c++ Access via iterators headaches part 2

[muteXe]

Hiya,

I have a method that takes in 2 objects, uses these objects to calculate some value, then returns the value. I do not change the original objects at all, so I thought it best to pass in by const reference:



double GetVal(const CPosition& pos1,const CPosition& pos2)
{
double fRtn = 0.0f;
// do the calculations here

return fRtn;
}


And when i call it i am iterating through a loop, in which the positions change:



std::vector<CPosition*>::iterator i;
for(i = clList.begin(); i != clList.end(); ++i)
{
CPosition sensorPos(0.0f,0.0f,0.0f);
double val = GetVal(sensorPos, (*i));
std::cout << val << std::endl;
}


But it's not liking the way I derefence 'i' and pass it into GetVal().

I know this is something to do with the fact that dereferencing i gives me a pointer to a CPosition, but my method takes a constant ref to a CPosition, but i am not sure how best to resolve this.

At the end of the day I am not modifying the position objects, just using their member variables to do some simple maths and return the result, so I would like to keep the input params const.

Hmm...I've just tried creating a local CPosition variable (after writing myself a proper CPosition copy constructor), like so:



CPosition sensorPos(0.0f,0.0f,0.0f);
CPosition mPos(**i);
double fVal= GetVal(sensorPos, mPos);
std::cout << fVal<< std::endl;


and this actually works! Can someone convince my tiny brain why


CPosition mPos(**i);


compiles and gives me the right numbers please?

My thanks in advance,

Tom

[dwhitney67]

You need to dereference the iterator to get to the element contained within the vector. That element happens to be a pointer to a CPosition object.

Thus when you want to pass this element by reference, you need to dereference the element's pointer. The following should suffice:


std::vector<CPosition*>::iterator i;
for(i = clList.begin(); i != clList.end(); ++i)
{
CPosition sensorPos(0.0f,0.0f,0.0f);
double val = GetVal(sensorPos, **i);
std::cout << val << std::endl;
}

Note the extra '*' associated with 'i'.

P.S. Since it seems your intent is to avoid modifying the contents of the vector, you should employ the use of the 'const_iterator' in lieu of the 'iterator'.

P.S. #2 Consider declaring the constructor for CPosition as the following:


CPosition(double x = 0.0, double y = 0.0, double z = 0.0);

This would obviate the need to pass coordinates representing the origin to the constructor. In other words, these would be equivalent:


CPosition sensorPos_1(0.0, 0.0, 0.0);

CPosition sensorPos_2;

[muteXe]

Thanks again dw, i get it now.
I was also not aware of 'const_iterator'. It seems that 3 years of C# at work has erased a lot of pointer/reference knowledge.

Thanks again mate,

Tom

In response to your PS#2: I have actually done that. That CPosition sensorPos_1(0.0f, 0.0f, 0.0f) is just an 'quick' test line. Cheers.