EECS402 Lecture 24. Something New (Sort Of) Intro To Function Pointers

Transcription

1 The University Of Michigan Lecture 24 Andrew M. Morgan Savicth: N/A Misc. Topics Something New (Sort Of) Consider the following line of code from a program I wrote: retval = oplist[whichop] (mainary, arysize); Any ideas as to what this odd syntax does? Consider, instead this line of code: retval = addemup(mainary, arysize); Now, we see that the above line is a function call retval is the return value of the function call addemup is the name of the function we are calling mainary is the the first parameter to the function arysize is the second parameter to the functions Notice the second example is the same as the first, with "oplist[whichop]" replaced with "addemup" Andrew M Morgan 2 Intro To Function Pointers In C++ function names in your program actually represent the address where the executable code for that function starts This means that we can set a pointer variable to the address of a function, and call it via the pointer These are called "function pointers" There are many potential uses for function pointers: Function pointers can be placed in an indexed array, thus allowing you to call a specific function depending on value used as index Function pointers can be passed as parameters to functions, allowing one function to call different functions in different circumstances Function pointer can be updated in a loop to perform several operations (function calls) on the same data, with only one LOC Andrew M Morgan 3 1

2 Use Of Function Pointers Any variable in C++ must be declared. Function pointer variables are no exception What type is a function pointer? Type must describe the return value and params of the function typedef int (*operfn) (int[], int); ^ ^ \ / This is the param list for the func ptr This is the name of the type we are creating return type of function being pointed to Says that we are defining a new type (for our function ptr) Now, variables we declare to be of type "operfn" are function pointers, with the described signature Andrew M Morgan 4 Use Of Function Pointers, Cot'd Let's assume we have the following functions defined: int addemup (const int ary[], int size); int multemup (const int ary[], int size); int findmin (const int ary[], int size); int findmax (const int ary[], int size); All of the above functions have the same parameter types and return type We can define a type for pointers to such functions typedef int (*operfn) (int[], int); Now, declare a function pointer variable: operfn fptr; The variable "fptr" can be assigned to the above functions fptr = addemup; // or fptr = multemup; etc... Andrew M Morgan 5 Interesting (and useful) C++ Stuff There are many elements of the C++ language not discussed This lecture is just a collection of C++ information These items are NOT necessarily related in any way More importantly, you should not assume that everything covered should be used often Remember, just because you are a good programmer, using good style, does not mean everyone else is You should know many features even if you don't plan on using them all, in case you come across them in code Every feature has a place where it is a good feature, and most have a place where it is a bad feature Andrew M Morgan 6 2

3 The Ternary Operator There is an operator in C++ called the ternary operator It is somewhat widely used in certain circumstances Can make your code more compact Avoid using it too much, though, as it could get hard to trace The ternary operator can replace the "if-then-else" structure in some situations The operator is "?:" Place an expression before the question mark Place a value to be used is the expression is true between the question mark and colon Place a value to be used otherwise after the colon Often used when assigning different values depending on a condition Andrew M Morgan 7 The Ternary Operator, Example int i = 7, j = 10, k = 4, s = 0; //Assign s to i if i is less than j. //Otherwise, assign s to j; //Method 1 if (i < j) s = i; else s = j; cout << "First - s: " << s << endl; First - s: 7 Second - s: 7 Max(7,10) = 10 Max(7,4) = 7 //Method 2 s = (i < j)?i:j; cout << "Second - s: " << s << endl; cout << "Max(" << i << "," << j << ") = " << ((i > j)?i:j) << endl; cout << "Max(" << i << "," << k << ") = " << ((i > k)?i:k) << endl; Andrew M Morgan 8 The Comma Operator The comma is another operator allowed in C++ A comma expression is a series of expressions separated by commas Since a comma expression is, in fact, an expression, it has a value - the value of the rightmost expression Each expression in a comma expression is evaluated in order from left to right Use of the comma can lead to major headaches in trying to interpret a program in some cases Use of the comma in a for loop is quite common though Andrew M Morgan 9 3

4 The Comma Operator, Example int i, j, k = 0, l = 0, m = 0; for (i = 0, j = 5; i < 5; i++, j--) cout << "i: " << i << " j: " << j << endl; if (k == 3) //FALSE m = k = 1; l = 2; cout << "k: " << k << " l: " << l << " m: " << m << endl; if (k == 0) //TRUE m = k = 3; l = 4; cout << "k: " << k << " l: " << l << " m: " << m << endl; if (k == 17) //FALSE m = (k = 5, l = 6); cout << "k: " << k << " l: " << l << " m: " << m << endl; i: 0 j: 5 i: 1 j: 4 i: 2 j: 3 i: 3 j: 2 i: 4 j: 1 k: 0 l: 2 m: 0 k: 3 l: 4 m: 3 k: 3 l: 4 m: 3 k: 7 l: 8 m: 8 if (k == 3) //TRUE m = (k = 7, l = 8); cout << "k: " << k << " l: " << l << " m: " << m << endl; Andrew M Morgan 10 Enumerated Types An enumerated type is a user defined data type Variables of enumerated types can be assigned one of an enumerated list of possible values Syntax: enum enumtypename VAL0, VAL1, VAL2, ; VAL0, VAL1, etc can be any user-defined identifier Typically in ALL CAPS Very similar to const int Internally, each enumerated value is assigned a unique integer First value is assigned 0 by default, then 1, etc Andrew M Morgan 11 Defining An Enumerated Type enum DayOfWeekEnum MON, TUE, WED, THU, FRI, SAT, SUN; void print(dayofweekenum myday) if (myday <= FRI) cout << "Its not the weekend!" << endl; else cout << "Its the weekend!" << endl; Andrew M Morgan 12 4

5 Printing An Enumerated Type If you don t overload the << operator, the integer value will be printed ostream& operator<<( ostream &os, DayOfWeekEnum rhs ) switch (rhs) case MON: os << "Monday"; case TUE: os << "Tuesday"; case WED: os << "Wednesday"; case THU: os << "Thursday"; case FRI: os << "Friday"; case SAT: os << "Saturday"; case SUN: os << "Sunday"; default: os << "**UNKNOWN DAY**"; Andrew M Morgan 13 int main() DayOfWeekEnum myday; DayOfWeekEnum tomorrow; DayOfWeekEnum thenextday; DayOfWeekEnum someday; Enumerated Type Example myday = WED; cout << "Today is " << myday << endl; print(myday); myday = SAT; cout << "Today is " << myday << endl; print(myday); tomorrow = (DayOfWeekEnum)((int)myDay + 1); cout << "Tomorrow is " << tomorrow << endl; thenextday = (DayOfWeekEnum)((int)tomorrow + 1); cout << "The next day is " << tomorrow << endl; Today is Wednesday Its not the weekend! Today is Saturday Its the weekend! Tomorrow is Sunday The next day is Sunday Someday is: Sunday Values beyond max assigned to max! Output without operator<< someday = (DayOfWeekEnum)(39); Today is 2 cout << "Someday is: " << tomorrow << endl; Its not the weekend! Today is 5 Its the weekend! Tomorrow is 6 The next day is 6 Andrew M Morgan Someday is: 6 14 Assigning Vals To Enumerated Types Enumerated types with values other than the defaults are allowed #include <iostream> using namespace std; enum MyType FIRSTVAL = 25, SECONDVAL = 50, THIRDVAL = 75; int main() MyType blah; First val: 25 Second val: 50 Third val: 75 blah = FIRSTVAL; cout << "First val: " << blah << endl; blah = SECONDVAL; cout << "Second val: " << blah << endl; blah = THIRDVAL; cout << "Third val: " << blah << endl; Andrew M Morgan 15 5

6 Function Objects You've used the ( ) operator many times when you make a function call Normally, it would be a bad idea to overload this operator However, there are cases where it might make sense to overload the function call operator When a class overloads ( ), an object of that class is called a "function object" I stress again - this is usually a bad idea Still, it is pretty interesting When using an overloaded ( ) operator, it appears you are calling a function Since the function call is actually a method of the class, it acts different, depending on the values of data members of the object Andrew M Morgan 16 class compareclass public: int cmpwith; char cmpop; ; bool operator() (int check) bool res; if (cmpop == '<') res = (cmpwith < check); else if (cmpop == '>') res = (cmpwith > check); return res; Function Object, Example 1: true 2: false 3: false 4: true bool a; compareclass compare; compare.cmpwith = 9; compare.cmpop = '>'; Andrew M Morgan 17 a = compare(5); cout << "1: " << (a?"true":"false") << endl; a = compare(12); cout << "2: " << (a?"true":"false") << endl; compare.cmpop = '<'; a = compare(5); cout << "3: " << (a?"true":"false") << endl; a = compare(12); cout << "4: " << (a?"true":"false") << endl; Labels Labels are exactly that - a way to identify (label) an area of your code If unreferenced, they are ignored during execution A label is just an identifier with a colon after it - see below cout << "Welcome to a label program" << endl; label1: cout << "Labels can appear in programs" << endl; blah: cout << "They do not interfere with" << endl; cout << "the flow or execution!" << endl; blip: cout << "The End!" << endl; exit(0); blop: cout << "See ya" << endl; Welcome to a label program Labels can appear in programs They do not interfere with the flow or execution! The End! Andrew M Morgan 18 6

7 Goto (gasp!) Labels by themselves are pretty boring, as they serve no purpose C++ does provide a goto statement although it is rarely used and always results in a nasty sneer by most programmers Still, there are situations, where you might see a goto in someone else's code (of course, you would never use one) "goto" is a C++ keyword The keyword is followed by a label, making a "goto statement" When a goto statement is reached, the flow of your program immediately jumps to the next statement after the label Using goto results in jumpy code that is difficult to trace and follow. Avoid the use of goto!! Andrew M Morgan 19 Goto (gasp!), Example (double gasp!) int fact = 1; int value = 5; goto mult; printres: cout << "result is: " << fact << endl; goto alldone; result is: 120 Hope you had fun! Bye mult: fact *= value; value--; goto whatnext; cout << "Current Val: " << fact << endl; (Yes, it actually works, but it is far from clear. This is a horrible use of goto - many uses are not so blatant and difficult) alldone: cout << "Hope you had fun! Bye" << endl; whatnext: if (value >= 1) goto mult; else goto printres; return (5); Andrew M Morgan 20 Bitwise Operators You all know that a byte is a collection of 8 bits Bits are just values that are either 0 or 1 Since an integer is a 4 byte value (usually) it contains 32 bits Each of those 32 bits can store a 0/1 value C++ provides bitwise operators, often used for storing flags, or attributes, of an object ~: Bitwise not - negates each bit in an expression &: Bitwise and - "multiplies" each bit in one expression with the corresponding bit in another expression : Bitwise or - "adds" each bit in one with corresponding bit in other >>: Bitwise shift right - Shifts each bit to the right <<: Bitwise shift left - Shifts each bit to the left Andrew M Morgan 21 7

8 Bitwise Operators, Example const unsigned int ATR1 = 0x0001; //2^0 => 0001 const unsigned int ATR2 = 0x0002; //2^1 => 0010 const unsigned int ATR3 = 0x0004; //2^2 => 0100 const unsigned int ATR4 = 0x0008; //2^3 => 1000 printenv(int env) //here, the & is used as a MASK cout << "ATR1: " << ((env & ATR1)?"on":"off") << endl; cout << "ATR2: " << ((env & ATR2)?"on":"off") << endl; cout << "ATR3: " << ((env & ATR3)?"on":"off") << endl; cout << "ATR4: " << ((env & ATR4)?"on":"off") << endl; int e = 0; //Initially empty environment e = e ATR1; //Set attribute 1 e = e ATR3; //Set attribute 3 printenv(e); e = ~e; //Invert all attributes printenv(e); e = e & ~ATR2; //Clear attribute 2 printenv(e); ATR1: on ATR2: off ATR3: on ATR4: off ATR1: off ATR2: on ATR3: off ATR4: on ATR1: off ATR2: off ATR3: off ATR4: on Andrew M Morgan 22 More Interesting Stuff Did you ever wonder why you could check for a file being in fail state with just the name of the object? int main() ifstream myfile("in.txt"); int i; int j; myfile >> i >> j; if (!myfile) cout << "myfile is in fail state!" << endl; What is the "value" of an object (i.e. myfile, or an object of a userdefined class)? Andrew M Morgan 23 Conversion Operator Andrew M Morgan 24 An object doesn't not result in any value unless you, the programmer, tell it what to evaluate to You can do so via the special member functions called "conversion operators" Conversion operators have NO return type The name of the function is "operator <type>() To have your object evaluate to a boolean value, implement the following member function: operator bool(); //No return type! Even though there is no return type, this function should return a value of type bool Other conversion operators must return the appropriate type The fstream class has a boolean conversion operator. When the file is in fail state, the conversion operator returns false, otherwise it returns true 8

9 class IntClass public: IntClass(int inval):val(inval) ; operator int() return (val); ; operator bool() return (val == 0?false:true); ; private: int val; ; Conversion Operator, Cot'd x: 17 IC1 WAS TRUE IC2 WAS FALSE int main() IntClass ic1(13); IntClass ic2(0); int x; x = 4 + ic1; cout << "x: " << x << endl; Andrew M Morgan 25 cout << "IC1 WAS " << (ic1?"true":"false") << endl; cout << "IC2 WAS " << (ic2?"true":"false") << endl; 9