Cycling through structure

This is a discussion on Cycling through structure within the C++ Programming forums, part of the General Programming Boards category; Suppose S is a structure: Code: typedef struct { type s1; type s2; // ... type sn; } S; S ...

  1. #1
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    Cycling through structure

    Suppose S is a structure:
    Code:
    typedef struct {
    
       type s1;
       type s2;
    // ...
       type sn;
    } S;
    
    S myS;
    Is there a way to assign values to myS without referring to s1, s2, ..., sn explicitly, i.e., instead of
    Code:
    myS.s1 = v1;
    myS.s2 = v2;
    //...
    myS.sn = vn;
    is something like
    Code:
    for (int i=1; i<=n; i++) {
           /* Do something like myS.i = vi */
    }
    possible? Thanks!

  2. #2
    C++ Witch laserlight's Avatar
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    Use an array or a container like std::vector instead of n separate member variables.
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  3. #3
    30 Helens Agree neandrake's Avatar
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    So is it correct to say that there is no way to enumerate over member variables in a struct, likewise in a class? The only way I could think of would be to access via pointers, but you would have to know the size of individual members. Are you guaranteed a correct layout from a C/C++ compiler in order for this to work?

    edit: By "correct layout" I mean are you guaranteed the order of members to be the same as defined?
    Last edited by neandrake; 04-10-2009 at 03:00 PM.
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  4. #4
    30 Helens Agree neandrake's Avatar
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    Followup:

    It looks like you are guaranteed the order of members defined in a struct, however the compiler may need to pad fields, depending on architecture. There is probably no good way to do this.

    Byte Alignment and Ordering in Message Definition
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  5. #5
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    Quote Originally Posted by neandrake View Post
    Followup:

    It looks like you are guaranteed the order of members defined in a struct, however the compiler may need to pad fields, depending on architecture. There is probably no good way to do this.

    Byte Alignment and Ordering in Message Definition
    True, although 99.9% of the programmers ignore this fact completely. How often haven't I seen fread(&file_header, 1, sizeof(file_header), some_file_type_handle);

    So it's generally ignored completely.

  6. #6
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    Quote Originally Posted by EVOEx View Post
    True, although 99.9% of the programmers ignore this fact completely. How often haven't I seen fread(&file_header, 1, sizeof(file_header), some_file_type_handle);

    So it's generally ignored completely.
    But for portable code, you can almost guarantee that the header is carefully constructed in such a way that it is unlikely to have any (unknown) gaps in it - either by asking the compiler to not add gaps, or by arranging the content in such a way that in almost all compilers the data is aligned properly no matter what compiler/processor architecture it is.

    Or the code is written such that it reads each element at a time.

    Also, as long as all elements in a struct are the same type (or a set of members are the same type and consecutive elements), you could use a pointer to that type - it should increment the same way as a pointer as the compiler arranges the elements. But if you have more than three of something, it's likely that you actually WANT an array anyways.

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    What are you trying to accomplish with this?

  8. #8
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    Quote Originally Posted by slippy View Post
    is something like
    Code:
    for (int i=1; i<=n; i++) {
           /* Do something like myS.i = vi */
    }
    possible? Thanks!
    It's technically possible given the struct in the OP, and ones like it.
    Code:
    #include <iostream>
    #include <cstdio>
    #include <boost/preprocessor/arithmetic/add.hpp>
    #include <boost/preprocessor/cat.hpp>
    #include <boost/preprocessor/repetition/repeat_from_to.hpp>
    #include <boost/preprocessor/list/at.hpp>
    
    typedef struct {
    
       int s1;
       int s2;
    // ...
       int s3;
    } S;
    
    // constructs "<struct>.<fieldname><number> = <value>;"
    #define INIT_EACH_MEMBER_STATIC_INT(NextDim, Number, Data) \
        BOOST_PP_LIST_AT(Data, 0).BOOST_PP_CAT(BOOST_PP_LIST_AT(Data, 1), Number) = BOOST_PP_LIST_AT(Data, 2);
    
    // loops from startElem to startElem+elements constructing the assignment for each with the above macro
    #define INIT_EACH_MEMBER_STATIC(structure, namePrefix, startElem, elements, value) \
        BOOST_PP_REPEAT_FROM_TO(startElem, BOOST_PP_ADD(startElem, elements), INIT_EACH_MEMBER_STATIC_INT, (structure, (namePrefix, (value, BOOST_PP_LIST_NIL))))
    
    // like the first macro, constructs
    // <struct>.<fieldname><number> = <varprefix><value>;
    #define INIT_EACH_MEMBER_VARIABLE_INT(NextDim, Number, Data) \
        BOOST_PP_LIST_AT(Data, 0).BOOST_PP_CAT(BOOST_PP_LIST_AT(Data, 1), Number) = BOOST_PP_CAT(BOOST_PP_LIST_AT(Data, 2), Number);
    
    #define INIT_EACH_MEMBER_VARIABLE(structure, namePrefix, startElem, elements, varPrefix) \
        BOOST_PP_REPEAT_FROM_TO(startElem, BOOST_PP_ADD(startElem, elements), INIT_EACH_MEMBER_VARIABLE_INT, (structure, (namePrefix, (varPrefix, BOOST_PP_LIST_NIL))))
    
    int main()
    {
        S myStructVar;
        // make every member equal 56
        INIT_EACH_MEMBER_STATIC(myStructVar, s, 1, 3, 56);
        printf("S members: s1 = %d, s2 = %d, s3 = %d\n", myStructVar.s1, myStructVar.s2, myStructVar.s3);
        // you could also use a macro for this to auto generate init variables 
        // and input for N values
        int initS1, initS2, initS3;
        std::cout << "Enter three ints: ";
        std::cin >> initS1 >> initS2 >> initS3;
        // make each member equal its eqivalent initS 
        INIT_EACH_MEMBER_VARIABLE(myStructVar, s, 1, 3, initS);
        printf("S members: s1 = %d, s2 = %d, s3 = %d\n", myStructVar.s1, myStructVar.s2, myStructVar.s3);
    }
    So it can be done, but it is has a limited applicability given the stringent requirements on naming / initialization etc and the amount of manual bookkeeping you have to contend with. Use an array or vector for a much easier life.

  9. #9
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    Quote Originally Posted by neandrake View Post
    It looks like you are guaranteed the order of members defined in a struct,
    In C++, that's only true for PoD struct types. For non-PoD types (which includes, among other things, structs with a declared destructor or a virtual function) the order is not guaranteed.

    Add to that the padding in structs, and the implementation defined layout of basic types (int, double, enums, pointers, etc) and it is usually better to write code that does not rely on specific layout of types in memory, if you care at all about portability.
    Right 98% of the time, and don't care about the other 3%.

  10. #10
    30 Helens Agree neandrake's Avatar
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    The only thing I could see where you would want to guarantee the layout would be in microchips -- in which you should know the architecture very well. Even then you would probably doing a lot of stuff in assembly if it really mattered.
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