CODEWITHSUNDEEP
c++classoop
Joydwip Mohajon
Joydwip Mohajon

Reputation: 15

Why can't I declare and define a member variable in different lines in a class?

As you can see, in the "Demo" class I declared "id". But I can't initialize it in the next line (I know, I could have done this in the same line, but still). I can do this same exact thing in case of "a" in the main function. Why???

#include <iostream>
using namespace std;

class Demo {
    public:
    int id;
    id = 90;
    int setID(int x)
    {
        id = x;
    }
};

int main() {
    int a;
    a = 90;
    Demo d;
    d.setID(50);
    cout<<a<<endl;

    return 0;
}

Upvotes: 0

Views: 412

Answers (2)

Brian Bi
Brian Bi

Reputation: 119382

A function body is a sequence of statements. A class body is a sequence of member declarations. Most kinds of statements can't go inside a class body, because they're not valid member declarations. You can only put "runnable code" inside a class if it's nested inside a function.

If there's a piece of code you want to have run every time a class is instantiated, put it in the constructor. If there's a piece of code you want to have run only once, when the class is defined, there's no mechanism for doing that in C++.

Upvotes: 3

Andrey Chernukha
Andrey Chernukha

Reputation: 21808

Because this is how the language works. main is a function. Inside a function commands get executed one by one:

1) Create variable a of type int. 2) Assign value of 90 to a 3) Create object d of type Demo and so on.

But class Demo { public: int id; int setID(int x) { id = x; } };

is not a function. It is a declaration of a class. It is nothing more than description of an entity. You cannot execute commands inside a description. You can say "this class has int a" and that's it

Upvotes: 2

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