[cairo] What is the closure in stream output to file based surfaces for?

[David Weiß]

Hey,

I want to implement file output for a rendering application and want to use C++ file streams using the following callback write function:

    cairo_status_t cairo_write_func (void *closure, const unsigned char *data, unsigned int length)
    {
        try
        {
            const std::string s(data, length);
            fout << s;
            fout.flush();
        }
        catch (std::exception &e)
        {
            return CAIRO_STATUS_WRITE_ERROR;
        }

        return CAIRO_STATUS_SUCCESS;
    }

I just can't figure out what the closure is for.
Can anyone explain the concept to me?
Is it comparable to variable capturing in C++11 closures?
Could I just pass nullptr as I do not use it anyway?

Thanks,
--
David Weiß

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