Is there a good way to allow a function to write to the console, a file, or a memory buffer without using the IOStream library?

Comments on my answer here have made me think about how one might implement the same pattern I've been doing with C++ streams. Specifically, I need to be able to have a function which can write to the console, a file, or a string/memory buffer. I don't need most of the f开发者_开发问答ormatting features and such that IOStreams provide. Is there some better design which would allow this type of redirection idiomatically that's commonly used in other circles?

for example - for testing I need to be able to write to a string, but for real program use it's always going to be going to a file or the console.

Totally untested, but you get the idea.

struct stdio_stream {
    enum { invalid_t, file_t, str_t } which;
    union {
        FILE *file_p;
        string *str_p;
    };

    int printf( char *fmt, ... );
    int scanf( char *fmt, ... );

    stdio_stream() : which( invalid_t ), file_p( NULL ) {};

     // etc
};

int stdio_stream::printf( char *fmt, ... ) {
    int ret;
    va_list args;
    va_start( args, fmt );

    if ( which == file_t ) {
        ret = vfprintf( file_p, fmt, args );
    } else if ( which == str_t ) {
        string_p->resize( vsnprintf( NULL, 0, fmt, args )+1, '\0' );
        ret = vsnprintf( &* string_p->begin(), string_p->size(), fmt, args );
    } else throw runtime_error( "uninitialized stream" );

    va_end( args );
    return ret;
}

Personally, I think iostream is just great. If I need to go fast, I'll write a fast parser. printf still has a lot of room for speed improvement… it's a nice middle ground, I suppose, but still just a compromise.

Well, you may use "CON" filename to write to console as a file, and I believe that's all we can do nice without going over iostream usage complexity.