Dynamically enumerating the members of a C++ struct

Applications Programming
1

In C++ there is a struct with the following structure:

typedef struct myStruct
{
    string s_date;
    float fA;
    float fB;
    float fC;
    .....
    float fZ;
} myData;

After some computations on the values of the struct members are inserted into a database table:

myData ind;
string sqlInsert = "INSERT INTO calculations (date, A, B, C, ..., Z) 
VALUES ('"+ind.s_date+"','"+ftostr(ind.fA)+"','"+ftostr(ind.fB)+"','"+ftostr(ind.fC)+"','"+ ... +"','"+ftostr(ind.fZ)+"')";

Given that the members of the struct can change, as well as currently close to 100 members, we are looking for a way to loop over the members in a dynamic fashion, without having to spell out each member individually.

Therefore the question is, is there a way to loop over the struct members so that the SQL that comes after the VALUES clause is updated based on however many struct members there are?

2

You can't do that sadly. What I usually see done when this is needed is one of two things:

  • Some sort of dynamic structure, like a map or hash or list, without predefined contents.

OR

  • Use a script to build the structure out of some sort of data file, and also build an array which describes its contents.
3

Thank you for your response. Do you have examples of C++ maps and/or C++ hash lists? And what exactly do you mean by "without predefined contents"?

4

Instead of having one big structure with hardcoded members like x.ytypeofvalue, have a container holding many structures with two members, name and value. You can store this in many kinds of data structures like lists, maps, vectors, arrays, most anything iterable.

#include <map>
#include <string>

#define foreach(var, container)                 \
                for(typeof((container).begin()) var = (container).begin(); \
                var != (container).end(); \
                ++var)

int main(void)
{
        std::map<std::string,std::string> mydata;

        mydata["a"]="b";
        mydata["c"]="d";

        foreach(x, mydata)
        {
                cout << (*x).first <<" "<<(*x).second<<"\b";
        }
}
5

Was not aware of this structure, but have read up more on it in the meantime. Thank you for your guidance.