1Parameter Passing Standard mechanisms Call by value Call by reference Other methods Call by value-result Call by name, resultTerms Function definition – where the details of the function are presented (type, name, parameters, body) - only one Function call – where the function is invoked (name, arguments) – zero or more (not interesting if no calls) Parameters (formal parameters) – names of local variables in function that are given values during call Local variables – variables in function body that are not parameters Arguments (actual parameters) – values provided for parameters2Terms Parameter passing – method(s) used to determine how argument values relate to parameters Overloading – when the same function name can have more than one set of parameters L-value – location of variable R-value – contents of variable (or result of expression)Terms Exampleint func1 (int a, char b, float& c) {int x,char y;…}func1(5 * 3, ‘A’, z);Local variablesParametersArgumentsFunction definitionFunction call3Call by Value Calling mechanism Arguments are evaluated for their values  Local variables created for each parameter Values resulting from arguments copied to new parameter variables When function call ends, parameter variables are discarded During function execution, value of parameters may diverge from argument values (function does not affect arguments)Call by Value Call by Value used in C Most C++ parameters Variables are changed in functions only indirectly  Pointer values are passed to functions Variables that the pointer point at may be changed in a function Characteristics: Variables may not directly be changed in function body (but changes in function do not change the values of arguments) Arguments can be complex expressions Mechanism is simple (easy to explain)4Call by Reference Calling mechanism Variable locations for arguments determined Parameter names added to the location for each argument When function call ends, extra names are discarded During function call, changes to referenced variables persist even after function endsCall by Reference Call by Reference used Pascal (var parameters) C++ (& parameters) Some versions of FORTRAN Characteristics: Changes to parameters change corresponding argument variables Arguments must be variables (cannot connect a reference to an expression)5Parameter Passing Example –Call by Valueint x = 1; // global xvoid func1 (int a) {// Location 2x = 2;// Location 3a = 5;// Location 4}void main () {// Location 1func1(x);// Location 5}Location 1: a undefined, x is 1Location 2: a is 1, x is 1Location 3: a is 1, x is 2Location 4: a is 5, x is 2Location 5: x is 2Parameter Passing Example –Call by Referenceint x = 1; // global xvoid func1 (int a) {// Location 2x = 2;// Location 3a = 5;// Location 4}void main () {// Location 1func1(x);// Location 5}Location 1: a undefined, x is 1Location 2: a is 1, x is 1Location 3: a is 2, x is 2Location 4: a is 5, x is 5Location 5: x is 56Call by Value-Result Calling mechanism Arguments are evaluated for their values Local variables created for each parameter Values resulting from arguments copied to new parameter variables When function call ends, values from parameters copied back to calling variables Operates somewhat like Call by Reference but differs under certain circumstances Used in some versions of FORTRAN, inout params in CORBAParameter Passing –Call by Value-Resultint x = 1; // global xvoid func1 (int a) {// Location 2a = 3;// Location 3x = 4;// Location 4}void main () {// Location 1func1(x);// Location 5}Location 1: a undefined, x is 1Location 2: a is 1, x is 1Location 3: a is 3, x is 2Location 4: a is 3, x is 4Location 5: x is 37Other Mechanisms Call by name Used in Algol Functions are a bit like a complex macro, the argument values replace the corresponding parameter names in the function body and then the body executed Call by result Value of parameter copied back to argument at end of function Out params in CORBAWhat About Java? Everything is Call by Value What about when we pass objects? An object variable in Java always holds a reference (pointer) to an instance in Java When called a copy of the reference (or l-value) is made Changes to the object pointed at can be made But the pointer to the object can then also be