C 或 C++ 中的单引号与双引号

Question

When should I use single quotes and double quotes in C or C++ programming?

Answer 1

In C and in C++ single quotes identify a single character, while double quotes create a string literal. 'a' is a single a character literal, while "a" is a string literal containing an 'a' and a null terminator (that is a 2 char array).

In C++ the type of a character literal is char , but note that in C, the type of a character literal is int , that is sizeof 'a' is 4 in an architecture where ints are 32bit (and CHAR_BIT is 8), while sizeof(char) is 1 everywhere.

Answer 2

Some compilers also implement an extension, that allows multi-character constants. The C99 standard says:

6.4.4.4p10: "The value of an integer character constant containing more than one character (eg, 'ab'), or containing a character or escape sequence that does not map to a single-byte execution character, is implementation-defined."

This could look like this, for instance:

const uint32_t png_ihdr = 'IHDR';

The resulting constant (in GCC, which implements this) has the value you get by taking each character and shifting it up, so that 'I' ends up in the most significant bits of the 32-bit value. Obviously, you shouldn't rely on this if you are writing platform independent code.

Answer 3

Single quotes are characters ( char ), double quotes are null-terminated strings ( char * ).

char c = 'x';
char *s = "Hello World";

Answer 4

• 'x' is an integer, representing the numerical value of the letter x in the machine's character set
• "x" is an array of characters, two characters long, consisting of 'x' followed by '\0'

Answer 5

I was poking around stuff like: int cc = 'cc'; It happens that it's basically a byte-wise copy to an integer. Hence the way to look at it is that 'cc' which is basically 2 c's are copied to lower 2 bytes of the integer cc. If you are looking for a trivia, then

printf("%d %d", 'c', 'cc'); would give:

99 25443

that's because 25443 = 99 + 256*99

So 'cc' is a multi-character constant and not a string.

Cheers

Answer 6

Single quotes are for a single character. Double quotes are for a string (array of characters). You can use single quotes to build up a string one character at a time, if you like.

char myChar     = 'A';
char myString[] = "Hello Mum";
char myOtherString[] = { 'H','e','l','l','o','\0' };

Answer 7

1. single quote is for character ;
2. double quote is for string .

Answer 8

在 C 中，单引号（例如'a'表示字符常量，而"a"是一个字符数组，总是以\0字符结尾

Answer 9

Double quotes are for string literals, eg:

char str[] = "Hello world";

Single quotes are for single character literals, eg:

char c = 'x';

EDIT As David stated in another answer, the type of a character literal is int .

Answer 10

A single quote is used for character, while double quotes are used for strings.

For example...

 printf("%c \n",'a');
 printf("%s","Hello World");

Output

a  
Hello World

If you used these in vice versa case and used a single quote for string and double quotes for a character, this will be the result:

  printf("%c \n","a");
  printf("%s",'Hello World');

output :

For the first line. You will get a garbage value or unexpected value or you may get an output like this:

�

While for the second statement, you will see nothing. One more thing, if you have more statements after this, they will also give you no result.

Note: PHP language gives you the flexibility to use single and double-quotes easily.

Answer 11

Single quotes are denoting a char, double denote a string.

In Java, it is also the same.

Answer 12

Use single quote with single char as:

char ch = 'a';

here 'a' is a char constant and is equal to the ASCII value of char a.

Use double quote with strings as:

char str[] = "foo";

here "foo" is a string literal.

Its okay to use "a" but its not okay to use ' foo'

Answer 13

While I'm sure this doesn't answer what the original asker asked, in case you end up here looking for single quote in literal integers like I have...

C++14 added the ability to add single quotes ( ' ) in the middle of number literals to add some visual grouping to the numbers.

constexpr int oneBillion = 1'000'000'000;
constexpr int binary = 0b1010'0101;
constexpr int hex = 0x12'34'5678;
constexpr double pi = 3.1415926535'8979323846'2643383279'5028841971'6939937510;

Answer 14

In C & C++ single quotes is known as a character ('a') whereas double quotes is know as a string ("Hello"). The difference is that a character can store anything but only one alphabet/number etc. A string can store anything. But also remember that there is a difference between '1' and 1. If you type cout<<'1'<<endl<<1; The output would be the same, but not in this case:

cout<<int('1')<<endl<<int(1);

This time the first line would be 48. As when you convert a character to an int it converts to its ascii and the ascii for '1' is 48. Same, if you do:

string s="Hi";
s+=48; //This will add "1" to the string
s+="1"; This will also add "1" to the string

Answer 15

different way to declare a char / string

char char_simple = 'a'; // bytes 1 : -128 to 127 or 0 to 255
signed char char_signed = 'a'; // bytes 1: -128 to 127
unsigned char char_u = 'a';  // bytes 2: 0 to 255

// double quote is for string.
char string_simple[] = "myString";
char string_simple_2[] = {'m', 'S', 't', 'r', 'i', 'n', 'g'};
char string_fixed_size[8] = "myString";
char *string_pointer = "myString"; 
char string_poionter_2 = *"myString";

printf("char = %ld\n", sizeof(char_simple));
printf("char_signed = %ld\n", sizeof(char_signed));
printf("char_u = %ld\n", sizeof(char_u));

printf("string_simple[] = %ld\n", sizeof(string_simple));
printf("string_simple_2[] = %ld\n", sizeof(string_simple_2));
printf("string_fixed_size[8] = %ld\n", sizeof(string_fixed_size));
printf("*string_pointer = %ld\n", sizeof(string_pointer));
printf("string_poionter_2 = %ld\n", sizeof(string_poionter_2));