Initializing struct, using an array

Question

I have a couple of array's:

const string a_strs[] = {"cr=1", "ag=2", "gnd=U", "prl=12", "av=123", "sz=345", "rc=6", "pc=12345"};
const string b_strs[] = {"cr=2", "sz=345", "ag=10", "gnd=M", "prl=11", "rc=6", "cp=34", "cv=54", "av=654", "ct=77", "pc=12345"};

which i then need to parse out for '=' and then put the values in the struct. (the rc key maps to the fc key in the struct), which is in the form of:

struct predict_cache_key {

pck() :
av_id(0),
sz_id(0),
cr_id(0),
cp_id(0),
cv_id(0),
ct_id(0),
fc(0),
gnd(0),
ag(0),
pc(0),
prl_id(0)
{ }

int av_id;
int sz_id;
int cr_id;
int cp_id; 
int cv_id;
int ct_id;
int fc;
char gnd;
int ag;
int pc;
long prl_id;
};

The problem I am encountering is that the array's are not in sequence or in the same sequence as the struct fields. So, I need to check each and then come up with a scheme to put the same into the struct.

Any help in using C or C++ to solve the above?

Answer 1

Probably I didn't get it correctly, but obvious solutions is to split each array element into key and value and then write lo-o-ong if-else-if-else ... sequence like

if (!strcmp(key, "cr"))
   my_struct.cr = value;
else if (!strcmp(key, "ag"))
   my_struct.ag = value;
...

You can automate the creation of such sequence with the help of C preprocessor, e.g.

#define PROC_KEY_VALUE_PAIR(A) else if (!strcmp(key,#A)) my_struct.##A = value

Because of leading else you write the code this way:

if (0);
PROC_KEY_VALUE_PAIR(cr);
PROC_KEY_VALUE_PAIR(ag);
...

The only problem that some of you struct fields have _id sufffix - for them you'd need to create a bit different macro that will paste _id suffix

Answer 2

Yeah, not sure there is an elegant solution. At the very least you could provide a constructor to your struct and do the parsing in there.

Answer 3

The problem is you dont have the metainformation to refer to the struct elements at run time (Something like structVar.$ElementName = ..., where $ElementName is not the element name but a (char?)variable containing the element name which should be used). My solution would be to add this metainformation. This should be an array with the offset of the elements in the struct.

Quick-n-Dirty solution: you add an array with the strings, the resulting code should look like this:

const char * wordlist[] = {"pc","gnd","ag","prl_id","fc"};
const int  offsets[] = { offsetof(mystruct, pc), offsetof(mystruct, gnd), offsetof(mystruct, ag), offsetof(mystruct, prl_id), offsetof(mystruct, fc)};
const int sizes[] = { sizeof(mystruct.pc), sizeof(mystruct.gnd), sizeof(mystruct.ag), sizeof(mystruct.prl_id), sizeof(mystruct.fc)}

to enter something you would then something like this:

index = 0;
while (strcmp(wordlist[index], key) && index < 5)
    index++;
if (index <5)
   memcpy(&mystructvar + offsets[index], &value, sizes[index]);
else
   fprintf(stderr, "Key not valid\n");

This loop for the inserts can get costly if you have bigger structures, but C doenst allow array indexing with strings. But the computer science found a solution for this problem: perfect hashes.

So it would afterwards look like this:

hash=calc_perf_hash(key);
memcpy(&mystruct + offsets[hash], &value, sizes[hash]);

But how to obtain these perfect hash functions (I called it calc_perf_hash)? There exist algorithms for it where you just stuff your keywords in, and the functions comes out, and luckily someone even programmed them: look for the "gperf" tool/package in your faviourite OS/distribution. There you would just input the 6 element names and he outputs you the ready to use C code for a perfect hash function (in generates per default a function "hash" which returnes the hash, and an "in_word_set" function which decides if a given key is in the word list). Because the hash is in different order, you have of course to initilize the offsetof and size arrays in the order of the hashes.

Another problem you have (and which the other answers doesnt take into account) is the type conversion. The others make an assignment, I have (not better) memcopy. Here I would suggest you change the sizes array into another array:

const char * modifier[]={"%i","%c", ...

Where each string describes the sscanf modifier to read it in. This way you can replace the assignment/copy by

sscanf(valueString, modifier[hash], &mystructVar + offsets(hash));

Cf course you can vary here, by including the "element=" into the string or similar. So you can put the complete string into value and dont have to preprocess it, I think this depends strongly on the rest of you parse routine.

Answer 4

Indeed, like many answered, there is a need to separate the parsing problem from the object construction problem. The Factory pattern is suited well for that.

The Boost.Spirit library also solves the parse->function problem in a very elegant way (uses EBNF notation).

I always like to separate the 'business logic' from the framework code.

You can achieve this by start writing "what you want to do" in a very convenient way and work to "how do you do it" from there.

  const CMemberSetter<predict_cache_key>* setters[] = 
  #define SETTER( tag, type, member ) new TSetter<predict_cache_key,type>( #tag, &predict_cache_key::##member )
  { SETTER( "av", int, av_id )
  , SETTER( "sz", int, sz_id )
  , SETTER( "cr", int, cr_id )
  , SETTER( "cp", int, cp_id )
  , SETTER( "cv", int, cv_id )
  , SETTER( "ct", int, ct_id )
  , SETTER( "fc", int, fc )
  , SETTER( "gnd", char, gnd )
  , SETTER( "ag", int, ag )
  , SETTER( "pc", int, pc )
  , SETTER( "prl", long, prl_id )
  };

  PCKFactory<predict_cache_key> factory ( setters );

  predict_cache_key a = factory.factor( a_strs );
  predict_cache_key b = factory.factor( b_strs );

And the framework to achieve this:

  // conversion from key=value pair to "set the value of a member"
  // this class merely recognises a key and extracts the value part of the key=value string
  //
  template< typename BaseClass >
  struct CMemberSetter {

    const std::string key;
    CMemberSetter( const string& aKey ): key( aKey ){}

    bool try_set_value( BaseClass& p, const string& key_value ) const {
      if( key_value.find( key ) == 0 ) {
        size_t value_pos = key_value.find( "=" ) + 1;
        action( p, key_value.substr( value_pos ) );
        return true;
      }
      else return false;
    }
    virtual void action( BaseClass& p, const string& value ) const = 0;
  };

  // implementation of the action method
  //
  template< typename BaseClass, typename T >
  struct TSetter : public CMemberSetter<BaseClass> {
    typedef T BaseClass::*TMember;
    TMember member;

    TSetter( const string& aKey, const TMember t ): CMemberSetter( aKey ), member(t){}
    virtual void action( BaseClass& p, const std::string& valuestring ) const {
      // get value
      T value ();
      stringstream ( valuestring ) >> value;
      (p.*member) = value;
    }
  };


  template< typename BaseClass >
  struct PCKFactory {
    std::vector<const CMemberSetter<BaseClass>*> aSetters;

    template< size_t N >
    PCKFactory( const CMemberSetter<BaseClass>* (&setters)[N] )
      : aSetters( setters, setters+N ) {}

    template< size_t N >
    BaseClass factor( const string (&key_value_pairs) [N] ) const {
      BaseClass pck;

      // process each key=value pair
      for( const string* pair = key_value_pairs; pair != key_value_pairs + _countof( key_value_pairs); ++pair ) 
      {
        std::vector<const CMemberSetter<BaseClass>*>::const_iterator itSetter = aSetters.begin();
        while( itSetter != aSetters.end() ) { // optimalization possible
          if( (*itSetter)->try_set_value( pck, *pair ) )
            break;
          ++itSetter;
        }
      }

      return pck;
    }
  };

Answer 5

I've written some little code that allows you to initialize fields, without having to worry too much about whether your fields are going out of order with the initialization.

Here is how you use it in your own code:

/* clients using the above classes derive from lookable_fields */
struct predict_cache_key : private lookable_fields<predict_cache_key> {
    predict_cache_key(std::vector<std::string> const& vec) {
        for(std::vector<std::string>::const_iterator it = vec.begin();
            it != vec.end(); ++it) {
            std::size_t i = it->find('=');
            set_member(it->substr(0, i), it->substr(i + 1));
         }
    }

    long get_prl() const {
        return prl_id;
    }

private:

    /* ... and define the members that can be looked up. i've only
     * implemented int, char and long for this answer. */
    BEGIN_FIELDS(predict_cache_key)
        FIELD(av_id);
        FIELD(sz_id);
        FIELD(gnd);
        FIELD(prl_id);
    END_FIELDS()

    int av_id;
    int sz_id;
    char gnd;
    long prl_id;
    /* ... */
};

int main() {
    std::string const a[] = { "av_id=10", "sz_id=10", "gnd=c",
                              "prl_id=1192" };
    predict_cache_key haha(std::vector<std::string>(a, a + 4));
}

The framework is below

template<typename T>
struct entry {
    enum type { tchar, tint, tlong } type_name;

    /* default ctor, so we can std::map it */
    entry() { }

    template<typename R>
    entry(R (T::*ptr)) {
        set_ptr(ptr);
    }

    void set_ptr(char (T::*ptr)) {
        type_name = tchar;
        charp = ptr;
    };

    void set_ptr(int (T::*ptr)) {
        type_name = tint;
        intp = ptr;        
    };

    void set_ptr(long (T::*ptr)) {
        type_name = tlong;
        longp = ptr;        
    };

    union {
        char (T::*charp);
        int  (T::*intp);
        long (T::*longp);
    };
};

#define BEGIN_FIELDS(CLASS)       \
    friend struct lookable_fields<CLASS>; \
    private:                      \
    static void init_fields_() {   \
        typedef CLASS parent_class;

#define FIELD(X) \
    lookable_fields<parent_class>::entry_map[#X].set_ptr(&parent_class::X)

#define END_FIELDS() \
    }                                                                              

template<typename Derived>
struct lookable_fields {
protected:
    lookable_fields() {
        (void) &initializer; /* instantiate the object */
    }

    void set_member(std::string const& member, std::string const& value) {
        typename entry_map_t::iterator it = entry_map.find(member);
        if(it == entry_map.end()) {
            std::ostringstream os;
            os << "member '" << member << "' not found";
            throw std::invalid_argument(os.str());
        }

        Derived * derived = static_cast<Derived*>(this);

        std::istringstream ss(value);
        switch(it->second.type_name) {
        case entry_t::tchar: {
            /* convert to char */
            ss >> (derived->*it->second.charp);
            break;
        }
        case entry_t::tint: {
            /* convert to int */
            ss >> (derived->*it->second.intp);
            break;
        }
        case entry_t::tlong: {
            /* convert to long */
            ss >> (derived->*it->second.longp);
            break;
        }
        }
    }

    typedef entry<Derived> entry_t;
    typedef std::map<std::string, entry_t> entry_map_t;
    static entry_map_t entry_map;

private:
    struct init_helper {
        init_helper() {
            Derived::init_fields_();
        }
    };

    /* will call the derived class's static init function */
    static init_helper initializer;
};

template<typename T> 
std::map< std::string, entry<T> > lookable_fields<T>::entry_map;

template<typename T> 
typename lookable_fields<T>::init_helper lookable_fields<T>::initializer;

It works using the lesser known data-member-pointers, which you can take from a class using the syntax &classname::member.

Answer 6

Were I to do this in straight C, I wouldn't use the mother of all if's. Instead, I would do something like this: typedef struct { const char *fieldName; int structOffset; int fieldSize; } t_fieldDef;

typedef struct {
    int fieldCount;
    t_fieldDef *defs;
} t_structLayout;

t_memberDef *GetFieldDefByName(const char *name, t_structLayout *layout)
{
    t_fieldDef *defs = layout->defs;
    int count = layout->fieldCount;
    for (int i=0; i < count; i++) {
        if (strcmp(name, defs->fieldName) == 0)
            return defs;
        defs++;
    }
    return NULL;
}

/* meta-circular usage */

static t_fieldDef metaFieldDefs[] = {
    { "fieldName", offsetof(t_fieldDef, fieldName), sizeof(const char *) },
    { "structOffset", offsetof(t_fieldDef, structOffset), sizeof(int) },
    { "fieldSize", offsetof(t_fieldDef, fieldSize), sizeof(int) }
};
static t_structLayout metaFieldDefLayout = { sizeof(metaFieldDefs)/sizeof(t_fieldDef), metaFieldDefs };

This lets you look up the field by name at runtime with a compact collection of the struct layout. This is fairly easy to maintain, but I don't like the sizeof(mumble) in the actual usage code - that requires that all struct definitions get labeled with comments saying, "don't effing change the types or content without changing them in the t_fieldDef array for this structure". There also needs to be NULL checking.

I'd also prefer that the lookup be either binary search or hash, but this is probably good enough for most cases. If I were to do hash, I'd put a pointer to a NULL hashtable into the t_structLayout and on first search, build the hash.

Answer 7

tried your idea and got an "error: ISO C++ forbids declaration of ‘map’ with no type" in linux ubuntu eclipse cdt.

I wish to notify that one should include in the "*.h" file in order to use your code without this error message.

**#include <map>**

// a framework

template struct entry { enum type { tchar, tint, tlong } type_name;

/* default ctor, so we can std::map it */
entry() { }

template<typename R>
entry(R (T::*ptr)) {

etc' etc'......