Thinking In C++. Volume 2: Practical Programming

The next string, heyMom, is initialized by the literal argument "Where are my socks?" This form of initialization uses a quoted character array as a parameter to the string constructor. By contrast, standardReply is simply initialized with an assignment. The last string of the group, useThisOneAgain, is initialized using an existing C++ string object. Put another way, this example illustrates that string objects let you do the following:^.

·         Create an empty string and defer initializing it with character data.

·         Initialize a string by passing a literal, quoted character array as an argument to the constructor.

·         Initialize a string using the equal sign (=).

·         Use one string to initialize another^.

//: C03:SmallString.cpp

#include

using namespace std;

int main() {

  string imBlank;

  string heyMom("Where are my socks?");

  string standardReply = "Beamed into deep "

    "space on wide angle dispersion?";

  string useThisOneAgain(standardReply);

} ///:~

These are the simplest forms of string initialization, but variations offer more flexibility and control. You can do the following:

 Use a portion of either a C char array or a C++ string.

 Combine different sources of initialization data using operator+.

 Use the string object’s substr( ) member function to create a substring^.

Here’s a program that illustrates these features.

//: C03:SmallString2.cpp

#include

#include

using namespace std;

int main() {

  string s1

    ("What is the sound of one clam napping?");

  string s2

    ("Anything worth doing is worth overdoing.");

  string s3("I saw Elvis in a UFO");

  // Copy the first 8 chars

  string s4(s1, 0, 8);

  cout << s4 << endl;

  // Copy 6 chars from the middle of the source

  string s5(s2, 15, 6);

  cout << s5 << endl;

  // Copy from middle to end

  string s6(s3, 6, 15);

  cout << s6 << endl;

  // Copy all sorts of stuff

  string quoteMe = s4 + "that" +

  // substr() copies 10 chars at element 20

  s1.substr(20, 10) + s5 +

  // substr() copies up to either 100 char

  // or eos starting at element 5

  "with" + s3.substr(5, 100) +

  // OK to copy a single char this way

  s1.substr(37, 1);

  cout << quoteMe << endl;

} ///:~

The string member function substr( ) takes a starting position as its first argument and the number of characters to select as the second argument. Both arguments have default values. If you say substr( ) with an empty argument list, you produce a copy of the entire string; so this is a convenient way to duplicate a string^.

Here’s the output from the program:

What is

doing

Elvis in a UFO

What is that one clam doing with Elvis in a UFO?

Notice the final line of the example. C++ allows string initialization techniques to be mixed in a single statement, a flexible and convenient feature. Also notice that the last initializer copies just one character from the source string^.

Another slightly more subtle initialization technique involves the use of the string iterators string::begin( ) and string::end( ). This technique treats a string like a container object (which you’ve seen primarily in the form of vector so far—you’ll see many more containers in Chapter 7), which uses iterators to indicate the start and end of a sequence of characters. In this way you can hand a string constructor two iterators, and it copies from one to the other into the new string:^.

//: C03:StringIterators.cpp

#include

#include

#include

using namespace std;

int main() {

  string source("xxx");

  string s(source.begin(), source.end());

  assert(s == source);

} ///:~

The iterators are not restricted to begin( ) and end( ); you can increment, decrement, and add integer offsets to them, allowing you to extract a subset of characters from the source string^.

C++ strings may not be initialized with single characters or with ASCII or other integer values. You can initialize a string with a number of copies of a single character, however^.

//: C03:UhOh.cpp

#include

#include

using namespace std;

int main() {

  // Error: no single char inits

  //! string nothingDoing1('a');

  // Error: no integer inits

  //! string nothingDoing2(0x37);

  // The following is legal:

  string okay(5, 'a');

  assert(okay == string("aaaaa"));

} ///:~

Operating on strings