Using same stream object to write to filestream or stringstream

I am trying to use an ostream object to write to either to a filestream of stringstream based user input (similar to fmemopen in Linux).

I realized that ostream doesnt take stringstream or fstream objects but instead takes stringbug or filebuf.

I tried the following code:

    char content[] = "This is a test";
    if (isFile)
    {
        filebuf fp;
        fp.open(filename, ios::out);
        ostream os(&fp);
        os << content;
        fp.close();
    }
    else
    {
        stringbuf str;
        ostream os(&str);
        os << content;
    }

This works fine, in the if else condition but I would like to use the ostream os, as os << content, outside the if else condition. The issue is however I am unable to globally define ostream os since there is no such constructor for ostream.

Is there a way to get around this?

This can be handled a couple of different ways.

Using a helper function:

void write(ostream &os, const char *content)
{
    os << content
}

...

char content[] = "This is a test";
if (isFile)
{
    ofstream ofs(filename);
    write(ofs, content);
}
else
{
    ostringstream oss;
    write(oss, content);
    string s = oss.str();
    // use s as needed...
}

Alternatively, using a lambda:

char content[] = "This is a test";
auto write = [](ostream &os, const char *content){ os << content; }

if (isFile)
{
    ofstream ofs(filename);
    write(ofs, content);
}
else
{
    ostringstream oss;
    write(oss, content);
    string s = oss.str();
    // use s as needed...
}

Using a pointer instead:

char content[] = "This is a test";
std::unique_ptr<ostream> os;

if (isFile)
    os = std::make_unique<ofstream>(filename);
else
    os = std::make_unique<ostringstream>();

*os << content;

if (!isFile)
{
    string s = static_cast<ostringstream*>(os.get())->str(); // or: static_cast<ostringstream&>(*os).str()
    // use s as needed...
}