string Assignment and Concatenation

Figure 18.1 demonstrates string assignment and concatenation. Line 7 includes header for class string. The strings string1, string2 and string3 are created in lines 1214. Line 16 assigns the value of string1 to string2. After the assignment takes place, string2 is a copy of string1. Line 17 uses member function assign to copy string1 into string3. A separate copy is made (i.e., string1 and string3 are independent objects). Class string also provides an overloaded version of member function assign that copies a specified number of characters, as in

targetString.assign( sourceString, start, numberOfCharacters );

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where sourceString is the string to be copied, start is the starting subscript and numberOfCharacters is the number of characters to copy.

Figure 18.1. Demonstrating string assignment and concatenation.

(This item is displayed on pages 886 - 887 in the print version)

 1 // Fig. 18.1: Fig18_01.cpp
 2 // Demonstrating string assignment and concatenation.
 3 #include 
 4 using std::cout;
 5 using std::endl;
 6
 7 #include  
 8 using std::string;
 9
10 int main()
11 {
12 string string1( "cat" );
13 string string2; 
14 string string3; 
15
16 string2 = string1; // assign string1 to string2 
17 string3.assign( string1 ); // assign string1 to string3
18 cout << "string1: " << string1 << "
string2: " << string2
19 << "
string3: " << string3 << "

";
20
21 // modify string2 and string3 
22 string2[ 0 ] = string3[ 2 ] = 'r';
23
24 cout << "After modification of string2 and string3:
" << "string1: "
25 << string1 << "
string2: " << string2 << "
string3: ";
26
27 // demonstrating member function at
28 for ( int i = 0; i < string3.length(); i++ )
29 cout << string3.at( i );
30
31 // declare string4 and string5
32 string string4( string1 + "apult" ); // concatenation
33 string string5;
34
35 // overloaded +=
36 string3 += "pet"; // create "carpet"
37 string1.append( "acomb" ); // create "catacomb"
38
39 // append subscript locations 4 through end of string1 to
40 // create string "comb" (string5 was initially empty) 
41 string5.append( string1, 4, string1.length() - 4 ); 
42
43 cout << "

After concatenation:
string1: " << string1
44 << "
string2: " << string2 << "
string3: " << string3
45 << "
string4: " << string4 << "
string5: " << string5 << endl;
46 return 0;
47 } // end main

 

 string1: cat
 string2: cat
 string3: cat

 After modification of string2 and string3:
 string1: cat
 string2: rat
 string3: car

 After concatenation:
 string1: catacomb
 string2: rat
 string3: carpet
 string4: catapult
 string5: comb

 

Line 22 uses the subscript operator to assign 'r' to string3[ 2 ] (forming "car") and to assign 'r' to string2[ 0 ] (forming "rat"). The strings are then output.

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Lines 2829 output the contents of string3 one character at a time using member function at. Member function at provides checked access (or range checking); i.e., going past the end of the string throws an out_of_range exception. (See Chapter 16 for a detailed discussion of exception handling.) Note that the subscript operator, [], does not provide checked access. This is consistent with its use on arrays.

Common Programming Error 18.2

Accessing a string subscript outside the bounds of the string using function at is a logic error that causes an out_of_range exception.

Common Programming Error 18.3

Accessing an element beyond the size of the string using the subscript operator is an unreported logic error.

String string4 is declared (line 32) and initialized to the result of concatenating string1 and "apult" using the overloaded addition operator, +, which for class string denotes concatenation. Line 36 uses the addition assignment operator, +=, to concatenate string3 and "pet". Line 37 uses member function append to concatenate string1 and "acomb".

Line 41 appends the string "comb" to empty string string5. This member function is passed the string (string1) to retrieve characters from, the starting subscript in the string (4) and the number of characters to append (the value returned by string1.length() - 4).