How to get struct member with a string using Macros C++

Question

Consider the following example:

struct MyStruct {
    int a;
    int b;
};

I can use macros to set a member from an instance of the struct by doing this:

#define setVar(x,y) instance.x = y

then in any function I can say:

setVar(a, 4)

How can I send in a as a string to the macro? Is that also possible?

setVar("a", 4)

EDIT: There are a bunch of predefined structs with members that are all of type double. I only know what struct I am using by an XML config file that is passed in. After parsing, I have a bunch of strings that are a list of all the data members and values that need to be set. I need to use this list to set values for each of the members in the struct.

Answer 1

It is only possible if you define the struct itself using some macro, for example:

#define MY_STRUCT_STRUCTURE FIELD(a) FIELD(b) FIELD(d) FIELD(e) FIELD(f)

struct MyStruct {

# define FIELD(name) int name;
    MY_STRUCT_STRUCTURE
# undef FIELD

  bool setVar(char* fieldname, int val)
  {
#   define FIELD(name) if(strcmp(#name,fieldname)==0){name=val; return true;};
    MY_STRUCT_STRUCTURE
#   undef FIELD
    return false; // name not found
  }
};


int main()
{
  MyStruct s;
  s.setVar("a",1);
  s.setVar("b",2);
  s.setVar("f",100);
}

Answer 2

I have coded some quick and dirty code, but could give you some ideas, hope that helps. The main trick here is too use unions.

struct MyStruct
{
int a;
double b;

MyStruct() 
    : a(0), b(0) {}
};

MyStruct instance;

union value 
{
    long value_a;
    double value_d;
} myvalue;


void blah_a(value v)
{
    instance.a = v.value_a;
}

void blah_b(value v)
{
    instance.b = v.value_d;
}

struct 
{
    (void)(*fn)(value);
    const char* key;
}

lookup_table[] = 
{
    { &blah_a, "a" },
    { &blah_b, "b" }
};

void setVar(const char* c, value v)
{
     for (int i = 0; lookup_table[i].fn; i++)
          if (c == lookup_table[i].key)
               (*(lookup_table[i].fn))(v);
}

int main(int argc, char* argv[])
{
    value v;
    v.value_a = 6;
    setVar("a", v);
    return 0;
}

Answer 3

Might not be what you are looking for but an alternative solution to macros etc.. would just be some encapsulation and OO design. You can change the Field class to a template later and you will be able to represent anything basically.

You can create a class

class Field
{
public:
    Field(const std::string& name, const std::string& type);
    virtual ~Field(void);
    std::string toString() const;
    std::string getName() const;
    int getValue() const { return value };
private:
    std::string name;
    std::string type;
    int value;
};

And then a structure class

#pragma once
#include <boost/ptr_container/ptr_deque.hpp>
#include <string>

class Field;

class MyStructure
{
public:
    typedef boost::ptr_deque<Field> FieldList;
    typedef FieldList::iterator FieldListIter;
    typedef FieldList::auto_type AutoField;

    MyStructure(void);
    MyStructure(const std::string& name);
    virtual ~MyStructure(void);

    void setName(const std::string& name);
    std::string getName() const;
    void addField( std::auto_ptr<Field> field );
    std::string getFieldValue( const std::string& name ) const;
    MyStructure::AutoField removeField( const std::string& name );
    std::string toString(void) const;

private:
    std::string struct_name;
    FieldList fields;
};

And then to use it:

auto_ptr<MySructure> struct_a(new MySructure("StructName1",0) );
struct_a->addField( auto_ptr<Field> ( new Field( "Field1",    1 ) ) );
struct_a->addField( auto_ptr<Field> ( new Field( var_str1,    2) ) );
struct_a->addField( auto_ptr<Field> ( new Field( getName(),   getVal() ) ) );