Explain different parameter passing mechanisms to a function with the help of example?
The different parameter-passing mechanisms are given below:
1. Call by value
2. Call by value-result
3. Call by reference
4. Call by name
In first call by value mechanism, the values of real parameters are passed to the called function. These values are allocated to the consequent formal parameters. If a function changes the value of a formal parameter, the modification is not reflected upon the consequent actual parameter. It is commonly utilized for built-in functions of the language. Its major advantage is its simplicity. The compiler can use formal parameter like a local variable. It simplifies compilation considerably.
Call by value-result: This mechanism extends the abilities of the call by value mechanism through copying the values of formal parameters back in consequent actual parameters at return. Such mechanism inherits the simplicity of the first mechanism i.e. but incurs higher overheads.
Call by reference: Now the address of an actual parameter is passed to the called function. If the parameter is an expression, its value is calculated and stored in a temporary location and the address of the temporary location is passed to the called function. If the parameter is an array component, its address is likewise calculated at the time of call. Such mechanism is very popular since this has 'cleaner' semantics than call by value-result.
Call by name: Such parameter transmission mechanism has similar effect as if every occurrence of a formal parameter in the body of the called function is replaced through the name of the consequent actual parameter. The actual parameter consequent to a formal parameter can change dynamically throughout the execution of a function. It makes the call through name mechanism immensely powerful. Though, the high overheads make this less attractive in practice.