global variables in c++

Your understanding of global variables is correct.

Your mistake.

Your problem is not global variables. As other users have already pointed out, your function mainis wrong; is incorrect because it should return int. The reason for this obligation is because (as the C++ language standard says ) otherwise your program will be incorrect, I show it to you directly extracted from the standard (translation and highlighting mine):

3.6.1.2 Functionmain

Implementations must not predefine main. This function should not be overloaded. It must be declared with a return of typeint , but otherwise its type is implementation-defined. Implementations must allow:

1. A function () returningint y
2. A function of (int, puntero a puntero a char) returningint

So your syntax error is because your function maindoesn't follow the standard. I bet even the compiler error is telling you that such a function should return int.

Regarding the global variables.

Try not to use global variables :

• They lack locality : Code is easier to understand when its scope is limited. Global variables can be read or modified from anywhere in the program, making it difficult to reason about their use or remember all the points at which they are used.
• They lack access control or constraint checking : A global variable can be read or written from anywhere in the program, various rules about its use can be forgotten or violated.
• Concurrency issues : If global variables can be accessed from multiple threads, you need to synchronize them. When working with dynamically linked modules that use global variables, the resulting system may not be secure even when the modules are secure independently.
• Namespace contamination : Global variables are in all contexts. You can end up using a global variable when you thought you were using a local one, or vice versa! (either due to ignorance, misspelling the name or forgetting to create the local variable). Also, if you link modules with global variables whose names are the same, if you're lucky, you'll get linking errors... if you're unlucky, the linker will consider the variables to be the same even if you didn't mean to.
• More problems in the link (in English) : I have translated the points that I thought were most relevant to your problem.

I see two errors here: first, you use void main(){where it should be int main, why? because you are going to return an integer. The second error I see is that you don't return it, that is, you don't add a return, as in the following code:

#include<cmath>
#include <stdio.h>
#include <iostream>

using namespace std;

int number=3;

int main(){

  cout<<number;
   return 0;
}

I recommend you read this: input_output in c++ and local and global variables in c++ int-main-vs-void-main-in-c

The problem is that the function mainis always of type intand must return a number.

#include<cmath>
#include <stdio.h>
#include <iostream>

using namespace std;

int number=3;

int main(){
    cout << number << '\n';
    return 0;
}

What do you want to achieve by using a global variable? Using global variables is not recommended and you may want to pass it by reference to your functions. For example:

#include <iostream>
using namespace std;

void suma(int &n);

int main(){
  int numero = 0;
  suma(numero);
  cout << numero << '\n';
  suma(numero);
  cout << numero << '\n';
}

void suma(int &n){
 n = n + 10;
}

Try this syntax , why? because it is a standard in C/C++, a int mainand its respective return 0.

#include<cmath>
#include <stdio.h>
#include <iostream>

using namespace std;

int number=3;

int main(){
    cout<<number;

    return 0;
}