Set new size of dynamic array from an external function/class?

Question

My main code declares a pointer that needs to be resized from an external function. Ex:

double *vect =NULL;
classobj object;

object.func(vect);

//--- print the outcome
for(int j=0; j<4; ++j)
   cout << vect[j] << " .. " << endl;

where function func() is part of classobj defined in an another file (say classobj.h and .cpp) as

void classobj::func(double *_vect){

  _vect  = new double[4];

  for(int j=0; j<4; ++j)
    _vect[j] = 3.0*j +1 ;

};

The problem is that vect has not been resized. I get segmentation fault. Any idea please on how to use pointers in this case?

Declare the function as void classobj::func(double*& _vect). That said, this is horrible design and is destined to cause even more issues down the line. — user657267
– user657267, Commented Sep 30, 2015 at 9:02

Thomas Sparber · Accepted Answer · 2015-09-30 09:05:43Z

3

You are passing a reference to your Array and then you are creating a new Array and assigning it to the pointer in the function. You Need to pass a pointer of the pointer and then reassign the address of that pointer:

void classobj::func(double **_vect){
    (*_vect)  = new double[4];

Although this solution will work I recommend using a std::vector for this purpose and passing it by reference to that function

edited Sep 30, 2015 at 9:05

answered Sep 30, 2015 at 9:02

Thomas Sparber

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2 Comments

rkachach Over a year ago

Well, if you use std::vector then there's no need of caring about resizing. It's performed automatically as you insert elements in the vector. Thus, you don't have to care about memory management.

Courier Over a year ago

are you sure it should not be (*_vect) = new double[4]?

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