(void)0 (+;) is a valid, but ‘does-nothing’ C++ expression, that’s everything. It doesn’t translate to the no-op instruction of the target architecture, it’s just an empty statement as placeholder whenever the language expects a complete statement (for example as target for a jump label, or in the body of an if clause).
From Chris Lutz’s comment:
It should be noted that, when used as a macro (say, #define noop ((void)0)), the (void) prevents it from being accidentally used as a value (like in int x = noop;).
For the above expression the compiler will rightly flag it as an invalid operation. GCC spits error: void value not ignored as it ought to be and VC++ barks 'void' illegal with all types.