Functions, Scope, and The Free Store Functions Functions must be declared by a function prototype before they are invoked, return_type Function_name(type, type, …); // Strong type checking The return_type: A function that does not return a value should declare a return_type of void. A return statement is not necessary in a function with a void return type (however, it can still be used to terminate the function), void Fun(){ if(cond) return; ….} The default return_type is int (e.g., in function main(), return 0; is found) Neither an array or a function can be specified as a return_type, int[10] fun(); // Error. The Argument_list: Functions are allocated storage on a structure called the program’s run-time stack, this stack is automatically popped when the function terminates. Arguments are passed by-value (the entire object is copied), or by-reference (only the address of the argument is copied) Arrays are passed by reference using a pointer to the zeroth element of the array Arguments can have default values, and functions can be overloaded

Functions, Scope, and The Free Store Default Values for the Argument_list A function may specify default values for all or only a subset of its arguments, int fun(int i= 0);// a function prototype with a default argument initializer int fun(int i = 0){//fun body….} // Error, a default value must be specified only once in a file, by convention it should be in the declaration The right most argument must have a default value before any argument to its left, fun(int i= 0, double x); // Error, default order is incorrect fun1(double x, int i= 0, char* str); // Error, the right most is not initialized fun1(double x, int i=0; char* str= “No Free Lunch”); // OK the invocation fun1(0.05,, “Hello”); // gives syntax error Function Overloading Several functions can be given the same name provided that each have a unique Argument_list, e.g. the following functions can exist in the same file, int max(int,int); // returns the max of two integers int max(const int *, int); // the first argument is a pointer int max(const List &); // returns the max of a List type object Avoid Ambiguous overloading: max(const int, int); // ambiguous with max(int,int) print(unsigned int); print (int); // suppose we have the invocation print(100);

Functions, Scope, and The Free Store Identifier Scoping and Storage Classes The scope of identifiers has three types, 1. File Scope: identifier is declared in the file and not within a class or within a function 2. Local scope: identifiers are declared within a function 3. Class scope: identifiers are declared within a class For example, enum Boolean {false,true}; // Boolean has a file scope class X{ // X has a file scope Boolean fun(); // fun has a class scope } const int Max = 100; // Max has a file scope X x_obj; // x_obj has a file scope main(){ Boolean Quit; // Quit has a local scope int fun();// fun has local scope} Boolean X::fun(){…} int fun(){…..}// now fun has the file scope // the above is not function overloading

Functions, Scope, and The Free Store File Scope The outer most scope which encloses both local scope and class scope The scope extends from the point of declaration in the file to the end of the file Storage classes static and extern are used to specify whether the same identifier can be declared and used in other files, e.g., in file x.cpp: extern const int Max = 100; // Max can be used in other files static int Max_size=1000; // Max_size can not be used in other // files, i.e., it does not have external linkage in file y.cpp: extern const int Max; // this does not allocate storage for Max // Max has already been defined in x.cpp extern int Max_size; // Error Max_size has static file scope Constants by default are static(i.e., no external linkage), while variables by default are extern. Class names can be used in other files but there are conditions or rules for that, it is better to put the class declarations in a header file and include this file in different files as shown in the lab. assignment examples typedef, enum, and template names can not have external linkage

Functions, Scope, and The Free Store Local Scope consists of function scope and block scope function scope: extern const int Max; // global constant max defined and initialized in another file funny(){ int I = 100; // local variable const int Max = :: Max * I; // the scoping operator is used to //access the global Max to initialize the local Max } Block Scope: while(cond){ int I = 0; I = 500; // this I is local to the while block if (cond) {int I = 0; // this is a different I local to the if block} } Static local variables {static int I=0; // I retains its most recent values and stays alive until the // program terminates I++; } // I will contain the number of times this block is executed

Functions, Scope, and The Free Store The following code will not produce a syntax error but will produce a dangling pointer, a pointer that points to no object, int * sqr(int x) { int y; y = x*x; return &y; }// the data object y does not exist after the termination of this function main(){ int * p = sqr(100); // p is a dangling pointer // and is initialized to point to an object which has been destroyed } // the above will different if y is declared as a static storage class using static int y; // y will still exist even after sqr() terminates