C中“3”和“3”之间的差异

Question

I tried to run the following program in C and got some output. Can you help me out why???

#include<stdio.h>

int main()
{
  char x='A';
  printf("%d%d%d",sizeof("3"),sizeof('3'),sizeof(3));
  return 0;
}

The output received is 2 4 4 using gcc in ubuntu 11.04 32 bit.

Similarly in other program:-

#include<stdio.h>

int main()
{
  char x='A';
  printf("%d%d",sizeof('A'),sizeof(x));
  return 0;
}

The output received is 4 1 using GCC in ubuntu 11.04 32 bit.

Can you help me out why the output is this way???

Answer 1

In C, char literals are of integer type, so sizeof('3') == sizeof(int) . See this C FAQ for details.

This is one of the areas where C and C++ differ (in C++ sizeof('3') is 1 ).

Actually, correcting my previous assertion, sizeof("3") will yield 2 because "3" is treated as a 2-element character array.

6.3.2.1/3

Except when it is the operand of the sizeof operator or the unary & operator, or is a string literal used to initialize an array, an expression that has type ''array of type'' is converted to an expression with type ''pointer to type'' that points to the initial element of the array object and is not an lvalue.

Answer 2

In C

3 : integer literal, type int
'3' : char literal, type int
"3" : string literal: type char[2]

In your second example, x denotes an object of type char .

Answer 3

"C" is

char mystring[2];
mystring[0] = 'C';
mystring[1] = '\0';

While 'C' is

int mychar;
mychar = 'C';

Answer 4

The '3' is converted to type int, which is 4 bytes. However "3" is a string with two bytes. The first byte is the char 3 and the second is the null terminator that gets appended to all strings.

Answer 5

'3' is a character constant. In C, character constants have type int (in C++, they have type char ). Thus, sizeof '3' == sizeof (int) .

"3" is a string literal. It is a 2-element array of char ( const char in C++) with the values {'3', '\\0'} . sizeof (char) == 1 by definition, thus, sizeof "3" == 2.

3 is an integer constant. It has type int . Thus, sizeof 3 == sizeof (int) .

The variable x is declared as char , thus sizeof x == 1.