Question

What is an “opaque value” in C++?

Answer 1

An example for an Opaque Value is FILE (from the C library):

#include <stdio.h>

int main()
{
    FILE * fh = fopen( "foo", "r" );
    if ( fh != NULL )
    {
        fprintf( fh, "Hello" );
        fclose( fh );
    }
    return 0;
}

You get a FILE pointer from fopen(), and use it as a parameter for other functions, but you never bother with what it actually points to.