How can i access variables in the main function from other function

Question

I've looked at other topics, the answers usually solve the problem using parameters, but this is not the way I want. I'm new to C++, I was developing NodeJS before. Can we make a variable that we can access from the region we want like in NodeJS?

string read_from_ram(string& key) {
   //how can I access and edit keysDB and valuesDB variables from here
}

int main(int argc, char* argv[]) {
    auto [keysDB, valuesDB] = read_from_disk();
}

Pass as arguments? Use classes and use member variables? Return values (objects) from functions? — Some programmer dude, Jul 20 '21 at 13:04
Your C++ textbook should have a chapter that gives an introduction to C++ classes, class members and methods. This should point you in the right direction. — Sam Varshavchik, Jul 20 '21 at 13:05
You can have global variables, but that's discouraged. I don't know any Node.js, so it's hard to tell if this is what you are looking for or not. — Yksisarvinen, Jul 20 '21 at 13:06
[Here's a list of good C++ books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). Please invest in some beginners books. — Some programmer dude, Jul 20 '21 at 13:07
You may find this answer useful ... https://stackoverflow.com/a/45550373/2785528 — 2785528, Jul 20 '21 at 13:36

Stefan Riedel · Answer 1 · 2021-07-20T13:09:24.857

1

You have to pass those variables to your function. That's something every textbook would have told you in the very first chapters.

Guessing the types, this could be what you want:

std::string read_from_ram(const std::string& key,
        const std::vector<std::string>& keys,
        const std::vector<std::string>& values)
{
   // access keys and values here
}

int main(int argc, char* argv[])
{
    auto [keysDB, valuesDB] = read_from_disk();
    auto foo = read_from_ram("foo", keysDB, valuesDB);
}

You could also make them global, but that would probably be a bad design choice.

edited Jul 20 '21 at 13:09

answered Jul 20 '21 at 13:07

Stefan Riedel

796
4
15

Doesn't this way cause extra cost in memory? – GreXLin85 Jul 20 '21 at 13:09
1

No, in this case you pass them by reference (hence the `&`). You're not copying those vectors. (well you end up with a pointer under the hood - probably - but that's negliable) – Stefan Riedel Jul 20 '21 at 13:10

score 1 · Accepted Answer · answered Jul 20 '21 at 13:08

No. You'll have to pass a pointer/reference to the function with the way you currently have it written, or set it as a global variable (which is typically frowned upon unless required by the problem scope).

Another potential idea would be to make an object.

class MyClass { 
  private:
    string key;
  public:            
    string read_from_ram(string& key) {};
};

In this case, key would be accessible from anywhere inside your class.

score 1 · Answer 3 · answered Jul 20 '21 at 13:09

If I understand you correctly, then you want to access a variable globally. If that is the case than you just have to define a global variable, which means define a variable outside of all functions, usually this is done under your includes. Warning: Usually you should avoid global variables because they can be manipulated from anywhere in the program and generic names can also be dangerous.

How can i access variables in the main function from other function

3 Answers3