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How to convert XmlRpc::XmlRpcValue into string?

asked 2011-12-13 04:45:20 -0600

Felix Tr gravatar image

updated 2011-12-14 01:32:40 -0600

Hello All,

I am trying to extract topic name as strings from a "yaml" file. The goal is depicted here:

I think that I have succeeded to receive the list as XmlRpc::XmlRpcValue class, but I am not succeeding to convert this to a string format.

I have tried to do it in two different ways:

    for (int32_t i = 0; i < my_list.size(); ++i) 
        printf ( "Topic %d is: %s.\n", i, static_cast<std::string>(my_list[i]) );

if ( my_list.stringFromXml ( topicsList, &offset ) == TRUE )
        printf ("\"topic_list = \"%s", topicsList );

The first way doesn´t compiles, and the second doesn´t run...

The two relevant links, I have found, that are connected to this problem are:

  1. (roll till the end of the page).

Both of them doesn´t really help me.

Here is also the code for getting my_list:

ros::NodeHandle nh;
XmlRpc::XmlRpcValue my_list;
nh.getParam("topic_list", my_list);
if ( my_list.stringFromXml ( topicsList, &offset ) == TRUE )
        printf ("\"topic_list = \"%s", topicsList );

While topic_list is the name of the list in "yaml" file:

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Thanks in advance, Felix.

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What exactly is the compilation error in the first case? Can you also include the code for getting my_list?
arebgun gravatar imagearebgun ( 2011-12-13 05:03:57 -0600 )edit
See my next post in this page please. Thank you.
Felix Tr gravatar imageFelix Tr ( 2011-12-13 05:16:20 -0600 )edit

2 Answers

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answered 2011-12-13 05:19:03 -0600

DimitriProsser gravatar image

updated 2011-12-14 01:59:46 -0600

I think the main problem is that printf's %s formatter expects a C-style string, not a C++ string. All you'd have to do to fix this is:

printf ( "Topic %d is: %s.\n", i, static_cast<std::string>(my_list[i]).c_str() );

Note the addition of the .c_str() function call.

Also, you might want to change printf to ROS_INFO. All of the formatting is the same, but it has cool ROS functionality.

EDIT: Here's a quick explanation. Since printf is the output format used in C, not in C++, it expects that you'd be using C-style data structures. The %s formatter in C expects that you will pass in a const char*, which is C's representation of a string - an array of characters. In C++, however, the std::string data structure is more complex than a const char* and is its own type entirely. It's essentially the same as trying to use a double with %s. It's the wrong datatype, and therefore will not work. The .c_str() method of a C++ std::string returns a const char* of the string so that you can use it with C functions.

The block of code static_cast<std::string>(my_list[i]) returns a std::string, and thus you cannot use it with %s. static_cast<std::string>(my_list[i]) is one of C++'s forms of casting my_list[i] to an std::string. static_cast<double>(my_list[i]) would cast it to a double.

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Dmitri, THANKS A LOT for this answer, it works :)!!! I will of course read about it more, but if you have time, can you explain your answer here (decompose into parts). I am writing mostly in C, so I am not sharp enough in C++. Thanks.
Felix Tr gravatar imageFelix Tr ( 2011-12-14 01:43:35 -0600 )edit
Thanks again :)
Felix Tr gravatar imageFelix Tr ( 2011-12-14 02:11:42 -0600 )edit

answered 2011-12-13 05:48:29 -0600

dornhege gravatar image

The code should work.

Can you verify this assertion: ROS_ASSERT(my_list.getType() == XmlRpc::XmlRpcValue::TypeArray);

Maybe you are not requesting the param in the right scope.

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There are two problems with that: 1. I don´t need the assertion. 2. It doesn´t compiles that way. Anyway, thanks a lot for your answers here, and anyway with the help from this forum, I have succeeded to solve the problem :).
Felix Tr gravatar imageFelix Tr ( 2011-12-14 02:20:47 -0600 )edit

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Asked: 2011-12-13 04:45:20 -0600

Seen: 4,970 times

Last updated: Dec 14 '11