How to define class attribute after creating class

Question

I am trying to figure out how to define a Variable which is a member of a class in C++, Outside of the normal class body. It may be inside of a Function, or outside of the class. Is this possible. What I need is that the variable should be a member of the class such that if I call Nodesecond.birthdate, it returns the birthdate. I am attempting to understand the language, there is no real-world application involved.

This was my attempt at doing it:

#include <iostream>
using namespace std;

struct Nodesecond {
public:
    int Age;
    string Name;
    //  I dont want to define Birthdate here. It should be somewhere else. 

    Nodesecond() {
        this->Age = 5;
        this->Name = "baby";
        this->birthdate = "01.01.2020";
    }
};

int main() {
    std::cout << "Hello, World!" << std::endl;
    Nodesecond mynode;
    cout << mynode.Age << endl << mynode.Name << mynode.Birthdate;
    return 0;
}

You can't. Everything that the class has, needs to be specified in the `member-specification` of the class. — Jason, Nov 22 '22 at 14:12
Unlike Python, you can't add attributes of an object dynamically. Get yourself a [good book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — molbdnilo, Nov 22 '22 at 14:19

Jason · Answer 1 · 2022-11-22T14:22:57.770

3

How to define class attribute after creating class

You can't. Everything that the class has, needs to be specified/declared in the member-specification of the class.

Note the emphasis on declared in the previous statement. We must declare everything first inside the class and then we can optionally define that afterwards outside the class.

edited Nov 22 '22 at 14:22

answered Nov 22 '22 at 14:13

Jason

36,170
5
26
60

But you can later on, outside of the class define extra functions using the [className]::[funcName] (parameters) {code}, if you wrote the function interface inside of the class. That is not possible in with variables right? – Flavio Nov 22 '22 at 14:20
1

@Flavio That is why I wrote **"declared"** and not "defined". Everything needs to be "declared" first inside the class and then you can optionally "define" that afterwards outside the class. – Jason Nov 22 '22 at 14:21

score 3 · Answer 2 · answered Nov 22 '22 at 14:15

3

This is not possible, since the class has to be complete by compiletime. However, under some circumstances a std::map<Key,Value> might do the job.

answered Nov 22 '22 at 14:15

Jakob Herk

157
11

273K · Accepted Answer · 2022-11-22T14:28:37.130

You can do something close to JavaScript objects.

#include <iostream>
#include <unordered_map>
using namespace std;

struct Nodesecond {
public:
    int Age;
    string Name;
    unordered_map<string, string> Fields;
    string& operator[](const string& name) {return Fields[name];}
    Nodesecond() {
        this->Age = 5;
        this->Name = "baby";
        *(this)["Birthdate"] = "01.01.2020";
    }
};

int main() {
    std::cout << "Hello, World!" << std::endl;
    Nodesecond mynode;
    cout << mynode.Age << endl << mynode.Name << mynode["Birthdate"];
    return 0;
}

How to define class attribute after creating class

3 Answers3