How to correctly declare a Polygon message

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You need to pre-allocate the points array before you assign members into it, or use a function that does the appropriate bounds-checking and resize for you. In particular, arrays in ROS messages in C++ are implemented as std::vectors, and std::vector::operator[] doesn't do any bounds checking.

If you just want to append items to your array, try:

geometry_msgs::Point32 tl;
polyStamp.polygon.points.push_back(tl);

If you already know how big your array is going to be, and you want to assign to specific indicies, try:

geometry_msgs::Point32 tl;
polyStamp.polygon.points.reserve(10); // ensure that there will be space for at least 10 points
polyStamp.polygon.points[0]=tl;