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elements in a srv or msg definition are assigned zero values by the default constructor. What about when I wish to assign a non-zero or other special value as the default initialization value to a msg element.
using a construct such as:
int32 x=100
regards x as a constant when the msg headers are generated.
Is what I'm looking for possible at all? Thanks much
Not in the message definition, itself. The problem is that message generation must support multiple languages.
For any specific language (like Python or C++), you can probably define your own derived class for the message with a constructor that fills in useful default values.
This is not a feature in ROS1 but this is a feature in ROS2. Interface definition using .msg / .srv / .action files Scroll down to the "Default values" section
In this ROS2 tutorial Creating custom msg and srv files They have you put the following in a .msg file
int64 num
By default the default value of a numerical datatype is 0. If you want to change the default value then put the desired default value after the name separated by a space. For example if you want to make the default value 1 then put the following in the .msg file.
int64 num 1
Now when you publish your message without defining the value of num it will publish num with a value of 1 instead of 0.
ros2 topic pub /numtopic tutorial_interfaces/msg/Num '{}'
You can confirm this by using ros2 topic echo.
ros2 topic echo /numtopic
num: 1
---
I just confirmed through experimentation that adding an = sign after any variable name in a ROS *.msg message definition file appears to be how to set constants, and all variables with an = after them show up in the autogenerated C++ *.h files as public static constexpr constants inside the autogenerated class (struct actually).
Yes, this is documented at wiki/msg: Constants.
But the OP here seems to ask for defaults, which constants cannot be used for.
As @joq mentions though: in C++, custom constructors can be used to provide alternatives to the default zero-initialisation of many fields. See wiki/roscpp/Overview/MessagesSerializationAndAdaptingTypes/Example: defining a custom constructor.
Asked: 2012-02-14 07:27:15 -0500
Seen: 9,356 times
Last updated: Feb 19 '21
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