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I assume you want to get the corners of your robot in the map frame.

The easiest way to do that is to use tf's transformPose. Create for poses in the robot's base_link frame and transform all of them to map. Have a look at the following code snippet (assuming that you already have an instance of tf::TransformListener bound to the variable transform_listener):

tf::Stamped<tf::Pose> corner1(
    tf::Pose(tf::Quaternion(0, 0, 0, 1), tf::Vector3(0.30, -0.45, 0.0)),
    ros::Time(0), "base_link");
tf::Stamped<tf::Pose> corner2(
    tf::Pose(tf::Quaternion(0, 0, 0, 1), tf::Vector3(0.30, 0.45, 0.0)),
    ros::Time(0), "base_link");
tf::Stamped<tf::Pose> corner3(
    tf::Pose(tf::Quaternion(0, 0, 0, 1), tf::Vector3(-0.30, -0.45, 0.0)), 
    ros::Time(0), "base_link");
tf::Stamped<tf::Pose> corner4(
    tf::Pose(tf::Quaternion(0, 0, 0, 1), tf::Vector3(-0.30, 0.45, 0.0)), 
    ros::Time(0), "base_link");

tf::Stamped<tf::Pose> transformed_corner_1;
transform_listener.transformPose("map", corner_1, transformed_corner_1);
tf::Stamped<tf::Pose> transformed_corner_2;
transform_listener.transformPose("map", corner_2, transformed_corner_2);
tf::Stamped<tf::Pose> transformed_corner_3;
transform_listener.transformPose("map", corner_3, transformed_corner_3);
tf::Stamped<tf::Pose> transformed_corner_1;
transform_listener.transformPose("map", corner_4, transformed_corner_4);