structure of a struct...

[avi123]

Suppouse I have a struct/class for example:

struct cMyStruct
{

int x;
char y;
double z;
char abc[5];
char ser[6];
};

and I'm sending it/or a pointer of it as a function parameter,
for example void MyFunc(cMyStruct* myStruct)
Is there a way for the function, to the know the structure of the struct?

Is there a way to know the offset of each field in the struct?

Thanks
avi

[Gabriel Fleseriu]

struct cMyStruct s;
int offset_of_abc = (char*)(&s.abc) - (char(*)(&s);

[Guysl]

giving

MyFunc(cMyStruct* myStruct)


the offset of each field is
(unsigned char*)&(myStruct->fieldName) - (unsigned char*)myStruct

[avi123]

but if I don't know the name of the field abc, is there a way to get all the class offsets?

Thanks
avi

[Assmaster]

Since you are passing the struct as a cMyStruct* to the function, how can you not know? The function will recieve a cMyStruct and since member-variable-names are known at compile time, just check the definition/documentation for the names. The number of, and the name of members can't magically change during run-time.

[avi123]

of course I can look at the structure definition...

but I wanted to know if there is a way to get the address of each of the fields without using thier names (I want it to be general for a few structs)

Thanks
avi

[Zaccheus]

How would you specify which field you wanted?

[avi123]

I don't know, that's what I want to know, is it possible?
Is there a function that may return the intrire strcture of the struct (I mean holds x fields, with the offsets/address...)

Anyone?

Thanks again
avi

[Zaccheus]

That information is not preserved by a C++ compiler.

The compiler just creates machine code instructions for each structure access you do.

Something like:

struct ms {int a; long b};
ms s;
s.b = 1;

becomes something like:

mov s(4), #1 (move 1 into 'the address that s is at plus 4').

So the compiler works out that member b is at offset 4 (because a is four bytes longs and b comes after it), and hard codes that whenever you access the structure.

[avi123]

ok, then is there a way to know the type of the fields in the structure, I mean is there a way to get for example: int, int, char[4], char[6]

for the struct:

struct cMyStruct
{
int x;
int y;
char z[4];
char z[6];
}

Thanks
avi

[MrViggy]

Nope. Once it's in machine code, you cannot. You might be able to guess at what the object is, by the length and the data it contains (that is, if you can tell what the data really is), but that's all you'd be doing is guessing.

Nothing about the structure is preserved (like Zaccheus said). So, how *could* you know what the fields are?

Viggy

[Guysl]

I'm not sure what you are trying to do,
this code might help. if you decide to use it, then you have
to implement the virtual function for each structure/class
according to its data members.

#include <vector>
#include <string>
#include <typeinfo.h>
#include <iostream.h>

using namespace std;


struct A{
	char a[20]; long b; short c; char d; 

        // vector contains pairs of member typename and offset
	// DO NOT FORGET A VPTR IS ADDED TOO TO AN OBJECT
        
        virtual vector<pair<string,int> > structure() 
	{
	  vector<pair<string,int> > descVec;

          descVec.push_back(make_pair(typeid(a).name(), (char*)&(this->a) - (char*)this));
          descVec.push_back(make_pair(typeid(b).name(), (char*)&(this->b) - (char*)this));
          descVec.push_back(make_pair(typeid(c).name(), (char*)&(this->c) - (char*)this));
          descVec.push_back(make_pair(typeid(d).name(), (char*)&(this->d) - (char*)this));

	  return descVec;
	}
};


int main()
{
   A obj;
   vector<pair<string,int> > descVec = obj.structure();

   for(int i=0; i < descVec.size(); i++)
	   cout << "TYPE: " << descVec[i].first.c_str() << endl << "OFFSET: " << descVec[i].second << endl << endl;

   return 0;

}

you can add information about sizeof() of each member, but then,
pair would have to be replaced by other container.

[Zaccheus]

Originally posted by Guysl

... you can add information about sizeof() of each member, but then, pair would have to be replaced by other container.

You could try this slightly wacky concoction:
pair<string, pair<int,size_t> >