ࡱ> pro5@ Q=bjbj22 (XXQ52222222F f!lF62!!!!!"""5555555$67R952"""""522!!5((("2!2!5("5(.((r?4T224!! mY 'v455064V:=(j:4FF2222:24""("""""55FF (FF Strings in C++ This is document was created by carefully reading the official C++ standard (ISO/IEC 14882, Programming Languages - C++, 1998-09-01) which has been adopted by ANSI, ISO, IEC, and ITI, and by testing everything on a seemingly reliable compiler (gnu v 2.95.4). Of course I may have made mistakes, but at least they will not be the usual mistakes that get copied blindly from one text-book or reference to the next. It is divided into four sections: General notes Ways of initialising strings (constructors) String operators (+, +=, =, ==, !=, <, >, <=, >=, []) String methods (special functions: everything else) Converting between strings and other things. Iterators are not covered here. 0. General Notes about Strings 0a. Internal Structure A C++ string is not the same thing as a C string. In C, string is just a descriptive name, not really part of the language; it means an array of chars terminated by a zero. In C++, string is a defined type. A C++ string is an object which includes both an array of characters and an independent record of the length; the array of characters is not zero-terminated, and may even contain zeros within it. 0b. Access #include is required to access C++ strings. and are completely different things, and must not be confused. 0c. Character Encoding The characters of a string are of type char; exactly what char means is permitted to vary from one implementation to another, but a char is practically invariably an 8 bit value. Whether the range is (-128 to +127) or (0 to +255) is also undefined, and really does vary between compilers. ASCII coding is not guaranteed, but can certainly be strongly expected. 1. Initialising and Creating strings: Constructors 1a. Default constructor, as in string s; Creates an empty string variable of effectively unlimited capacity, but empty. Like s=; 1b. Copy constructor, as in string s(t); // where t is also a string, a char* or an array of chars or string s=t; // where t is also a string, a char* or an array of chars or string s(init); or string s=init; Creates a new string which is an exact copy of t (or the quoted string). 1c. Substring constructor (beware! case 1d can easily be confused with this case) string s(t, pos); // where t is also a string, pos is an int or string s(t, pos, len); // where t is also a string, pos, len are ints The new string is a copy of t, with the first pos characters skipped. If len is supplied then only len characters are copied. That is, it copies the substring from position [pos] to position [pos+len-1], or to the end of the string, whichever comes first. 1d. Array of characters constructor string s(t, len); // where t is a char* or an array of chars or string s(init, len); // and len is an int The new string is made from the first len characters of t. The actual length of t is completely ignored: string s(abc, 6) still copies six characters starting with the a, b, c, and \0. This may cause a memory access violation if the length is wrong. 1e. Repeated Character constructor string s(len, ch); // where len is an int, and ch is a single char or string s(len, c); // where len is an int Creates a new string exactly len characters long, consisting of the character ch repeated len times. 2. String Operators 2a. Assignment of another string following this declaration: string s; s=t; // where t is also a string. Makes an exact copy of the source string t. s is a copy of t, not a reference (or alias), so later modifying s can not have any effect on t, or vice-versa. 2b. Assignment of an array of characters or a char-star pointer following this declaration: string s; s=t; // where t is a char* or an array of chars or s=val; The characters of t are copied into s, up to but not including the first NUL \0. If t is not properly NUL-terminated, a memory access violation may result. The number of characters copied, and hence the resulting length of s, is the value that strlen(t) would produce. 2c. Assignment of a single character following this declaration: string s; s=ch; // where ch is a char or s=c; s becomes a string of length 1, and its one character is set to c. This is true even if c is NUL, \0. s=c is effectively equivalent to s=c 2d. Comparisons: Relational Operators The operators ==, !=, <, >, <=, and >= may be used to compare two strings, a string with a char*, or a char* with a string; they do not perform string comparisons between two char*s. Remember that constant strings in quotes, like this are char*s, not C++ strings. following this declaration: string s, t; the expressions s==t s!=t st s<=t s>=t, s==xxx s!=xxx sxxx s<=xxx s>=xxx, xxx==t xxx!=t xxxt xxx<=t xxx>=t, all behave in the naturally expected way. 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