CODEWITHSUNDEEP
c++pointersvector
Cynizm
Cynizm

Reputation: 99

How to access members of type std::vector

my program has a function to calculate some sum, for this i need to access attributes of objects in vector:

declaration of vector: 

class trilateration {
public:
    ...
    std::vector<tip> *potential;
    ...
};

then in constructor its initialized:

trilateration::trilateration()
{
...
potential = new std::vector<tip>();
...
}

class tip looks like this:

class tip {
public:

double sum;
Point2d *pt;
tip();
tip(double x, double y);
virtual ~tip();
};

tip constructor:

tip::tip(double x, double y)
{
pt = new Point2d(x,y);
sum=0;
}

objects are added to vector in some function like this:

potential->push_back(tip1);

then i want to access some objects in vector like this:

void trilateration::get3points()
{

for(int i=0; i<potential->size(); ++i)
{
    for(int j=0; j<potential->size(); ++j)
    {
        potential[i].sum=potential[i].sum+normalize(potential[i].pt,potential[j].pt);
    }

}
}

while compilation im getting follwoing error:

error: ‘class std::vector<tip>’ has no member named ‘sum’
error: ‘class std::vector<tip>’ has no member named ‘pt’

how can i acess these attributes from vector?

EDIT:

after changing potential to be a member of trilateration and pt to be member of tip, program compiled but when it encounters 

potential.push_back(tip1);

throws:

*** glibc detected *** ./loktest: malloc(): memory corruption: 0x00792f10 ***

Upvotes: 4

Views: 414

Answers (2)

Mr.C64
Mr.C64

Reputation: 43004

Unless there are strong reasons to have pointer data members and to allocate your objects on the heap using new, don't do that.

Your code seems to use a kind of Java-style, but this is C++, not Java. Enjoy the automatic resource management of C++, and the power of C++ destructors. Just define data members without using pointers and dynamic allocations.

In class trilateration, change from vector<tip>* to vector<tip>:

class trilateration {
public:
    ...
    // WAS std::vector<tip> *potential;
    std::vector<tip> potential;
    ...
};

In trilateration's constructor, you don't need to create the vector dynamically; just delete the line allocating the vector with new:

trilateration::trilateration()
{
    ...
    // REMOVED:
    // potential = new std::vector<tip>();
    ...
}

Note that in your previous code, when you allocated the vector with new, you had to properly delete it in the destructor, and provide proper copy operations (copy constructor and copy assignment), or ban copies marking the aforementioned operations =delete.

Instead, you don't need all this complicated stuff if you have an ordinary simple non-pointer data member.

The same goes for your tip class.
Unless there is a strong reason to have a Point2d* data member, just use a Point2d (non-pointer) data member:

class tip {
public:

  double sum;

  // WAS: Point2d *pt;
  Point2d pt; // <-- note: no pointers here

  tip();
  tip(double x, double y);

  // NOTE: Do you really need a virtual destructor here?? 
  virtual ~tip();
};

Change the constructor as well:

tip::tip(double x, double y)
    : pt(x, y), sum(0)
{
  // REMOVE:
  // pt = new Point2d(x,y);
  //sum=0;
}

Your code gets simplified, and you'll avoid some bugs and headaches.

Upvotes: 4

YSC
YSC

Reputation: 40150

Since trilateration::potential is a std::vector<tip>*, potential[i] is a std::vector<tip> located at trilateration::potential + i. This is not what you want.

for(int j=0; j<potential->size(); ++j)
{
    (*potential)[i].sum=potential[i].sum+normalize((*potential)[i].pt,(*potential)[j].pt);
}

is a possible hack. But what you really want is to get rid of all those useless pointers. Make the vector a member variable of your class and stop using new where it is not needed.

Upvotes: 3

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