Use 'NULL' the 'nullptr' is for higher versions of Visual Studio.
You are able to define 'nullptr' as NULL too but useless, Upgrade Source to vs17 or use the Source Blouflash has released... Everything other is just **** because you dont know what kind of Person work on it... u dont know about Backdoors or some higher exp at some mobs or maybe drops at one/more mobs...
PS: if you got the defines you are also able to use 'nullptr' , '0' and custom text's like 'nll'