C++, fstream objects passed to functions as reference, const?

Question

I know that generally, references to data should be isolated as a constant within a function so that the function can't change it, does the same hold true for fstream objects that are only being used for input?

Such as...

void doFoo(fstream &fileName)
{
  fileName.open("data.txt", ios::in);
} 

IF is it advisable, does it follow the same logic of most everything else?

Such as...

void doFoo(const fstream &fileName)
{
  fileName.open("data.txt", ios::in);
} 

also curious about output streams as well

I'm just wondering if it matters, and if so, why?

thanks!

Answer 1

The constantness of a file does not translate to the constant attribute of the object used to access the file. Even for just reading a file, you must modify internal buffers, the current read position etc. The same applies to output, so basically a const stream is not useful.

BTW: If you want to make clear that a function only reads from a stream, pass it a std::istream&. Firstly, the "i" in istream makes reasonably sure that you don't write to it. Secondly, the missing "f" of fstream allows the use with stringstreams, too, or streams like cin.