service problem

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Hi, I defined a server called DetectsObs.srv

float32 x
float32 y
float32 theta
float32 threshold
---
Points obstacles
bool obs
bool processFinished

and two msg type
Point.msg

float32 x
float32 y

and Points.msg

Point[] points

I have a service whose response is obstacle_detection::DetectsObs::Response &res (obstacle_detection is my package) I want to set the field obstacles of my response but if I do

obstacle_detection::Point obstacle;
obstacle.x = 0;
obstacle.y = 0;
res.obstacles[0] = obstacle;

I get this error

error: no match for ‘operator[]’ in ‘res->obstacle_detection::DetectsObsResponse_<std::allocator<void> >::obstacles[0]’

What should i do to fix it.
Thank you in advance