声明字符时char和int之间的区别

Question

I just started learning C and am rather confused over declaring characters using int and char.

I am well aware that any characters are made up of integers in the sense that the "integers" of characters are the characters' respective ASCII decimals.

That said, I learned that it's perfectly possible to declare a character using int without using the ASCII decimals. Eg. declaring variable test as a character 'X' can be written as:

char test = 'X';

and

int test = 'X';

And for both declaration of character, the conversion characters are %c (even though test is defined as int ).

Therefore, my question is/are the difference(s) between declaring character variables using char and int and when to use int to declare a character variable?

Answer 1

The difference is the size in byte of the variable, and from there the different values the variable can hold.

A char is required to accept all values between 0 and 127 (included). So in common environments it occupies exactly one byte (8 bits). It is unspecified by the standard whether it is signed (-128 - 127) or unsigned (0 - 255).

An int is required to be at least a 16 bits signed word, and to accept all values between -32767 and 32767. That means that an int can accept all values from a char, be the latter signed or unsigned.

If you want to store only characters in a variable, you should declare it as char . Using an int would just waste memory, and could mislead a future reader. One common exception to that rule is when you want to process a wider value for special conditions. For example the function fgetc from the standard library is declared as returning int :

int fgetc(FILE *fd);

because the special value EOF (for End Of File) is defined as the int value -1 (all bits to one in a 2-complement system) that means more than the size of a char. That way no char (only 8 bits on a common system) can be equal to the EOF constant. If the function was declared to return a simple char , nothing could distinguish the EOF value from the (valid) char 0xFF.

That's the reason why the following code is bad and should never be used:

char c;    // a terrible memory saving...
...
while ((c = fgetc(stdin)) != EOF) {   // NEVER WRITE THAT!!!
    ...
}

Inside the loop, a char would be enough, but for the test not to succeed when reading character 0xFF, the variable needs to be an int.

Answer 2

The char type has multiple roles.

The first is that it is simply part of the chain of integer types, char , short , int , long , etc., so it's just another container for numbers.

The second is that its underlying storage is the smallest unit, and all other objects have a size that is a multiple of the size of char ( sizeof returns a number that is in units of char , so sizeof char == 1 ).

The third is that it plays the role of a character in a string, certainly historically. When seen like this, the value of a char maps to a specified character, for instance via the ASCII encoding, but it can also be used with multi-byte encodings (one or more char s together map to one character).

Answer 3

Usually you should declare characters as char and use int for integers being capable of holding bigger values. On most systems a char occupies a byte which is 8 bits. Depending on your system this char might be signed or unsigned by default, as such it will be able to hold values between 0-255 or -128-127.

An int might be 32 bits long, but if you really want exactly 32 bits for your integer you should declare it as int32_t or uint32_t instead.

Answer 4

在大多数体系结构中， int大小为4个字节，而char的大小为1个字节。

Answer 5

I think there's no difference, but you're allocating extra memory you're not going to use. You can also do const long a = 1; , but it will be more suitable to use const char a = 1; instead.