Question

I am creating my own streambuf subclass and using the C++03 spec (ISO/IEC 14882:2003) as a reference.

In section 27.5.2.4.2 [lib.streambuf.virt.buffer], the specifications for both basic_streambuf::seekoff() and basic_streambuf::seekpos() say:

Default behavior: Returns pos_type(off_type(-1)).

Now, I thought off_type was supposed to be a signed integral type and pos_type was supposed to be an unsigned integral type, so it seems to me this expression has to be equivalent to just pos_type(-1).

But supposing I am mistaken, and these types might be some other combination of signed and unsigned… Then I still cannot figure out any possible use for this double cast.

For example, if both are signed, then again the expression is equivalent to pos_type(-1).

If pos_type were signed and off_type were unsigned — which makes no sense, but bear with me — then at best this double cast would shove some huge value into pos_type and at worst it would invoke implementation-defined behavior by assigning a too-large value to the signed pos_type integer.

Does anyone know the standard’s rationale for specifying pos_type(off_type(-1)) here instead of just pos_type(-1)? If not, can you even imagine a plausible rationale?

Answer 1

I believe that pos_type by default boils down to a std::streampos, which is typically defined like this:

typedef fpos<mbstate_t> streampos;

std::fpos is usually an offset combined with a multi-byte state object.

Additionally, off_type by default will boil down to a std::streamoff which is also a signed integral type (long or something like that)

So I think the mentality here is that you can initialize a offset type with the sentinal -1 value, then that can be used to initialize a position object which is a more complex object. Because, fpos‘s constructor takes streamoff type, not a long, if streamoff could be a more complex type than a long or similar, it would make the cautious-ness necessary.

Imagine an implementation where streamoff was defined like this:

struct streamoff {
    explicit streamoff(long offset);
    // rest of the stuff here...
};

Then just pos_type(-1) wouldn’t work because there would be no implicit conversion.

So I think it’s just being cautious.