Announcements. Lecture 04b Header Classes. Review (again) Comments on PA1 & PA2. Warning about Arrays. Arrays 9/15/17

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1 Announcements Lecture 04b Sept. 14 th, 2017 Midterm #1: Sept. 26 th (week from Tuesday) Code distributed one week from today PA2 test cases & answers posted Quiz #4 next Tuesday (before class) PA3 due next Tuesday Any questions? 9/15/17 CS253 Fall 2017 Bruce Draper 1 9/15/17 CS253 Fall 2017 Bruce Draper 2 Comments on PA1 & PA2 Lots of questions (Piazza/ ) that were answered in class Lots of Tuesday night questions A few people not using objects No one has ever passed the class this way Reminder: all code must be your own Includes not using another student s previous assignments 9/15/17 CS253 Fall 2017 Bruce Draper 3 Review (again) Local variables are stored on the stack They might be primitives They might be objects Constructor are invoked Where local variable are declared Where parameters are passed by value Destructors are invoked... When local variables fall out of scope When non-reference parameters fall out of scope References are not themselves objects Do not trigger constructors/destructors 9/15/17 CS253 Fall 2017 Bruce Draper 4 An array is A contiguous block of memory Containing instances of 1 type of data The data type of an array is a pointer Quagga* and Quagga[] are the same data type The Brackets [] are used to offset into arrays Arr[0] : 1 st element Arr[1] : 2 nd element Arr[n-1]: N th element Arrays Array of 3 Quaggas Quagga #1 Quagga #2 Quagga #3 Warning about Arrays Arrays are primitive data types (pointers) They are not objects (class instances) You cannot ask them their size The bracket operator adds offsets This is why indices start at 0! The bracket operator does not check bounds Imagine the following: Quagga qarray[5]; Quagga quik = qarray[9]; 9/15/17 CS253 Fall 2017 Bruce Draper 5 9/15/17 CS253 Fall 2017 Bruce Draper 6 1

C-Style Strings Before object oriented programming was invented, C programmers needed strings Strings in C are just arrays of char Problem: how to keep track of their length?

Remember the main? * int main(int argc, char* argv[]) The argv argument is an array of c-style strings Can be written as char* argv[] Can be written as char** argv * Any history buffs in the audience?

2 C-Style Strings Before object oriented programming was invented, C programmers needed strings Strings in C are just arrays of char Problem: how to keep track of their length? Solution: arrays of char end in a NULL byte Convention, but very strong convention Only applies to arrays of char C-string libraries assume NULL byte endings Functions such as strlen & strcmp Remember the main? * int main(int argc, char* argv[]) The argv argument is an array of c-style strings Can be written as char* argv[] Can be written as char** argv * Any history buffs in the audience? 9/15/17 CS253 Fall 2017 Bruce Draper 7 9/15/17 CS253 Fall 2017 Bruce Draper 8 Argv in memory > PA3 foo.pgm bar.pgm argv P A 3 <--- argc ---> f o o. p g m b a r. p g m Dynamic Data This is how we can allocate dynamic data. Image file headers begin by telling you the image width and height Read the file header, discover the image is 512x512 int* image = new int[width * height]; Now you have an array whose size was not known until run-time 9/15/17 CS253 Fall 2017 Bruce Draper 9 9/15/17 CS253 Fall 2017 Bruce Draper 10 What About Vectors? Vector is a header class Header classes are Memory Management Strategy #1 A simple vector class might look like class intvector private: Header classes manage heap memory They allocate & de-allocate memory Properly implemented, they allow you to treat dynamic memory like a stack variable To do this, they rely on * Constructors Destructors The Assignment operator * Your book calls these The Big 3 9/15/17 CS253 Fall 2017 Bruce Draper 11 9/15/17 CS253 Fall 2017 Bruce Draper 12 2

3 Example: An Int Vector class intvector ; Destructor Declaration Example (continued) intvector::intvector(int sz) : size(sz) // Allocate memory when intvector data = new int[size]; // comes into scope (is created) intvector::~intvector() delete [] data; data = NULL; size = 0; // Deallocate memory when intvector // drops out of scope (or is deleted) // Setting data to NULL is good style int intvector::at(int index) const if ((index >= 0) && (index < size)) return data[index]; else throw std::exception(); // At methods check bounds (unlike [ ]) 9/15/17 CS253 Fall 2017 Bruce Draper 13 9/15/17 CS253 Fall 2017 Bruce Draper 14 Using intvector #include<intvector.h> #include<iostream> using std::cin; int main(int argc, char* argv[]) int data_size; cin >> data_size; //todo: error check intvector ivec(data_size); return 0; Destructor deletes memory on heap Constructor allocates memory on heap The goal of a header class is to make dynamic data structures act like local variables intvectors are local variables They are objects on the stack That contain a pointer to data on the heap Like all local variables, their constructor is called when they come into scope Under the hood, their constructor dynamically allocates heap memory When they leave scope, their destructor is called Under the hood, this deletes the heap memory For those of you using vectors and/or strings in PA1/2/3, this is what you were doing! 9/15/17 CS253 Fall 2017 Bruce Draper 15 9/15/17 CS253 Fall 2017 Bruce Draper 16 Improving intvector class intvector ; Is this legal? Two methods with the same name and arguments? The hidden argument isn t the same It is of type const intvector in one It is of type intvector in the other Why declare both methods? The 1 st lets you get value from a constant vector The 2 nd lets you change values in a (nonconstant) vector 9/15/17 CS253 Fall 2017 Bruce Draper 17 9/15/17 CS253 Fall 2017 Bruce Draper 18 3

4 Using Both Methods void Foo(intvector& a, const intvector& b) int test; if (b.size() > 0) test = b.at(0); if (a.size() > 0) a.at(0) = test; Make intvector Dynamic class intvector void push_back(int value); ; 9/15/17 CS253 Fall 2017 Bruce Draper 19 9/15/17 CS253 Fall 2017 Bruce Draper 20 Implementing push_back void intvector::push_back(int value) int* temp = data; data = new int[size+1]; for(int i=0; i< size; i++) data[i] = temp[i]; data[size] = value; size++; delete [] temp; // discuss Problem: why does this crash? void Foo(intvector a) void Quagga::Bar() intvector iv(0); iv.push_back(37); Foo(iv); 9/15/17 CS253 Fall 2017 Bruce Draper 21 9/15/17 CS253 Fall 2017 Bruce Draper 22 Copy Constructors When an object is passed by value, its copy constructor is called. Copy constructors take a constant reference to same-type object as their arguments Quagga::Quagga(const Quagga& src) The default copy constructor copies the values of the parameter to the new instance Including pointers Fortunately, you can redefine the copy constructor Fixing intvector class intvector intvector(const intvector& src); void push_back(int value); ; 9/15/17 CS253 Fall 2017 Bruce Draper 23 9/15/17 CS253 Fall 2017 Bruce Draper 24 4

5 Implementing the Copy Constructor intvector::intvector(const intvector& src) size = src.size; data = new int[size]; for(int i=0; i < size; i++) data[i] = src.data[i]; Are these legal? Yes. Size and data are protected, which means only intvectors can access them. It would be legal if they were private, too. 9/15/17 CS253 Fall 2017 Bruce Draper 25 5

Announcements. Lecture 05a Header Classes. Midterm Format. Midterm Questions. More Midterm Stuff 9/19/17. Memory Management Strategy #0 (Review)

Announcements. Lecture 05a Header Classes. Midterm Format. Midterm Questions. More Midterm Stuff 9/19/17. Memory Management Strategy #0 (Review) Announcements Lecture 05a Sept. 19 th, 2017 9/19/17 CS253 Fall 2017 Bruce Draper 1 Quiz #4 due today (before class) PA1/2 Grading: If something is wrong with your code, you get sympathy PA3 is due today