Thread: Using pointers to access variables in structures

Hey guys. I have this project where I need to use a structure without knowing what its name is nor how its members are named. I decided to force the user to add a static const char* Format variable which holds information on the type of the members only (i = int, d = double..) and this way I can somewhat get around this problem. However, it seems that some of my calculations are wrong even though when I tried it with a dummy class and a few pointers in the main() function it worked, now that I integrated all of this into my class, there's a problem. I stepped through with the debugger and all the values are correct; however, the debugger also revealed that only the first variable is modified (i.e. the pointer is not shifted properly to the next variable). How is this possible according to the following code ? I have checked back my algorithm and it seems correct to me.

Code:

	template <typename ArgT>
	void ProcessVar(void* ptr, ArgT* var, std::string tmp)
	{
		StrToT(var, tmp);
		*((ArgT*)ptr) = *var; // This sets the variable pointed to to *var
		ptr = ((ArgT*)ptr + 1); // This is the line shifting the pointer
	}

Thanks.

I have this project where I need to use a structure without knowing what its name is nor how its members are named. I decided to force the user to add a static const char* Format variable which holds information on the type of the members only (i = int, d = double..) and this way I can somewhat get around this problem.

Your idea seems to work, but that's what I thought <typeinfo> was there for. You might want to use that instead. As for your problem, have you tried something like:

((ArgT*) ptr)++;

That should shift the void * properly.

Using the post-increment operator makes the compiler complain about the expression not being an l-value and using the typeid operator isn't of much use, if it works as I understand it, because I do not even know what type of variable will be the next variable, thus I cannot cast a pointer to this type and typeid() cannot retrieve any useful information. I don't know if you know what I mean but I'm fairly sure typeid() can't do the job here. Thanks for the suggestions though.

>I have this project where I need to use a structure without knowing what its name is nor how its members are named.
That severely limits what you can do with it. Can you be more specific about what this project entails?

> void ProcessVar(void* ptr, ArgT* var, std::string tmp)
You probably need to pass ptr as a reference. Otherwise the change to ptr won't be seen in the calling function.

Code:

	void ProcessVar(void* &ptr, ArgT* var, std::string tmp)