Question

I have a set of 3 nodelets running which publish and subscribe to topics amongst themselves. To optimize the process I want to use the shared memory functionality for zero-copy data being shared between nodelets through publish and subscribe. I tried following this example. The example uses a very basic data type for std_msgs::String::ConstPtr. I am using a custom defined message and I get a compile time error saying that I cannot assign values to boost::shared_ptr.

For e.g. if my message type is the following (MyMessage.msg)-

Header header
int dataA
string dataB

In my publish method I do something like this -

void publishTopic()
{
    std::string str("my message");
    MyMessage::ConstPtr msg;
    msg->dataA = 5;
    msg->dataB = str;

    myPublisher.publish(msg);
}

Lines msg->dataA = 5 and msg->dataB = str are giving me the errors. I know I'm doing something wrong with how I'm assigning values to pointers, but cannot think of a solution off the top of my head. Any help will be appreciated.

Can you please also refer a documentation for using boost::shared_ptr in ROS if any exists?

Asked by ajain on 2014-12-23 14:05:20 UTC

Answer 1

Your problem is that you're using const pointers. const pointers cannot be mutated, which is why you are getting errors.

When you are generating a message for publishing it needs to be a standard shared pointer. When you receive it in your callback it will be a const pointer or else it will be copied.

Note: When you publish a message with a shared pointer you're not allowed to modify it after it has been sent as other parts of the process might be accessing it.

For information on boost pointers I suggest the boost documentation: http://www.boost.org/doc/libs/1_57_0/libs/smart_ptr/shared_ptr.htm Or general reference sites like stack overflow: http://stackoverflow.com/questions/569775/smart-pointers-boost-explained

Asked by tfoote on 2014-12-23 18:12:46 UTC

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Thanks a lot for a quick answer. It works if I instantiate msg as boost::shared_ptr<MyMessage> msg.

Asked by ajain on 2014-12-23 19:01:01 UTC

Implement shared memory data publishing between nodelets

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