macro for looping through a templated map

This is a discussion on macro for looping through a templated map within the C++ Programming forums, part of the General Programming Boards category; Is it possible to make a macro to loop through any kind of maps? Normally, it's Code: map<int, string>::iterator it; ...

  1. #1
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    macro for looping through a templated map

    Is it possible to make a macro to loop through any kind of maps?

    Normally, it's
    Code:
    map<int, string>::iterator it;
    for(it = somemap.begin(); it != somemap.end(); it++){
       // do schtuff
    }
    For an xml class I have, I've actually made some looping macros which replaces
    Code:
    int i;
    for(i = 0; i < xmlnode.size(); i++){
       xmlnode& tec = xmlnode[i];
       // do schtuff
    }
    into
    Code:
    xmlloop(xmlnode)
       // do stuff with "tec" as a xmlnode&
    xmlloopend
    So yeah, can the same be done with maps without having to specify their types? that is, I don't want:
    Code:
    maploop(int, string, somemap)
       // do stuff with it, which is an iterator..
    maploopend

  2. #2
    C++ Witch laserlight's Avatar
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    You could take a look at Boost.Foreach. In the next version of C++, there will probably be a range based for loop that would make this redundant.

    Of course, you could also consider applying an appropriate generic algorithm instead of using a convenience macro.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    Shouldn't std::for_each() work too?
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  4. #4
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    Quote Originally Posted by cpjust
    Shouldn't std::for_each() work too?
    That is what I mean by "applying an appropriate generic algorithm", though I hope that an even more appropriate algorithm is chosen, if feasible (or maybe a std::map specific algorithm should be used). The Boost.Foreach documentation notes that this "forces us to move our logic far from where it will be used", unless lambda expressions are used.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    I see.. even with combining a #define macro with for_each it will still need to know the type.. I guess it's probably good for type checking.

    anyways thanks!

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by underthesun
    even with combining a #define macro with for_each it will still need to know the type..
    If you are going to restrict the usage to std::map (or perhaps to standard library containers in general), then this is not necessary since value_type typedef in the scope of the std::map would provide the necessary type information. However, you would then loose the covariance feature provided by BOOST_FOREACH, although you gain in convenience.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
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    Hmm.. care to explain that last bit? How exactly do I use value_type for automatically getting an iterator type given a map?

    Code:
    map<int, int> themap;
    
    themap::iterator it; // <-- is this where I can use value_type somehow?
    for(it = themap.begin(); it != themap.end(); it++){
       blah();
    }

  8. #8
    C++ Witch laserlight's Avatar
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    hmm... looks like I am wrong. Since one cannot use a function template, the value_type macro is not useful, since one still needs to provide the type of the container to the macro in order to access it.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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