"Pavol Droba" <droba@topmail.sk> wrote in message news:20041008202443.GA22250@lenin.felcer.sk... | > that's not what null objects are for. | | So what is the difference between your null-object and 0 pointer? | I see none, except in your case I will have to define special class/state | for every type I would like to use the smart_container with. the point is basically to avoid all if( !null ) logic. | > | > > Otherwise, you will have pay the comparison cost every time you perform an | > > operation | > > on the null object, and that might be on far more places. | > > | > not understood. | | Try to ask Jeff Garland, how many complications it brought when a date class in date_time | library was enriched with the special values. Every operation must be aware of it. | yes, the 0 pointer is a special value and we want to avoid the complications it brings. | > I need some real code to work with to be able to see this. Then I can rewrite | > it to compare. | > | | // Some classes to work with | class Base | { | virtual int Op1() =0; | ... | virutal int Op100() =0; | }; | | | class A : public Base | { | virtual int Op1() { // do something reasonable and return a valid value // } | .. | virtual int Op100() { // do something reasonable and return a valid value // } | }; | | | class Null : public Base | { | virtual int Op1() { return special_value1; } | .. | virtual int Op100() { return special_value100; } | }; I would return 0 here, in all cases. | // Now some operations, that use the object of Base. | // Assume we have 0 pointers | | int foo1(Base* pObj) | { | // Simple case -> check if oObj is not null | if(pObj==0) | return special_value; | else | return pObj->Op1() + pObj()->Op2() + ...... + pObj()->Op100(); | } int foo1( Base& pObj ) { return pObj->op1() + ... + pObj->op100(); } | This example might look realy stupid, but it illustrates well the problem I'm refering to. | You cannot assume, that null object will work in the same way as an ordinary one. Otherwise | you only pospone the checking one level further -> to its operations and the objects, that | uses it. Yeah, that is really the issue here: are we just forcing users to defer the comparinson for "nullness" yet another level. Am eager to be proven wrong, but It's going to take more convincing examples. I would like to see real examples and then see if I can recode them so we can compare. But it has to be real examples (you must have plenty from your work? Just take one where you use shared_ptr). But seriously, I don't recall the need for 0's for a long time. A long with that code I need to see why there is a need to put a 0 in the container in the first place and it would also be nice to know the density of them. br Thorsten