I don't think this is enough for a lot of common cases. Let's say you want to write a 'sort()' function. It takes a 'vector' and a comparator function. Unless that comparator function is declared 'const', you can't be sure that it won't destroy your 'vector'.

Another example. Let's say your 'foo()' function calls 'bar.toString()'. How can the compiler tell whether this call should be allowed when 'bar' is 'const'? You would need to know whether 'toString()' modifies 'bar' or not, but you don't have a way to tell. You might be able to figure this out if you look inside the code of 'toString()', but what if 'toString()' is virtual? You can't look at the code then.