Identifiers, Types, and Literals
Identifiers are names that are used in C++ programs for functions, parameters, variables, constants, classes, and types. An identifier consists of a sequence of letters, digits, and underscores that does not begin with a digit. An identifier cannot be a reserved keyword. See Appendix A for a list of them. The standard does not specify a limit to the length of an identifier, but certain implementations of C++ only examine the first 31 characters to distinguish between identifiers.
A literal is a constant value that appears somewhere in a program. Since every value has a type, every literal has a type also. It is possible to have literals of each of the native data types, as well as character string literals. Table 1.1 shows some examples of literals.
|
Literal |
Meaning |
|---|---|
|
5 |
an int literal |
|
5u |
u or U specifies unsigned int |
|
5L |
l or L specifies long int after an integer |
|
05 |
an octal int literal |
|
0x5 |
a hexadecimal int literal |
|
true |
a bool literal |
|
5.0F |
f or F specifies single precision floating point literal |
|
5.0 |
a double precision floating point literal |
|
5.0L |
l or L specifies long double if it comes after a floating point |
|
'5' |
a char literal (ASCII 53) |
|
"50" |
a const char* containing the chars '5', '0', and '' |
|
"any" "body" |
"anybody" |
|
'a' |
alert |
|
'\' |
backslash |
|
'' |
backspace |
|
' ' |
carriage return |
|
'" |
single quote |
|
'"' |
double quote |
|
'f' |
formfeed (newpage) |
|
' ' |
tab |
|
' ' |
newline char literal |
|
" " |
newline followed by null terminator (const char*) |
|
'' |
null character |
|
'v' |
vertical tab |
|
"a string with newline " |
another const char* |
String literals are special in C++, due to its historical roots in the C language. Example 1.6 shows how certain characters need to be escaped inside double-quoted string delimiters.
Example 1.6. src/early-examples/literals.cpp
|
#include #include int main() { using namespace std; const char* charstr = "this is one very long string " "so I will continue it on the next line"; string str = charstr; <-- 1 cout << str << endl; cout << " A b\c'd"" << endl; return 0; } (1)STL strings can hold onto C-style char* strings. |
We compile and run the way we described earlier:
src/early-examples> g++ -ansi -pedantic -Wall literals.cpp src/early-examples> ./a.out
The output should look something like this:
this is one very long string so I will continue it on the next line A bc'd"
Notice that this program shows a way to avoid very long lines when dealing with string literals. They can be broken at any white space character and are concatenated automatically using this syntax.
Exercises: Identifiers, Types, and Literals
Modify Example 1.6 so that, with a single output statement, the output becomes:
| 1. |
GNU stands for "GNU's Not Unix". |
| 2. |
Title 1 "Cat Clothing" Title 2 "Dog Dancing" |