Advanced Program Design with C++ COMP 345 - Advanced Program Design with C++ Advanced Program Design with C++ Part 2: Data types Joey Paquet, 2007-2016

Data types Simple data types Pointers Type checking Type coercion COMP 345 - Advanced Program Design with C++ Data types Simple data types Pointers Type checking Type coercion Joey Paquet, 2007-2016

Highly similar to Java data types COMP 345 - Advanced Program Design with C++ Data types Highly similar to Java data types Basic types are not classes (like Java) Pit trap: different compilers will have different ranges for most basic data types Some programs potentially will behave differently across different platforms Hence, lack of portability of C++ programs User-defined data types using struct (as in C), as well as class (object-oriented programming) Both are allowed in the same program In fact, they are almost equivalent, but struct was kept for backward compatibility A struct can have data members, methods, constructors, destructors, etc One difference is that a struct sets its members as public by default Joey Paquet, 2007-2016

Data types: simple types size, range and precision COMP 345 - Advanced Program Design with C++ Data types: simple types size, range and precision Joey Paquet, 2007-2016

Data types: simple types size, range and precision COMP 345 - Advanced Program Design with C++ Data types: simple types size, range and precision Joey Paquet, 2007-2016

COMP 345 - Advanced Program Design with C++ Variable declaration A variable can be declared for any type valid in the current scope. int x; double y; myClass mc; Multiple variables of the same type can be declared on the same declaration: int x,y,z; Any declared name can be referred to thereafter in the scope in which it is declared. Joey Paquet, 2007-2016

All of these are widely used and apparently equivalent. However: COMP 345 - Advanced Program Design with C++ Variable declaration Declarations can include an optional initialization, which can use different syntactical forms: Type a1 {v}; Type a2 = {v}; Type a3 = v; Type a4(v); All of these are widely used and apparently equivalent. However: Some are restricted to use in certain situations. Only the first one is universally usable, and is actually safer, as it implicitly does some checking of the value passed versus the specified type. int a1 = 1.5; //allowed using truncation int a1 {1.5}; //not allowed, as truncation would happen Joey Paquet, 2007-2016

Data types: type checking and type coercion COMP 345 - Advanced Program Design with C++ Data types: type checking and type coercion C++ uses a manifest typing strategy Variables and values are assigned types explicitly in the source code Values can only be assigned to variables declared as having the same type However, C++ allows type coercion, i.e. implicitly or explicitly changing the type of variables or values This loophole, among other things, makes C++ a weakly typed language Type mismatches General Rule: Cannot place value of one type into variable of another type int var = 2.99; // 2 is assigned to var! Only the integer part "fits", so that’s all that goes Called "implicit type casting" or "automatic type conversion" When using pointers or classes, much more problematic! Joey Paquet, 2007-2016

Literals Data types: literals COMP 345 - Advanced Program Design with C++ Data types: literals Literals 2, 5.75, ‘Z’, "Hello World“ Considered "constants": can’t change in program All literals have an inherent type that can be determined during lexical analysis Like many other languages, C++ uses escape sequences for string literals: Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables Variables contain a specific value, e.g., an integer. Pointer variables hold memory addresses as their values. A pointer contains the memory address of a portion of memory that in turn contains a specific value. For any type T, T* is the type “pointer to T”, i.e. a variable of type T* can hold the address of an object of type T. int i = 99; int* p = &i; cout << *p << endl; Two operators on pointers: Dereferencing operator: *, e.g. *p refers to the object pointed to by the pointer. Address operator: &, e.g. &i refers to the address of the first memory cell holding an object. i (int) 99 p (*int) &i Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables Consider: int *p1, *p2, v1, v2; Pointer assignment: p1 = &v1; Sets pointer variable p1 to "point to" variable v1 "p1 equals address of v1" Or "p1 points to v1“ v1 = 0; *p1 = 42; p1 and v1 refer to same memory cell Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables Pointer assignment vs value assignment: int v1 = 42; int v2 = 9; int *p2 = &v2; int *p1 = &v1; Pointer assignment: p2 = p1; Assigns one pointer to another "Make p2 point to where p1 points“ Value assignment: *p2 = *p1; Assigns "value pointed to" by p1, to "value pointed to" by p2 Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables Dynamic variables Allocated with new operator, deallocated with the delete operator Allocated and destroyed explicitly while program runs Local variables Declared within function definition Not dynamic Allocated on the stack when code block is entered (e.g. function call) Destroyed when code block is exited (e.g. function call completes) Often called "automatic" variables Allocation and deallocation controlled for you Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables The operator new creates dynamically allocated values that can then be pointed to by pointer variables. The value created is a nameless pointer value. Allocated on the heap, or freestore through the runtime system’s interaction with the operating system. All dynamically allocated variables need to be carefully managed by the programmer. C++ does not have garbage collection. Dynamically allocated variables need to be allocated and deallocated manually Similar to C’s malloc Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables int *p1, *p2; p1 = new int; *p1 = 42; p2 = p1; Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables *p2 = 53; p1 = new int; *p1 = 88; Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables If the type used as parameter is of class type: Constructor is called for new object Can invoke different constructor with initializer arguments: MyClass *myPtr; myPtr = new MyClass(32.0, 17); Can still initialize non-class types: int *n; n = new int(17); //Initializes *n to 17 Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables Pointers are full-fledged types Can be used just like other types Can be function parameters Can be returned from functions Example: int* findOtherPointer(int* p); This function declaration: Has "pointer to an int" parameter Returns "pointer to an int" Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables Potential problem if freestore runs out of memory Older compilers: Test if null returned by call to new: int *p; p = new int; if (p == NULL) { cout << "Error: Insufficient memory.\n"; exit(1); } Later compilers (C++98 and after) : new throws exception bad_alloc try { int * myarray= new int[1000]; } catch (bad_alloc&) { cout << "Error allocating memory." << endl; } Joey Paquet, 2007-2016

Data types: pointer variables COMP 345 - Advanced Program Design with C++ Data types: pointer variables To deallocate dynamic memory, use the delete operator When value no longer needed Returns memory to freestore Example: int *p; p = new int(5); //allocate memory … //Some processing… delete p; //deallocate memory p = NULL; //prevents dangling pointer errors Deallocates dynamic memory "pointed to by pointer p“ p is then a dangling pointer If not deleted before the variable goes out of scope, memory is not freed, which creates a memory leak. Plus, dereferencing a dangling pointer leads to unpredictable results, ranging from getting a seemingly random value to program crash. Managing dangling pointers and deallocating dynamically allocated memory is a very important aspect of proper C++ programming. Joey Paquet, 2007-2016

Can perform arithmetic operations on pointers COMP 345 - Advanced Program Design with C++ Pointer arithmetic Can perform arithmetic operations on pointers Used to navigate arrays (covered later) Example: int *d; d = new int[10]; d refers to: address of new int[10] d + 1 refers to: address of new int[10] + 1*sizeof(int) d + 2 refers to: address of new int[10] + 2*sizeof(int) d[i] == *(&d[0]+i) == *(d+i) Joey Paquet, 2007-2016

References COMP 345 - Advanced Program Design with C++ Joey Paquet, 2007-2016

Reference Pointers are very powerful, as they allow: COMP 345 - Advanced Program Design with C++ Reference Pointers are very powerful, as they allow: A variable to refer a value held by another variable. A variable to refer to different values held by different variables in time. Pass information around without having to copy it. However, due to their power, pointers bring additional complexities: Must use a special syntax (*, &, ->) Possibility of dangling pointers, wild pointers, null pointers. References are pointer variables that eliminate some of the disadvantages of pointers, at the cost of eliminating some of their power. Pointer arithmetic cannot be applied to a reference. Any operation applied to a reference is actually applied onto the variable it refers to, including assignment. Hence, references must be initialized upon declaration and cannot be changed afterwards. Furthermore, given a reference int& r {v1}, &r returns a pointer to the object referred to by r. Thus, we cannot even have a pointer to a reference. Joey Paquet, 2007-2016

Type casting COMP 345 - Advanced Program Design with C++ Joey Paquet, 2007-2016

Data types: explicit type casting COMP 345 - Advanced Program Design with C++ Data types: explicit type casting C++ provides operators for explicit type coercion, or type casting static_cast<double>(intVar) Explicitly "casts" intVar to double type doubleVar = static_cast<double>(intVar1/intVar2); Casting forces double-precision division to take place among two integer variables. Equivalent in meaning to the following C syntax, even though the C++ cast operation is checked at compile time and is thus less prone to runtime errors doubleVar = (double)intVar1/intVar2; Joey Paquet, 2007-2016

Data types: explicit type casting COMP 345 - Advanced Program Design with C++ Data types: explicit type casting Different kinds of explicit type casting operations: static_cast<Type>(expression) General-purpose type casting const_cast<Type>(expression) Cast-out “constantness” dynamic_cast<Type>(expression) Runtime-checked conversion of pointers and references within a single class hierarchy. Used for downcasting from a superclass to a subclass reinterpret_cast<Type>(expression) Implementation-dependent casting, performs a binary copy and assigns the new type to the resulting binary copied value. Highly unsafe and error-prone. Joey Paquet, 2007-2016

Data types: upcasting and downcasting COMP 345 - Advanced Program Design with C++ Data types: upcasting and downcasting When dealing with classes and subclasses, one can declare objects of a supertype and manipulate them as one of its subclasses Problem: subclass members are undefined in superclass GeometricObject ---------------------- area perimeter void displayGeometricObject(GeometricObject& g) { cout << "The radius is " << g.getRadius() << endl; cout << "The diameter is " << g.getDiameter() << endl; cout << "The width is " << g.getWidth() << endl; cout << "The height is " << g.getHeight() << endl; cout << "The area is " << g.getArea() << endl; cout << "The perimeter is " << g.getPerimeter() << endl; } Circle ------------ radius diameter Rectangle ------------- width height Joey Paquet, 2007-2016

Data types: upcasting and downcasting COMP 345 - Advanced Program Design with C++ Data types: upcasting and downcasting May want to use static_cast: This successfully compiles, but will fail at runtime if the object passed was originally of a type that does not contain the members referred to in the code. static_cast makes a static (compile-time) type cast, but correct runtime behavior is not verified. void displayGeometricObject(GeometricObject& g) { GeometricObject* p = &g; cout << "The radius is " << static_cast<Circle*>(p)->getRadius() << endl; cout << "The diameter is " << static_cast<Circle*>(p)->getDiameter() << endl; cout << "The width is " << static_cast<Rectangle*>(p)->getWidth() << endl; cout << "The height is " << static_cast<Rectangle*>(p)->getHeight() << endl; cout << "The area is " << g.getArea() << endl; cout << "The perimeter is " << g.getPerimeter() << endl; } Joey Paquet, 2007-2016

Data types: upcasting and downcasting COMP 345 - Advanced Program Design with C++ Data types: upcasting and downcasting Use dynamic_cast to downcast into a subclass dynamic_cast works on pointers Does runtime checking to verify that the cast is successful Also deals with polymorphic types and the virtual methods table at runtime Joey Paquet, 2007-2016

COMP 345 - Advanced Program Design with C++ References Y. Daniel Liang, Introduction to Programming with C++ (Chapter 1, 11, 13, 15), Peason, 2014. Bjarne Stroustrup, The C++ Programming Language (Chapter 6, 7, 11, 22), Addison-Wesley, 2013. Joey Paquet, 2007-2016