Question: What's the difference between:

```
void function();

int main()
{......}

void function()
{......}

```

vs

```
void function()
{.......}

int main();

```

It seems odd to declare a function before main then define it after main when you could just declare and define it before main. Is it for aesthetic purposes? My teacher writes functions like the first example.

Answer: 
It's just for code organization purposes ("aesthetics", I guess). Without forward declarations you'd need to write every function before it's used, but you may want to write the bodies of a function in a different order for organizational purposes.

Using forward declarations also allows you to give a list of the functions defined in a file at the very top, without having to dig down through the implementations.

Forward declarations would also be necessary in the case of mutually recursive functions. Consider this (silly) example:

```
bool is_odd(int);  // neccesary

bool is_even(int x) {
  if (x == 0) {
    return true;
  } else {
    return is_odd(x - 1);
  }
}

bool is_odd(int x) {
  return is_even(x - 1);
}

```
Look at this example:

```
int max(int num1, int num2) 
{
   // local variable declaration
   int result;

   if (num1 > num2)
      result = num1;
   else
      result = num2;

   return result; 
}

```

where, `int max(int num1, int num2)` is behaving as a function.

Now, here we have a program

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

// function declaration
int max(int num1, int num2);

int main ()
{
   // local variable declaration:
   int a = 100;
   int b = 200;
   int ret;

   // calling a function to get max value.
   ret = max(a, b);

   cout << "Max value is : " << ret << endl;

   return 0;
}

// function returning the max between two numbers
int max(int num1, int num2) 
{
   // local variable declaration
   int result;

   if (num1 > num2)
      result = num1;
   else
      result = num2;

   return result; 
}

```

I kept `max()` function along with `main()` function and compiled the source code. While running final executable, it would produce the following result:

```
Max value is : 200

```

**Calling a Function:**

While creating a C++ function, you give a definition of what the function has to do. To use a function, you will have to call or invoke that function.

When a program calls a function, program control is transferred to the called function. A called function performs defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.

To call a function, you simply need to pass the required parameters along with function name, and if function returns a value, then you can store returned value.