Functions in C++

To manage larger programs, programmers break them into functions, which are subprograms designed for specific tasks. Functions can be compiled, tested separately, and reused, simplifying the main program. The C++ standard library includes pre-defined functions accessed via header files, while users can also create their own functions. 

Function Declaration and returntype 

 return-type function_name(list of parameters)  

   { 

       local variable declarations; 

       body of the function;  

       return statement; 

   } 

The value returned by the return statement must match the return type. (If function never returns a value, you may leave out return statement and use a return type of void.) A C++function should have a prototype, declaring its arguments and return type. 

Default arguments 

Functions that take values are called arguments or parameters and are optional. When values are initialized while specifying arguments, they are called default arguments and can be specified in the prototype. The default parameters should be trailing, that is the ending arguments should be given default arguments first. 

Techniques used to pass variables into C++ Functions 

1. Pass by value: A function receives a local copy of a variable, leaving the original unchanged. This ensures simplicity and protects the calling routine's variable but is inefficient for large data structures. 
2. Passing by reference using a pointer: A pointer to the variable passed to a function allows changing the original variable's value in the calling routine. Although the function cannot change the pointer but it can manipulate the memory itself to which it points.
3. Passing by reference using a reference: In C++, a reference is an alias to a variable. Changes to the reference affect the original. Passing by reference efficiently modifies multiple variables without creating local copies.

Function Overloading 

A function is overloaded when declared multiple times with different argument signatures. This allows similar operations to share a name, providing alternate implementations. The compiler selects the correct function based on the number and type of arguments. 

Inline Functions 

An inline function has its code inserted directly into the caller's code stream, like a #define macro, enhancing performance by avoiding call overhead and saving memory. Declare it normally but prepend the definition with inline and place it in a header file. 

Syntax: inline void function(int x, chary) 

                 {  

                    //function code ...... . .  

                 } 

Recursion (Recursive functions) 

When a function calls itself within its body, it is called as recursive function. But note that the value of the argument and the passed argument must be different. Recursion is also called circular definition. Note that all the functions can't be made recursive.