how to use a function parameter in the function body?

[ardavan]

I want to use object M in the function body but It dose not implement,when I create another object from MobileNode class and equal two objects again it dose not implement, what is wrong with it?
==================================================================
void MacTdma::NodeLocation(MobileNode* M)
{
double a=0.0;
a=M->X();
printf("the X position is: %f \n ",a);
}
==================================================================
void MacTdma::NodeLocation(MobileNode* M)
{
MobileNode* m3=new MobileNode;
m3=M;
double a=0.0;
a=m3->X();
printf("the X position is: %f \n ",a);
}

[Repeater]

If you told us the error message, we could tell you what it means.

[ardavan]

the content of M is nill, so I can't use it in a=M->X();

[Repeater]

If M is null (C++ doesn't recognise "nil"; if your pointer is deliberately set to not be pointing at something, it is a NULL pointer), then you never set it to point at a MobileNode object. Make it point at the appropriate MobileNode object.

You need to create an object of type MobileNode, and then if you want a pointer to that object, create a pointer of type MobileNode* and then make that pointer point at the MobileNode object.

I suspect that you actually don't need to use pointers at all for what you're trying to do. I also suspect that you don't understand pointers. Here's something that explains them in simple terms: http://www.cplusplus.com/articles/EN3hAqkS/

[keskiverto]

This in itself seems ok:

void MacTdma::NodeLocation( MobileNode* M )
{
  double a = 0.0;
  a = M->X();
  printf( "the X position is: %f \n ", a );
}

However:
* We don't know whether MobileNode has member function X().
* You don't check whether the pointer is null (nullptr).
* You do use C-library's I/O, rather than C++ iostreams.
* Unnecessary statements. Why initialize to 0.0 and then assign a new value?

void MacTdma::NodeLocation( MobileNode* M )
{
  if ( M ) {
    double a = M->X();
    printf( "the X position is: %f \n ", a );
  }
}

Or

void MacTdma::NodeLocation( MobileNode* M )
{
  if ( M ) {
    std::cout << "the X position is: " <<  M->X() << '\n';
  }
}

The other one is a horror story:

void MacTdma::NodeLocation(MobileNode* M)
{
  MobileNode* m3  // you declare a local variable m3 that is a pointer
                 = new MobileNode; // you create object in free store
  // then you initialize the pointer m3 with the address of the object
  // that is ok in itself

  m3 = M; // you assign address stored in M into m3
  // this overwrites the address that the m3 was initialized with
  // you now longer have the address of the object that is in free store
  // you have "leaked" the memory of the object

  double a=0.0;
  a=m3->X();
  printf("the X position is: %f \n ",a);
}

We could make that shorter:

void MacTdma::NodeLocation( MobileNode* M )
{
  new MobileNode; // useless allocation of memory,
  // construction of object,
  // and a leak

  double a = 0.0;
  a = M->X();
  printf( "the X position is: %f \n ", a );
}

If you would like to copy value from object pointed to by M into object pointed to by m3, you would:

void MacTdma::NodeLocation(MobileNode* M)
{
  if ( M ) {
    MobileNode* m3 = new MobileNode( *M ); // copy construction
    // or
    *m3 = *M; // copy assignment of dereferenced objects

    double a = m3->X();
    printf("the X position is: %f \n ",a);
    delete m3; // deallocate memory of the object
  }
}