Update features using an updated cursor
AddXY.cpp
// Copyright 2011 ESRI
// 
// All rights reserved under the copyright laws of the United States
// and applicable international laws, treaties, and conventions.
// 
// You may freely redistribute and use this sample code, with or
// without modification, provided you include the original copyright
// notice and use restrictions.
// 
// See the use restrictions.
// 

  
#include "AddXY.h"

int main(int argc, char **argv)
{

  std::cout << "AddXY - ArcObjects C++ SDK Developer Sample" 
            << std::endl << std::endl;

  if (argc != 2)
  {
    std::cerr << "Usage: AddXY [input shapefile]" << std::endl;
    AoExit(0);
  }
  
  char* data = argv[1];

  // Process arguments
  CComBSTR dataPath;
  CComBSTR dataName;
  HRESULT hr = GetParentDirFromFullPath(data, &dataPath);
  if (FAILED(hr) || dataPath.Length() <= 0)
  {
    std::cerr << "Couldn't get input data path." << std::endl;
    AoExit(0);
  }
  hr = GetFileFromFullPath(data, &dataName);
  if (FAILED(hr) || dataName.Length() <= 0)
  {
    std::cerr << "Couldn't get input data file name." << std::endl;
    AoExit(0);
  }

  if (!InitializeApp())
  {
    AoExit(0);
  }

  // Add the x and y fields 
  hr = AddXYValues(dataPath, dataName);
  if (FAILED(hr))
  {
    std::cerr << "AddXY failed." << std::endl;
    AoExit(0);
  }
  else
    std::cerr << "Done!" << std::endl;

  ShutdownApp();
  AoExit(0);
}

HRESULT AddXYValues(BSTR dataPath, BSTR dataName)
{
  IWorkspaceFactoryPtr ipWkspFac(CLSID_ShapefileWorkspaceFactory);
  IWorkspacePtr ipWksp;
  HRESULT hr = ipWkspFac->OpenFromFile(dataPath, 0, &ipWksp);
  if (FAILED(hr) || ipWksp == 0)
  {
    std::cerr << "Couldn't open workspace factory." << std::endl;
    return E_FAIL;
  }
  IFeatureWorkspacePtr ipFeatureWksp(ipWksp);

  IFeatureClassPtr ipFeatureClass;
  hr = ipFeatureWksp->OpenFeatureClass(dataName, &ipFeatureClass);
  if (FAILED(hr) || ipFeatureClass == 0)
  {
    std::cerr << "Couldn't open feature class." << std::endl;
    return E_FAIL;
  }
  esriGeometryType shape;
  ipFeatureClass->get_ShapeType(&shape);

  if (shape != esriGeometryPoint)
  {
    std::cerr << "This is not a shapefile of points." << std::endl;
    return E_FAIL;
  }
  else std::cerr << "Adding X & Y value columns to the shapefile of points..."
                 << std::endl;

  // Check to see if X, Y fields are on the class. If not, add them.
  long found;
  ipFeatureClass->FindField(CComBSTR(L"textX"), &found);
  if (found == -1)
  {
    std::cerr << "Making the shape field textX..." << std::endl;
    IFieldPtr ipField(CLSID_Field);
    IFieldEditPtr ipFieldEdit(ipField);
    ipFieldEdit->put_Name(CComBSTR(L"textX"));
    ipFieldEdit->put_Type(esriFieldTypeDouble);
    ipFeatureClass->AddField(ipField);
  }
  else
    std::cerr << "textX field already exists -- no X values being added." 
              << std::endl;

  ipFeatureClass->FindField(CComBSTR(L"textY"), &found);
  if (found == -1)
  {
    std::cerr << "Making the shape field textY..." << std::endl;
    IFieldPtr ipField(CLSID_Field);
    IFieldEditPtr ipFieldEdit(ipField);
    ipFieldEdit->put_Name(CComBSTR(L"textY"));
    ipFieldEdit->put_Type(esriFieldTypeDouble);
    ipFeatureClass->AddField(ipField);
  }
  else
    std::cerr << "textY field already exists -- no Y values being added." 
              << std::endl;
  
  // Loop through the features and populate the new fields
  IFeatureCursorPtr ipFeatureCursor;
  ipFeatureClass->Update(0, VARIANT_FALSE, &ipFeatureCursor);
  
  std::cerr << "Getting the positions of the fields to update..." << std::endl;
  long lXField, lYField;
  ipFeatureCursor->FindField(CComBSTR(L"textX"), &lXField);
  ipFeatureCursor->FindField(CComBSTR(L"textY"), &lYField);
  
  std::cerr << "Processing the features..." << std::endl;
  IFeaturePtr ipFeature;
  ipFeatureCursor->NextFeature(&ipFeature);
  IPointPtr ipPoint;
  IGeometryPtr ipGeom;
  while (ipFeature != 0)
  {
    HRESULT hr = ipFeature->get_Shape(&ipGeom);
    ipPoint = ipGeom;

    double x, y;
    ipPoint->get_X(&x);
    ipPoint->get_Y(&y);
    ipFeature->put_Value(lXField, CComVariant(x));
    ipFeature->put_Value(lYField, CComVariant(y));

    ipFeatureCursor->UpdateFeature(ipFeature);
    ipFeatureCursor->NextFeature(&ipFeature);
  }

  return S_OK;
}