C++ Handle Dynamic mapping between member variable name and its value - c++

Question

There is a class say Person which has member variables name, email, mobile, gender.
Now, we are getting this information in an array.
std::vector<std::string> a[] = {"XYZ", "xyz#mail.com", "1234567890", "Male"};
Person p;
Now, instead of writing it like:
p.name = a[0]; p.email = a[1]....
I want something like this for dynamic allocation as well as for reducing code lines:-
std::vector<std::string> b[] = {"name", "email", "mobile", "gender"};
int len = a.size();
for (int i=0; i < len ; i++)
{
set_value(p, b[i], a[i]);
}
How to write a function like set_value, or is there any way to do something like this in c++ ?
Right now in above example we have a vector size of 4, but it can be in size of 10 or 50 or more also. In that case if we don't have the way of setting the values dynamically then we might have to write same number of lines for setting every member variable .. .?

Answer 1

I think one of the solutions in which you are talking about is to implement all the fields of the Person class as a map<K, V>, which will store all of yours properties. In this case you will be able to refer to the value by the key of a map.
I would not recommend this solution for you. You can think of set_value function as a way in which the names from array a should refer to the name of the properties or rather how does those names should be linked to the functions which could invoke a proper property-right set method.
If those setting functions you are interested in use the same base type as a parameter, you could create map of strings and function pointers.
#include <map>
#include <vector>
#include <string>
#include <iostream>
using namespace std;
class Person{
std::map<std::string, std::string> _properties;
public:
Person() = default;
~Person() = default;
void set_values(vector<string> & fields, vector<string> & values){
if(fields.size() != values.size()) return;
for(int i = 0; i < fields.size(); ++i){
_properties.insert_or_assign(fields[i], values[i]);
}
}
void print(){
for (auto const &it : _properties)
std::cout << it.first << " => " << it.second << '\n';
}
};
int main(){
Person p;
vector<string> fields = {"email", "name"};
vector<string> values = {"a#b.com", "Andrew"};
p.set_values(fields, values);
p.print();
vector<string> fields2 = {"name"};
vector<string> values2 = {"Tom"};
p.set_values(fields2, values2);
p.print();
return 0;
}
This minimal working example shows what you want to get and it has a lot of constraints, such as it assumes that all of the values are stored as a std::string which can not be a good in every case. It produces a following output after applying the first set of fields and values:
email => a#b.com
name => Andrew
After the second one, the email stays the same but name is updated as follows:
email => a#b.com
name => Tom
I've compiled it using g++ version 7.1 using using following command:
g++ --std=c++1z main.cpp on my Fedora 26.

Answer 2

Well, there are no easy way to set a variable by name... There are library that allows to do something like that (for ex. boost fusion) but in practice, you could also write a simple function if you have only a few functions to write on a few classes.
One simple possibility assuming you want to reuse the code that fill a object:
void set_values_of(Person &p, const std::vector<std::string> &data)
{
assert(data.size() == 4); // put whatever error handling you want...
p.name = data[0];
p.email = data[1];
p.mobile = data[2];
p.gender = data[3];
}
And if you prefer, you might change second argument for another source (for ex. a stream or one line a string). At that point, it really depends on your application.
If your prefer immuable objects, you might also consider having a factory free function:
Person create_from(const std::vector<std::string> &data) { ... }