Problem in C++: error: expected primary-expression before ‘.’ token

Question

In this member function of class transcation, I use the "distance" function from another cpp program and that function is also a member of class "GPS_DD". My code is like below:

double

Transaction::getDistance()
`{ return GPS_DD.distance(tr_src_where, tr_dst_where);}

This function calculates two locations' distance from source to destination.

Then I get the error message and I have no idea to apply this function.

This is the "distance" function from class "GPS_DD":

double
GPS_DD::distance(GPS_DD& another)  {return GeoDataSource_distance(this->latitude, this->longtitude, 
 another.latitude, another.longtitude, 'M');

}

Regardless of the "GeoDataSource_distance, what code should be implemented in order to apply in the "getDistance" function?

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Your call doesn't even match the prototype. I suspect that you need `tr_src_where.distance(tr_dst_where)` and [a good book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — molbdnilo, Apr 20 '20 at 06:17
Okay! I just realized that tr_src_where itself is the GPS_DD class, and the argument is used for destination. I run this code and it successes, thanks! — William Wang, Apr 20 '20 at 06:33

score 1 · Answer 1 · answered Apr 20 '20 at 06:26

You need an instance of GPS_DD class, or transform GPS_DD::distance in a static method (but you cannot use this inside a static method).

for example: instead of:

Transaction::getDistance()
{return GPS_DD.distance(tr_src_where, tr_dst_where);}

you need to instantiate the GPS_DD class like this:

Transaction::getDistance()
{ GPS_DD anInstanceOfGPS_DD;
  return anInstanceOfGPS_DD.distance(tr_src_where, tr_dst_where);}

score 0 · Answer 2 · answered Apr 20 '20 at 06:31

It's pretty clear from the names involved that what you are looking for is

double Transaction::getDistance()
{
    return tr_src_where.distance(tr_dst_where);
}

to calculate the distance between tr_src_where and tr_dst_where.

The right syntax to call a method on an object is pretty basic. Time to revise the fundamentals?

Problem in C++: error: expected primary-expression before ‘.’ token

2 Answers2