errno - Boost.Outcome documentation

# errno

The idiom of returning, upon failure, a special value and storing an error code (an `int`) inside a global (or thread-local) object `errno` is inherited from C, and used in its Standard Library:

    int readValue(const char * filename)
    {
      FILE* f = fopen(filename, "r");
      if (f == NULL)
        return 0; // special value indicating failure
                  // keep errno value set by fopen()

      int i;
      int r = fscanf(f, "%d", &i);
      if (r == 0 || r == EOF) { // special values: i not read
        errno = ENODATA;        // choose error value to return
        return 0;

      fclose(f);
      errno = 0;  // clear error info (success)
      return i;
    }

One advantage (to some, and a disadvantage to others) of this technique is that it uses familiar control statements (`if` and `return`) to indicate all execution paths that handle failures. When we read this code we know when and under what conditions it can exit without producing the expected result.

### Downsides

Because on failure, as well as success, we write into a global (or thread-local) object, our functions are not pure: they have side effects. This means many useful compiler optimizations (like common subexpression elimination) cannot be applied. This shows that it is not only C++ that chooses suboptimal solutions for reporting failures.

Whatever type we return, we always need a special value to spare, which is sometimes troublesome. In the above example, if the successfully read value of `i` is `0`, and we return it, our callers will think it is a failure even though it is not.

Error propagation using `if` statements and early `return`s is manual. We can easily forget to check for the failure, and incorrectly let the subsequent operations execute, potentially causing damage to the program state.

Upon nearly each function call layer we may have to change error code value so that it reflects the error condition adequate to the current layer. If we do so, the original error code is gone.

Last revised: January 16, 2019 at 01:05:39 +0100