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CS253 String Literals

Literals

• Literals are constants, like 42, 1.2e-24, 'x', "foo", true, or nullptr.
  • const variables are not literals. They’re variables that don’t vary.
• This is a char: 'X'.
  • Single quotes for a single character.
• This is a C-style string literal: "alpha beta gamma".
  • Its type is const char [], or const char *.
• C++ 2011 has no std::string literals.
• C++ 2014 has "foobar"s, with the trailing s.
  • Its type is const std::string.

String Literals

A "string literal" is an anonymous array of constant characters. These are equivalent:

cout << "FN-2187";

FN-2187

const char whatever[] = "FN-2187";
cout << whatever;

FN-2187

const char whatever[] = "FN-2187";
const char *p = &whatever[0];
cout << p;

FN-2187

• A "string literal" is like an anonymous array.
• An array name is the same as the address of its first element.

Escape Sequences:

Sequence	Meaning	Sequence	Meaning
\a	bell	\'	'
\b	backspace	\"	"
\f	form feed	\\	\
\n	newline	\0ddd	0–3 octal digits
\r	carriage return	\xdd	1–∞ hex digits
\t	horizontal tab	\udddd	Unicode U+dddd
\v	vertical tab	\Udddddddd	Unicode U+dddddddd

String Pasting

Two adjacent string literals are merged into one at compile-time:

cout << "alpha beta "  "gamma delta "
        "epsilon\n";

alpha beta gamma delta epsilon

cout << "Business plan:\n\n"
        "1. Collect underpants\n"
        "2. ?\n"
        "3. Profit\n";

Business plan:

1. Collect underpants
2. ?
3. Profit

Raw Strings

• Sometimes, you want a string to actually contain a backslash.
• If so, you double the backslash.
• This can get tedious.

Raw Strings

A raw string starts with R"( and ends with )". The parens are not part of the string.

cout << R"(Don't be "afraid" of letters:
\a\b\c\d\e\f\g)";

Don't be "afraid" of letters:
\a\b\c\d\e\f\g

Cool! Quotes inside of quotes!

However …

What if the string contains a right paren? I want to emit:

    A goatee!  :-)"  Cool!

cout << "A goatee!  :-)"  Cool!";

c.cc:1: warning: missing terminating " character
c.cc:1: error: missing terminating " character

That didn’t work. The )" at the bottom of the face was taken to be the end of the raw string.

Solution

A raw string starts with:

R"whatever-you-like-up-to-sixteen-chars(

and ends with:

)the-same-up-to-sixteen-chars"

cout << R"X(A goatee!  :-)"  Cool!)X";

A goatee!  :-)"  Cool!

cout << R"WashYourHair(What the #"%'&*)?)WashYourHair";

What the #"%'&*)?

cout << R"(The degenerate case)";

The degenerate case

Comparing C-Style Strings

if ("foo" < "bar")
    cout << "😢";

c.cc:1: warning: comparison with string literal results in unspecified behavior
😢

• Look—“unspecified behavior”! Remember that?
• This will not compare the letters in the strings. It will, instead, compare the addresses. Which is at the lower address? Who knows‽
• Are the arrays "marx" and "marx" two arrays or one? Who knows‽
• g++ -Wall will detect this deplorable code.

Comparing C-style strings properly.

• To compare C-style strings, use the function strcmp.
• It has a peculiar return value. strcmp(a,b) returns:
  • Some value <0 if a<b.
  • 0 if a==b.
  • Some value >0 if a>b.
• Why are you even considering using C-style strings?
  • Well, sometimes, you have to.

Comparing C++ std::strings

• To compare C++ std::string values, or to compare a std::string with a C-style string, use the usual operators:

    <  >  <=  >=  ==  !=

• Only Java geeks use std::string::compare(), which has the same three-way return value as strcmp.
• This language has operator overloading. Be thankful!

Example

string name = "Conan O’Brien";
if (name == "Conan O’Brien")
    cout << "good 1\n";
if (name < "Zulu")
    cout << "good 2\n";
if (name > "Andy Richter")
    cout << "good 3\n";
if (name == name)
    cout << "good 4\n";

good 1
good 2
good 3
good 4

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