Manually Declare Global Variable

Question

Is it possible to manually declare a global variable that is inside of a function. The only current way I know how to do so is to declare that variable within the setup() function or simply outside of any function, but I was hoping there was a way to manually declare a variable as global so that I may call one variable from within one function in another function.

Answer 1

You need to research 'Scope'.

Your variable has to be at the same level as all functions that will use it. So at a simple level the answer has to be no.

int variableOne = 0;
void setup ()
{
  variableOne = 1;
}
void loop ()
{
  variableOne = 2;
}

The above code will work, but if you move int variableOne = 0; inside either function then it won't work, because the variable will not be in scope in the other function.

You can use parameters to functions.

void AFunction (const int& variable);

Will let you pass a single integer value to the function from the parent. You can't change that value, because of the const qualifier. If you remove the const then the & means pass the parameter by reference, which means you are passing in a reference (pointer) to the variable in the parent function and you can change it. A better way to change the parent's value is to return a value from the call.

int AFunction (const int& variable);

So you could use this like this:

int AFunction (const int& variable)
{
  return (++variable);
}
void loop (void)
{
  int value = 0;
  value = AFunction(value);
  // value now == 1.
}

You could use a Singleton class, but this is hardcore and I'm not sure you would see any real benefit on an Arduino. A class is a grouping of related function and variables that you see in libraries and a Singleton class means there will only ever be one during the lifetime of the program. In this way you can declare an instance of the class (i.e. CSingleInstanceClass onlyOne;) and then retrieve/set data from the setup and loop function, i.e.

void setup ()
{
  CSingleInstanceClass onlyOne;  // Single instance class
  onlyOne.SetVariableOne (1);
}
void loop ()
{
  CSingleInstanceClass onlyOne;  // Single instance class
  Serial.println(onlyOne.GetVariableOne());// Prints 1
}

Hope that helps.

Answer 2

even though I understand this takes up a bit of dynamic memory

There is no way around that. They have to go somewhere and that place is RAM (exception: constants could be stored in flash).

Another variation (which goes beyond your question) is that a parent function could declare and build a collection of data in a local array or struct and pass a pointer to it to any sub-functions. It still takes RAM, but it will be on the parent function's stack, so only for the duration of parent function's execution(s); it will be released when the parent function exits.

Answer 3

Please note that 'Scope' of a variable is limited inside the block ( a block starts at { and ends at } ), and that variable cant be accessed outside the block.

consider life time of a variable as well. using the keyword static may can help you some times. refer this link.