Some other issues in C tsaiwncsie nctu edu

Some other issues in C++ 蔡文能 tsaiwn@csie. nctu. edu. tw tsaiwn@cs. nctu. edu. tw 交通大學資訊 程學系 2006/08/23 1 交通大學資訓 程學系 蔡文能

http: //tsaiwn. net/cpp/ Agenda http: //www. csie. nctu. edu. tw/~tsaiwn/cpp • • Namespace static ? const ? cast in C++ Class Library STL – Standard Template Library Exception handling 2 交通大學資訓 程學系 蔡文能

Namespaces • variables with same name and different scopes can overlap – need to distinguish them • a namespace defines a scope for local and global identifiers. – body delimited by braces {} – use (: : ) to access namespace members: 例如 std: : cout namespace_name: : member – or, a using statement must occur before name is used using namespace_name; -members of the namespace do not need a prefix – not guaranteed to be unique – can be nested using namespace std; //for STL 交通大學資訓 程學系 蔡文能 3

神奇的 “static” ü On a global variable or a function static long my. Data[38]; //information hiding static void my. Function(float); – Tells the linker not to export the variable or function. – Makes the identifier “file scope, ” as the linker will not use it fulfill dependencies from other files. ü On a local variable in a function void some. Func(void) { static int array[4000]; } Places the variable off the stack. This has the side-effect that it retains it value across calls. It is often used when a variable is too large to be put on the stack. (auto變數則在 stack) ü On a class member data or member function (next slide) 4 交通大學資訓 程學系 蔡文能

Static Global 變數 #include <stdio. h> 參考K&R課本 4. 6節 #define BUFSIZE 100 static char buf[BUFSIZE]; int bufp = 0; 別的檔案中任何function都 int getch( ) { 看不見這個 static 的變數 /*. . . */ } void ungetch(int c) { /*. . . */ 也參考stack的push和pop寫在同一獨 } 立 file 中, push和pop共享 data 交通大學資訓 程學系 蔡文能 5

再談Static Global 變數 #include <stdio. h> 參考K&R課本 4. 6節 #define RAND_MAX 65535 static unsigned long seed=0; int rand( ) { seed = seed * 1103515245 + 12345; return seed % (RAND_MAX+1); } void srand(int newseed) { seed = newseed; } 交通大學資訓 程學系 蔡文能 Pseudo random number 6

Static Local變數 (1/2) #include <iostream. h> int fa( ) { int x = 1; return x++; /*先取其值, 再做 ++ */ } int fb( ) { static int x = 1; /* 注意 static int x = 1; */ return x++; } int main( ) { cout << "fa( )=" << fa( )<<fa( ) << endl; cout << "fb( )=" << fb( )<<fb( ) << endl; return 0; /* 0 in main( ) means OK */ } return x++; 和 return ++x; 不同 ! 7 交通大學資訓 程學系 蔡文能

Static class member • static member不需要透過任何的object存取，在無任何 object時可透過member selection operators (就是 : : 或Java 仍用. )來存取. • Static member functions – Also known as class function (class methods) – Can be called without any instance, for example: m = Mankind: : howmany( ); – Can NOT access instance variables • Static member data – Also known as class variable – All objects of a class share one copy of a static data member (non-static member data = instance variable) – Usage example: x = Mankind: : data_which_is_static; – They are not global variables; they have class scope All member functions are shared between objects, even the non-static member functions 交通大學資訓 程學系 蔡文能 10

神奇的 “const” ü Const object is NOT modifiable ü const x = 3. 14159; // 在 C 與較舊C++ compile會過; 問題在哪裡? ü Const parameter can NOT be modified in the function Return. Type Function. Name(long x, const Student y){ /*… to modify y is NOT allowed! 即 y 不可當左值 */ }; ü Const member function can NOT modify the object Return. Type Function. Name(param 1, param 2…) const; Return. Type Function. Name(param 1, param 2…) const { /*… */}; ü Constructors / Destructors cannot be const - They need to initialize variables (therefore modifying them) const 是不准變的變數! 也佔一塊記憶體! 那 #define nsize 99 呢? 交通大學資訓 程學系 蔡文能 11

Review: Constants 常數 • #define MAX_LOOP 500 – 此稱 define macro 巨集 – 是C的前處理器之句子, 結尾不必有分號 – 之後用到 MAX_LOOP 會被換成 500 • const double delta = 0. 0001; ^^^^^^^^ 不可變的變數叫const • const PI = 3. 14159; /* Error result? PI為 3*/ 12 交通大學資訓 程學系 蔡文能

Character Literal • 單引號夾住是 char – 'A' 相當於 65 (assume using ASCII) – 'a' 相當於 97 (assume using ASCII) – '0' 相當於 48 (assume using ASCII) • Wide character ? • Escape Sequence 逃脫序列串 – 'n' – '101' – 'x 41' == '12 ' == 'A' 13 交通大學資訓 程學系 蔡文能

Escape Sequence 逃脫序列串 • • • 'a' 'b' 'f' 'n' 'r' 't' 'v' ' \' '? ' '' ' '" ' == '07' == CTRL_G == Alert (Bell) == '10' == CTRL_H == Back. Space == '14' == CTRL_L == Form. Feed == '12' == CTRL_J == New. Line == '15' == CTRL_M == Carriage RETURN == '11' == CTRL_I == TAB'60' (Horizontal Tab) == == Vertical Tab == == backslash 反斜線 'x 30' == == question mark == == single quote ( apostrophe ) == == double quote (quote) 參考K&R課本 2. 3節 交通大學資訓 程學系 蔡文能 14

Cast in C++ • C++ has 4 more separate, specific casts – Conventional static cast (C-Style): (type)expression ü static_cast - conversion between types • • type checking at compile time standard conversions: void* to char*, int to float, etc. base class pointers to derived class pointers Format: static_cast<type to convert to>(object to convert) ü const_cast - cast away const or volatile • cannot be used directly to cast away const-ness; use pointers ü dynamic_cast – for safe navigation of an inheritance hierarchy. • Can be used only for pointer or reference types ü reinterpret_cast - for nonstandard casts • one pointer type to another pointer type, void* to int*, etc. • cannot be used for standard casts (int to double, etc. ). 15 交通大學資訓 程學系 蔡文能

C ++ class Library <algorithm> <exception> <ios> <iterator> <map> <ostream> <set> <typeinfo> <cmath> <bitset> <fstream> <iosfwd> <list> <memory> <queue> <sstream> <utility> <complex> <functional> <iostream> <locate> <new> <streambuf> <stack> <valarray> <deque> <iomanip> <istream> <limits> <numeric> <string> <stdexcept> <vector> 16 交通大學資訓 程學系 蔡文能

Standard C Library <stdio. h> <math. h> <stdlib. h> <string. h> <ctype. h> <time. h> <assert. h> <float. h> <limits. h> <stdarg. h> <stddef. h> <errno. h> <locale. h> <signal. h> <setjmp. h> • open, read, write, close 等不是 standard C Library, 而是 system call • C Library 在 UNIX 手冊第三章, System call 在 UNIX 手冊 第二章( 用 man 會看到 (2) ) • 所有 Library function 參看 K&R 附錄B 19 交通大學資訓 程學系 蔡文能

The C++ String Class • The C-style strings (arrays of char) that you’ve been using are not the only way of managing character data. • C++ allows you to work with a string class that eases many of the problems you’ve encountered with C-style strings. • In particular, you don’t have to worry about managing memory for the string data. 20 交通大學資訓 程學系 蔡文能

C++ String Basics • The basic character template class is basic_string<>. • It has two specializations (generated using typedefs), string and wstring. • string corresponds to the C-style string (I. e. , const *char). • wstring is an extension for languages that use characters. • You can copy, assign, and compare strings just like you would copy, assign, and compare primitive types (int, double) string a = “Hello”; string b = string(a); string c = a; bool d = (a==b); 21 交通大學資訓 程學系 蔡文能

using the String types • In the header file <string> – (Note, no. h) • All such functions and other names are in the namespace std. • The basic class name is a template class, basic_string<>. • String constructors string( ) // empty string(string s) // copy of s string(string s, int start) // substring(string s, int start, int len) // substring(char* a) // C-string(int cnt, char c) // one or more chars string(char* beg, char* end) // [beg, end) 22 交通大學資訓 程學系 蔡文能

Accessing Individual Characters • It’s almost like for a C-string s = “Harry”; s[0] is ‘H’ s[1] is ‘a’ … s[5] is undefined!!!! – no ‘’ at the end 23 交通大學資訓 程學系 蔡文能

C++ string Operations • = is used to assign a value (char, C-string, or string) to a string. • += is used to append a string, character, or C-string to a string. • + is used to concatenate two strings or a string with something else • Boolean operations do a dictionary order comparison • << and >> are used for input and output. On input, leading whitespace is skipped, and the input terminates with whitespace or end of file. 24 交通大學資訓 程學系 蔡文能

Other C++ string Operations swap(a, b); // swap the guts of a with b s. append(s 2); // append s 2 to s s. c_str( ); // return a C-string s. push_back(c); // append a char s. erase(various); // erases substrings s. insert(various); // inserts substrings s. clear(); // removes all contents s. resize(cnt); // change the size of s to cnt s. replace(various); // replaces characters s. size(); // how many characters? s. length(); // how many characters? s. max_size(); // maximum number of char? s. empty(); // is s empty? s. capacity(); // size without reallocation s. reserve(cnt); // reserves memory 25 交通大學資訓 程學系 蔡文能

Using c++ string s = "Harry "; s. data() // returns s as a data array, no ''. s. c_str() // returns s as a C-string with '' int i = atoi(s. c_str()); // conversion char *carray = new char[80]; s. copy(carray, 79); // copies up to 79 char 26 交通大學資訓 程學系 蔡文能

STL Algorithms <algorithm> • binary_search • sort • count: count(list. begin(), list: end(), val, num); • equal: compares containers • for_each: applies ftn to each element • copy: copies container • reverse • min/max • Some in <numeric> 27 交通大學資訓 程學系 蔡文能

Using vector <int> v; v. reserve (100); // allocate space for 100 integers // capacity 100, size 0 int i; while (cin >> i) // read from the standard input v. push_back (i); // will expand v if needed for (i = 0; i < v. size (); ++i) cout << v[i]; try { // use checked access cout << v. at (100); // may throw } catch (std: : out_of_range&) { cout << "doesn't have 101 elements" << endl; } for (int i = 0; i < v. size () / 2; ++i) v. pop_back (); // remove second half vector <int> v 1 (v); // copy to v 1. insert (v 1. begin ()+1, 117); // insert after first 29 交通大學資訓 程學系 蔡文能

Utilities: pair • a helper class template std: : pair, used a lot by the standard library template <typename T 1, typename T 2> // simplified struct pair { T 1 first; T 2 second; pair () : first (T 1()), second (T 2()) {} // init pair (const T 1&, const T 2&); . . . // etc. }; • provides a full range of comparison operations: ==, <, etc. • a related helper function template to make pairs: std: pair <int, double> w = std: : make_pair (1, 1. 2); 30 交通大學資訓 程學系 蔡文能

Sequences • There are several kinds of sequences; choose ü vectors when – there are random access operations, – most insertions and removals are at the end of the container ü deques when – there are frequent insertions and deletions at either end, – there are random access operations ü lists when – there are frequent insertions and deletions at positions other than at the end – there are few random access operations – want to guarantee iterators are valid after modifications 31 交通大學資訓 程學系 蔡文能

Deques • deques are similar to vectors • deque iterators are random access • additionally two operations to insert/remove elements in front: push_front () add new first element pop_front () remove the first element • deques do not have operations capacity () and reserve () 32 交通大學資訓 程學系 蔡文能

Linked Lists list <char> s; // empty list s. insert (s. end (), 'a'); s. insert (s. end (), 'b') // s has (a, b) list <char> s 1; // empty list // copy s to s 1: s 1. insert (s 1. end (), s. begin (), s. end ()); s. clear (); cout << s 1. front (); s 1. erase (s 1. begin ()); // remove first element 33 交通大學資訓 程學系 蔡文能

Iterators again • an iterator provides access to objects stored in a container (points to an element); every iterator it has to support: *it to access the element currently pointed to by the iterator ++it to move to the next element of the container it = = it 1 to compare two iterators for pointer equality it ! = it 1 to compare two iterators for pointer inequality • every container type provides one or more iterators in a uniform way as standardized type names: std: : vector <string>: : iterator std: : vector <string>: : const_iterator begin () end () returns an iterator pointing to the first element returns an iterator pointing past the end; serves as a sentinel, i. e. , end marker. 35 交通大學資訓 程學系 蔡文能

Iterators again (cont. ) • the iterator operations are sufficient for iterating over any Container: Container c; . . . Container: : iterator it; for (it = c. begin (); it != c. end (); ++it) {. . *it. . it->op (). . } • a sequence of consecutive values in the container is determined by an iterator range, defined by two iterators, [first, last) • last is assumed reachable from first by using the ++ operator, and all iterators, including first but excluding last can be dereferenced • two iterators can be compared for equality and inequality 36 交通大學資訓 程學系 蔡文能

Associative Containers • STL associative containers: – represent sorted collections – have keys, and are sorted on these keys • A map is a mapping from keys to values: – it stores pairs (key, value) – there may be at most one pair with the same key – e. g. : (123, "Bush") and (124, "Kerry") • A multimap: – may have more than one pair with the same key – e. g. , (123, "Bush") and (123, "Kerry") 37 交通大學資訓 程學系 蔡文能

map usage // map: string to doubles map <string, double> salaries; // map: string to doubles sorted by greater <string> map <string, double, greater <string> > salaries 1; • to access an element of a map, you can use an overloaded index operator [] (key): salaries 1 ["John"] = 10000; salaries 1 ["Mary"] = 20000; • operator [] can not be used for multimaps; for example multimap <string, string> mm; mm ["color"] = "blue"; • instead, you have to use insert (): mm. insert ("color", "blue"); mm. insert ("color", "green"); // compile-time error 38 交通大學資訓 程學系 蔡文能

STL e. g. : using map • Goal: store grades for group of students – ordered set of assignments – maybe students with same name • For each student, have grades – want fast access to each grade – use vector of chars – typedef vector<int> Grades; • First, create map of students: • map<string, Grades> roster; 39 交通大學資訓 程學系 蔡文能

Using map: add a student to map • map represents a function: key maps to value • map, basically: set of ordered pairs – pair<x, y> template • void Roster: : add. Student(const string &name) { //check if already exists if (roster. find(name) != roster. end()) return; //check for room if (roster. size() == MAX) wait. List. push_back(name); else { Grades grades; roster. insert( pair<string, Grades>(name, grades)); } 40 交通大學資訓 程學系 蔡文能

Using map: drop a student • void Roster: : drop. Student(String &name) { if (roster. find(name) == roster. end()) return; roster. erase(name); if (wait. List. size() > 0) { string wait = wait. List. pop_front(); wait. List. pop(); Grades grades; roster. insert( pair<string, Grades>(name, grades)); } } 41 交通大學資訓 程學系 蔡文能

Using map: set a grade • void Roster: : set. Grade(const &string name, const int assign, const char grade) { map<string, Grades> stud = roster. find(name); if (stud == roster. end()) { cerr << “not foundn”; return; } if (stud->second(). size() <= assign) stud->second(). resize(assign+1); stud->second[assign] = grade; } 42 交通大學資訓 程學系 蔡文能

Associative Containers (cont. ) • set is a map for which the key and the value are identical therefore it stores logically single elements rather than pairs • multisets are sometimes called bags; they are like sets except there may be more than one occurrence of an element with the same key • elements in associative containers are sorted by the value of a key • associative containers use a binary predicate less <Key>, which compares two Key elements, using Key: : operator<. • the less predicate also determines the meaning of two keys being equivalent: two keys of an associative container are equivalent if neither is less than the other 43 交通大學資訓 程學系 蔡文能

Container Operations 47 交通大學資訓 程學系 蔡文能

http: //www. csie. nctu. edu. tw/~tsaiwn/cpp/ Thank You! 謝謝捧場 tsaiwn@csie. nctu. edu. tw 蔡文能 交通大學資訓 程學系 蔡文能 51