i don't get it - if do

  int foo(int a) {
    assert(a > 5);
    int b = a * 10;
    assert(b > 50);
    return b;
  }

do you think those asserts don't "run automatically"?

You define the invariants once for the class and they are run around every public function. Done manually you'd probably use a helper object that calls the invariants in its constructor and destructor (have to handle exceptions) that you have to add to every public function's definition.