Function as arguments

Function as arguments

A function is a group of statements that forms together to perform a task. Every C++ program has at least one function, which is main(), and all the other most trivial programs can define additional functions.

A function declaration tells the compiler about a function’s name, return type, and parameters. A function definition provides the actual body of the function.

Defining a Function:

The general form of a C++ function definition is given as follows:

return_type function_name( parameter list )
{
   body of the function
}

A function declaration can have the following parts:

return_type function_name( parameter list );

For the above defined function max(), following is a function declaration :

int max(int num1, int num2);

Parameter names are not important in function declaration only their type is required, so the given below is also valid declaration:

int max(int, int);

Function declaration is required when you define a function in 1 source file and you call that function in another file. In such case, you should declare the function at the top of the file calling that function.

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. For example:

#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; 
}

here max() function kept along with main() function and compiled the source code. While running final executable, the result will be :

Max value is : 200

Function Arguments:

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.

The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways that arguments can be passed to a function:

Call Type	Description
call by value	This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
call by pointer	This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.
call byreference	This method copies the reference of an argument into the formal parameter. Inside the function, the reference is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

By default, C++ uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function and above mentioned example while calling max() function used the same method.

Default Values for Parameters:

When you define a function, you can specify a default value for each of the last parameters. This value will be used if the corresponding argument is left blank when calling to the function.

let us consider following example for this :

#include <iostream>
using namespace std;
 
int sum(int a, int b=20)
{
  int result;

  result = a + b;
  
  return (result);
}

int main ()
{
   // local variable declaration:
   int a = 100;
   int b = 200;
   int result;
 
   // calling a function to add the values.
   result = sum(a, b);
   cout << "Total value is :" << result << endl;

   // calling a function again as follows.
   result = sum(a);
   cout << "Total value is :" << result << endl;
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Total value is :300
Total value is :120