Question

I want to create a class one of whose private: members is a struct point(see below). The public members ndim and numparticles are
set at run-time by the user, which are used to create corresponding arrays inside the class . However I am getting a compiler error. I don’t understand where I messed up.

The compiler error being displayed:

nbdsearch.h:25: error: ‘point’ does not name a type
nbdsearch.h:24: error: invalid use of non-static data member ‘nbdsearch::ndim’
nbdsearch.h:31: error: from this location
nbdsearch.h:31: error: array bound is not an integer constant before ‘]’ token

The class code:

class nbdsearch{
 public: 
  int ndim,numparticles;
  point particlevec[numparticles];


 private: 
  struct point{
    string key;
    double cood[ndim];
              };

};

Answer 1

nbdsearch.h:31: error: array bound is not an integer constant before ‘]’ token

double cood[ndim];

Array size needs to be a compile time constant and ndim clearly isn’t. ndim is a variable.

error: ‘point’ does not name a type

point particlevec[numparticles];

On line 25, compiler doesn’t know what point is. The structure is defined at a later point. Compilers in C++ work on a top to bottom approach(Not sure whether C++0X relaxes this rule). So, what ever types being used should be known to it before hand.

Try this –

class nbdsearch{
 private: 
  struct point{
    string key;
    std::vector<double>cood;
  };
 public: 
    int ndim,numparticles;
    std::vector<point> particlevec;   
};