How do you exit from a void function in C++?

You mean like this?

void foo ( int i ) {
    if ( i < 0 ) return; // do nothing
    // do something
}

Use a return statement!

return;

if (condition) return;

You don't need to (and can't) specify any values, if your method returns void.

void foo() {
  /* do some stuff */
  if (!condition) {
    return;
  }
}

You can just use the return keyword just like you would in any other function.

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