In LLDB, can I call methods and make instances of C++ classes?

Question

I am trying to explore the behaviour of a complex C++ application using Clang and LLDB. I have set a breakpoint in my applicaiton. Once I reach that breakpoint, I would like to create an instance of a simple C++ class and then call a method in the context of that breakpoint.

For example, here is my application:

#include <iostream>
#include <vector>

struct Point {
  int x;
  int y;
};

int main() {
  std::vector<Point> points;
  points.push_back(Point{3, 4});
  // <--------- Breakpoint here
  int total = 0;
  for (const auto& p : points) {
    total += p.x * p.y;
  }
  std::cout << "Total: " << total << std::endl;
  return 0;
}

Inside the breakpoint above, I would like to:

1. Clear the points vector
2. Create a new Point instance
3. Add it to the vector
4. Continue execution

This example is trivial, but often I have a considerably bigger application. Is this possible using expr?

---

Update

I receive this error when trying to clear the points:

(lldb) expr points.clear()
warning: could not load any Objective-C class information. This will significantly reduce the quality of type information available.
error: Couldn't lookup symbols:
  __ZNSt3__16vectorI5PointNS_9allocatorIS1_EEE5clearEv

I can create an object, which is good!

(lldb) expr auto $x = Point{1, 2}
(lldb) expr $x
(Point) $x = {
  x = 1
  y = 2
}

However, I cannot push it into my vector:

(lldb) expr points.push_back($x)
error: Couldn't lookup symbols:
  __ZNSt3__16vectorI5PointNS_9allocatorIS1_EEE9push_backERKS1_

Answer 1

You can create objects in the debugger. The trick to telling the debugger you want to create a persistent object in the expression parser is to give it a name starting with a "$" when you create or refer to it. Then lldb will make sure the object persists.

Note, however, the caveats when working with STL classes mentioned in:

Printing/Debugging libc++ STL with XCode/LLDB