How can I call a method of an object in a list iterator in C++?

Question

I have a list of Point objects in C++11 and I want to combine each point's string value into a longer string. I have a method to get the point as a string in a certain format, but I can't figure out how to call that method while iterating through the list.

This is my code:

  list<Point> points;
  string results = "";
  
  for (int i = 0; i < num_points; i++) {
    Point ptemp = Point(random01(), random01());
    points.push_back(ptemp);
    results += ptemp.getStr();
  }
  
  string log_output = "";
  string sep = ",";
  list<Point>::iterator iter;
  for (iter = points.begin(); iter != points.end(); ++iter) {
    // get the first three points in the log.txt format
    log_output += *iter.getRawStr(",") + " , ";  
  }

I'm getting this error:

error: ‘std::__cxx11::list<Point>::iterator’ {aka ‘struct std::_List_iterator<Point>’} has no member named ‘getRawStr’
  177 |     log_output += *iter.getRawStr(",") + " , ";

I've tried taking away the asterisk, using to_string() instead, and reformatting the loop to use an increasing integer i with the iterator methods called separately inside the loop, but it still doesn't work. I've also tried researching it, but I've only seen cout << *iter, not how to specifically call methods from the iterator. What should I do?

. operator has higher precedence than *. You need (*iter).getRawStr() or iter->getRawStr() — Yksisarvinen
– Yksisarvinen, Commented Oct 18, 2022 at 19:32

Silvio Mayolo · Accepted Answer · 2022-10-18 19:34:07Z

2

It's a case of operator precedence. The . binds more tightly than *, so this

*iter.getRawStr(",")

is actually this

*(iter.getRawStr(","))

That is, it tries to call getRawStr on the iterator (not the thing it's pointing to), and then dereference the result. You want

(*iter).getRawStr(",")

which is equivalent^[1] to

iter->getRawStr(",")

^[1] Technically, operator -> can be overridden, so a pathological class might define it to do something different than unary *, but no normal, sane C++ class should do this.

answered Oct 18, 2022 at 19:34

Silvio Mayolo

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Comments

Vlad from Moscow · Accepted Answer · 2022-10-18 19:34:51Z

1

Either write

log_output += ( *iter ).getRawStr(",") + " , ";

or

log_output += iter->getRawStr(",") + " , ";

Otherwise this expression

*iter.getRawStr(",")

is equivalent to

* ( iter.getRawStr(",") )

answered Oct 18, 2022 at 19:34

Vlad from Moscow

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fabian · Accepted Answer · 2022-10-18 19:38:56Z

1

The issue here is the . operator having higher precedence than the unary * operator, so the compiler looks for a member getRawStr in std::list::iterator. You need to use () to change this. Alternatively you could use a range-based for loop allowing you to skip the iterator-related code:

for(auto& point : points)
{
    log_output += point.getRawStr(",") + " , ";
}

answered Oct 18, 2022 at 19:38

fabian

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