Concatenation & Comparison

The <string> library redefines several standard operators using a C++ feature called operator overloading. When you use the + operator with numbers, it means addition, but, when you use it with the string type, it means concatenation.

string s1 = "hello", s2 = "world";
string s2 = s1 + " " + s2;          // "hello world"

The shorthand += assignment operator has also been overloaded. It concatenates new text to the end of an existing string. You may concatenate char values to a string object, but you cannot concatenate numbers to string objects as you could in Java.

string s{"abc"};  // uniform initialization
s += s;           // ok, "abcabc"
s += "def";       // literal ok, "abcabcdef"
s += 'g';         // char ok, "abcabcdefg"
s = s + 2;        // ERROR; no conversion

You cannot concatenate two string literals: "a" + "b" is illegal. However, separating them with whitespace, like "a" "b", is legal. Use this to join long lines together.

Comparisons

C++ overloads the relational operators so that you can compare string values just like primitive types. To see if the value of str is equal to "quit", just write this:

if (str == "quit") . . .

There is no need to use equals() or compareTo() as in Java.

Strings are compared using lexicographic ordering. Informally that means a string is smaller if it would appear earlier in the dictionary. However, when doing comparisons, case is significant, so "abc" is not equal to "ABC". Upper-case characters are "smaller" than lower-case characters, because they have smaller ASCII values.