Can [std::streampos] be converted to [size_t] implicitly?

Question

#include <fstream>

int main()
{
    std::ifstream fin{ "" };
    size_t n = fin.tellg(); // ok
}

The code compiles ok. However, according to cppreference, I find fin.tellg() is a type of std::fpos, which doesn't define the ability to convert itself to size_t implicitly.

Any explanations?

`std::streampos` is an integer type that just *might* be compatible with `size_t` on your system. Just because there is no guarantee, doesn't mean that it *never* works. — Bo Persson, Jan 18 '17 at 08:18
You should be particularly careful with such conversion if the file happens to be very large. — Sven Nilsson, Jan 18 '17 at 08:21
It is theoretically possible that `std::fpos` is 64-bit while `size_t` is 32-bit. — rustyx, Jan 18 '17 at 08:25
@BoPersson `basic_istream::tellg()` returns `pos_type`; `pos_type` is `Traits::pos_type`; `char_traits::pos_type` is `streampos`; `streampos` is `fpos`, which is not an integral type? — BoBTFish, Jan 18 '17 at 08:26
@BoB - Ok, I missed one step. `fpos` is implicitly convertible to `streamoff`, which is the integer type. — Bo Persson, Jan 18 '17 at 08:31
Your question raises another: why do you need to convert it to a `size_t` in the first place? — user657267, Jan 18 '17 at 08:49

score 6 · Accepted Answer · answered Jan 18 '17 at 08:33

You are right about the fact that it returns an std::fpos. Now let's look at how it's defined:

template<class _Statetype>
    class fpos {<...>}

fpos also does have a conversion operator for converting into the streamoff type which is "stream offset" type:

__CLR_OR_THIS_CALL operator streamoff() const
    {   // return offset
    return ((streamoff)(_Myoff + _FPOSOFF(_Fpos)));
    }

On my machine streamoff happens to be defined as typedef _Longlong streamoff;, I would believe it's something similar on your machine. This is why it can be converted to type_t, however nothing prevents it from being larger than type_t, so be careful.

score 4 · Answer 2 · answered Jan 18 '17 at 08:30

"OK" on your platform. Not necessarily OK on all platforms. Whether in practice that means much depends on the circumstances. For example, on a 32-bit system, the file may be 4GB or larger, and thus not fit in the 32-bit size_t, where std::fpos is a 64-bit value and does hold the size of the file.

If the n is used to determine the length of the file or some such, serious problems may occur if you misjudge the total size - overwriting old data, or if you load the file and then save it based on that, you'd lose some or all of the data.

Just a minor note: The problem with loading and saving the file is more complicated - and cannot really be solved by a different type. The reason is that if the file-size doesn't fit in size_t then the file contents is unlikely to fit in memory. — Hans Olsson, Jan 18 '17 at 10:12

Can [std::streampos] be converted to [size_t] implicitly?

2 Answers2