C结构编译错误

Question

Why does the following code produce a compile-time error? I cannot seem to see why the types are mismatched.

typedef char f_string[MAX_CHARS+1] ;    /* string for each field */

/*
 * A parsed CSV line, with the number of fields and upto MAX_FIELDS themselves.
*/

typedef struct {
    int nfields ;               /* 0 => end of file */
    f_string field[MAX_FIELDS] ;        /* array of strings for fields */
} csv_line;

....

csv_line sut;
sut.field[0] = "Name, "; //Compile-time error.

Error being:

error: incompatible types in assignment

Answer 1

You are trying to assign a const char * to a char[] , which is not quite the same thing. This would work if your f_string were defined as

typedef const char * f_string;

What you are looking for here is

strcpy ( sut.field[0], "Name, " );

Or use strncpy so that you can specify the size of the destination buffer ..

strncpy ( sut.field[0], "Name, ", MAX_CHARS )

That will keep you from overrunning your buffer.

Answer 2

You'll need to use something like:

strcpy( sut.field[0],"Name, ");

You can't assign strings like you tried other except as an initializater at declaration time.

Answer 3

the type of sut.field[0] is array-of-char of size MAX_CHARS+1 - you cannot assign a string pointer to an array of characters.

You'd either need to change the type of csv_line::field to a const char*, or just do a string copy of the literal "Name, " to the target array.

Note that both strcpy() and strncpy() alone are unsafe: the first might overflow your buffer, and the second might leave it without a NUL terminator. You must be aware of BOTH of these circumstances even if you "know" that your string in question won't ever overflow.

Use a helper function to do this safely:

char * strncopy(char *dst, const char *src, int dstsize)
{
    strncpy(dst, src, dstsize-1);
    dst[dstsize-1] = '\0';

    return dst;
}

Then:

strncopy(sut.field[0], "Name, ", sizeof sut.field[0]);

Answer 4

sut.field[0] is a char[MAX_CHARS+1]

"Name, " is a const char*

Try this:

strcpy(sut.field[0], "Name, ");

Answer 5

The type of sut.field[0] is indeed char [MAX_CHARS+1] . However, most of the other answers have the type of "Name, " wrong - it is actually of type char [7] (use sizeof "Name, " for an easy demonstration of this).

Nonetheless, you still cannot directly assign a char [7] to a char [MAX_CHARS+1] . You cannot even directly assign a char [7] to another char [7] ( initialisation is treated differently from assignment in this way).

The answer is probably just to use a coyping function - for example, if you are certain that MAX_CHARS >= 6 , then you can just use strcpy() . If you cannot be sure about the length being correct, then you can use strncat() as as truncating string copy:

sut.field[0][0] = '\0';
strncat(sut.field[0], "Name, ", MAX_CHARS);

(Note that despite the name, strncpy() is not suitable for this, and in fact is very rarely the desired function at all).

---

It is worth pointing out, however, that you can indirectly assign arrays (of the same type) if they are wrapped up inside a struct . This means that the following will work (if you have a C99 compiler):

typedef struct { char s[MAX_CHARS+1] } f_string;    /* string for each field */

csv_line sut;
sut.field[0] = (f_string){"Name, "};