizeof(X)));
    if (!p) last_err = -1;     // error if memory is exhausted
    // ...
}

Note

C-style error handling is based on the global variable errno, so it is essentially impossible to avoid this style completely.

Enforcement

Awkward.