[ROS2] How to extract header from a ROS2 message? [closed]

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Is there a way to check if the message type has header or not? I only want to get the header if it has one.

Depends on you usecase. Where do you want to do the check? You should know the message type since you have to specify it in your subscriber (if you are checking the header there). For instance if you check this example:

auto node = rclcpp::Node::make_shared("listener_float");
auto sub = node->create_subscription<std_msgs::msg::Float32>(
    "chatter", chatterCallback);

You can see that the message is std_msgs::msg::Float32 and this message does not have a header (has only float32 data). It is always good to know the message types. You can also try to extract the msg type in your code as shown in this question.

Can it be done programmatically? I am writing a template class which accepts any message type and want to check if it has header at runtime.

runtime or compile-time? Your mention of templates makes it seem like compile-time would make more sense.

Afaik this is currently not possible, but I'm not an expert ROS 2 user. There is a rosidl_generator_cpp/traits.hpp header that gets specialised for each generated msg/srv header, but that doesn't appear to contain a trait for messages that have a header. You could perhaps check for a field header, but there is no requirement for a field with such a name to exist (it could be called hdr or something else).

Oh yes I meant compile time. Thanks a lot for your explanation. Do you think explicitly checking for the header field would be the closest workaround for the purpose? Main goal of my application is to perform some operation with the header timestamp if the message has a header, and the class needs to be able to take any kind of message, thus a template class and I need compile time checking.