ifstream and istream issue

Hello,
I found following source code:

Code:

void func_read(istream&, vector<string>&);
void func_read_file(ifstream&, vector<string>);

//and implementations

void func_read(istream& in, vector<string>& v)
{
	string s;
	while( in >> s )
	{
		v.push_back(s);
	}
}

void func_read_file(ifstream& in, vector<string> v)
{
	string s;
	while( in >> s )
	{
		v.push_back(s);
	}
}

I really don't know why this is written like this, I tested func_read:

Code:

int main()
{
	vector<string> v;

	ifstream in("test.txt");

	func_read( in, v );
	
	vector<string>::const_iterator it;

	for(it = v.begin(); it != v.end(); ++it)
		cout<<*it<<endl;

}

and everything seem to be OK, and it works fine since istream can accept referece to ifstream. So I assume there is no need to write special function to read from file (ifstream&).

Can you confirm this?
Is there any situations where this approach is necessary?

Thanks very much

and everything seem to be OK, and it works fine since istream can accept referece to ifstream. So I assume there is no need to write special function to read from file (ifstream&).

Yes, using the istream version, func_read(), is sufficient. func_read_file() is broken anyway, since the vector is passed by value.

Is there any situations where this approach is necessary?

When you want more flexibility, e.g. option of using cin or an istringstream instead of an ifstream.