example.cpp

// do this
vector<ARAnchor> mats;

for(int i = 0; i < session.currentFrame.anchors.count; ++i){
  mats.push_back(session.currentFrame.anchors[i]); 
}

// then access transforms from the "mats" variable.

// if you grab a transform from "mats", you shouldn't run into an out of bounds error because 
// ARKit won't be automatically changing things in the "mats" variable.