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Asked: 2026-06-14T20:21:15+00:00

I am trying to learn C++ and I’m having some difficulties learning pointers and

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I am trying to learn C++ and I’m having some difficulties learning pointers and references. I’m trying to understand why some of the below do not work, and I cannot seem to figure out the difference between “double *const ptd” and “const double *ctd” 

double d;
  const double r; //bad; r must be initialised
  const double pi = 3.1416;
  double *ptr = π //illegal to point to a constant, because otherwise one could change the value of the constant which defies the purpose of a constant
  double *const cpt; //bad; cpt must be initialised
  double *const ptd = &d;
  const double *ctd = &d;
  const double *ptc = π
  double *const ptp = π //illegal
  const double *const ppi = π 
  double * const * pptr1 = &ptc;
  double * const * pptr2 = &ptd;

  void F () {
    ptr = new double;
    r = 1.0;
    *ptr = 2.0;
    cpt = new double;
    *cpt = 3.0;
    ptc = new double;
    *ptc = 4.0;
    ptd = new double;
    *ptd = 5.0;
    ctd = new double;
    *ctd = 6.0;
    ptp = new double;
    *ptp = 7.0;
    ppi = new double;
    *ppi = 8.0;
  }
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1 Answer

  1. Editorial Team

    double * const (read as “constant pointer”) means that the pointer is constant. You can’t modify it, but you can change the object that it points to.

    const double * or double const * (read as “pointer to constant”) means that the pointer can’t be used to change the object it points to; but the pointer itself can be modified.

    In general, the const qualifies the thing immediately before it; or the first thing, if there’s nothing before it.

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