class :: return value (void)

[spamme]

Hello!

I want to create a general data type "Parameter", which is used in a list "ParameterList" in class TestData. It works, and I obtain the value in Parameter for example via "get_typecast_value<int>()". I would like to simplify the get-value-call in parameter by using just "get_value()". However the compiler doesn't like my construction. What's my fault?

Thanks a lot!

class Parameter{
 private:
  void * const value;  
  const int type;

 public:

  template<class T> T get_typecast_value(){T *_v = (T*)value; return *_v;};
 
  void get_value(){    // doesn't work
    if      (TYPE_INT    == type) {get_typecast_value<int>();}
    else if (TYPE_DOUBLE == type) {get_typecast_value<double>();}
    else if (TYPE_Float  == type) {get_typecast_value<float>();}
    else if (TYPE_String == type) {get_typecast_value<string>();}
    else error_msg();
  }
};

class TestData{
 private:
  
 public:

  list<Parameter> ParameterList; 
}

void output_liste(list<Parameter> Liste)
{
  double value;

  typedef list<Parameter>::iterator LI;
   for(LI i=Liste.begin() ; i != Liste.end() ; i++){

     // doesn't work
     value = i->get_value();   

     // it's fine
     value = i->get_typecast_value<double>();
   }
}

Error Message of the Compiler:

In function ‘void output_liste(std::list<Parameter, std::allocator<Parameter> >)’:
src//template_field.cc:84: error: void value not ignored as it ought to be

[R0mai]

void get_value(); -> returns void
You have to strictly decide what your function will return, it can't be decided at run time. (With a small exception to inheritance). In your current implementation get_value() should be able to return either int, double, float, string, or possibly nothing. That's not going to work.

[spamme]

Hi R0mai,

thanks and ... it's pity with the data types.

[R0mai]

Depending on what is your final goal, we could come up with a solution. There are some boost libraries which could be useful for you.
boost::any : http://www.boost.org/doc/libs/1_43_0/doc/html/any.html
boost::variant : http://www.boost.org/doc/libs/1_43_0/doc/html/variant.html