What is the best way to return an image or video file from a function using c++?

Question

I am writing a c++ library that fetches and returns either image data or video data from a cloud server using libcurl. I've started writing some test code but still stuck at designing API because I'm not sure about what's best way to handle these media files. Storing it in a char/string variable as binary data seems to work, but I wonder if that would take up too much RAM memory if the files are too big. I'm new to this, so please suggest a solution.

Answer 1

You can use something like zlib to compress it in memory, and then uncompress it only when it needs to be used; however, most modern computers have quite a lot of memory, so you can handle quite a lot of images before you need to start compressing. With videos, which are effectively a LOT of images, it becomes a bit more important -- you tend to decompress as you go, and possibly even stream-from-disk as you go.

The usual way to handle this, from an API point of view, is to have something like an Image object and a Video object (classes). These objects would have functions to "get" the uncompressed image/frame. The "get" function would check to see if the data is currently compressed; if it is, it would decompress it before returning it; if it's not compressed, it can return it immediately. The way the data is actually stored (compressed/uncompressed/on disk/in memory) and the details of how to work with it are thus hidden behind the "get" function. Most importantly, this model lets you change your mind later, adding additional types of compression, adding disk-streaming support, etc., without changing how the code that calls the get() function is written.

The other challenge is how you return an Image or Video object from a function. You can do it like this:

Image getImageFromURL( const std::string &url );

But this has the interesting problem that the image is "copied" during the return process (sometimes; depends how the compiler optimizes things). This way is more memory efficient:

void getImageFromURL( const std::string &url, Image &result );

This way, you pass in the image object into which you want your image loaded. No copies are made. You can also change the 'void' return value into some kind of error/status code, if you aren't using exceptions.

Answer 2

If you're worried about what to do, code for both returning the data in an array and for writing the data in a file ... and pass the responsability to choose to the caller. Make your function something like

/* one of dst and outfile should be NULL */
/* if dst is not NULL, dstlen specifies the size of the array */
/* if outfile is not NULL, data is written to that file */
/* the return value indicates success (0) or reason for failure */
int getdata(unsigned char *dst, size_t dstlen,
            const char *outfile,
            const char *resource);