不了解++和-的指针算法

Question

So, I am learning about pointers via http://cplusplus.com/doc/tutorial/pointers/ and I do not understand anything about the pointer arithmetic section. Could someone clear things up or point me to a tutorial about this that I may better understand.

I am especially confused with all the parentheses things like the difference between *p++ , (*p)++ , *(p++) , and etc.

Answer 1

*p++

For this one, ++ has higher precedence then * so it increments the pointer by one but retrieves the value at the original location since post-increment returns the pointer and then increments its value.

(*p)++

This forces the precedence in the other direction, so the pointer is de-referenced first and then the value at that location in incremented by one (but the value at the original pointer location is returned).

*(p++)

This one increments the pointer first so it acts the same as the first one.

An important thing to note, is that the amount the pointer is incremented is affected by the pointer type. From the link you provided:

char *mychar;
short *myshort;
long *mylong;

char is one byte in length so the ++ increases the pointer by 1 (since pointers point to the beginning of each byte).

short is two bytes in length so the ++ increases the pointer by 2 in order to point at the start of the next short rather than the start of the next byte.

long is four bytes in the length so the ++ increases the pointer by 4.

Answer 2

I found useful some years ago an explanation of strcpy, from Kernighan/Ritchie (I don't have the text available now, hope the code it's accurate): cpy_0, cpy_1, cpy_2 are all equivalent to strcpy:

char *cpy_0(char *t, const char *s)
{
    int i = 0;
    for ( ; t[i]; i++)
        t[i] = s[i];
    t[i] = s[i];
    i++;
    return t + i;
}
char *cpy_1(char *t, const char *s)
{
    for ( ; *s; ++s, ++t)
        *t = *s;
    *t = *s;
    ++t;
    return t;
}
char *cpy_2(char *t, const char *s)
{
    while (*t++ = *s++)
        ;
    return t;
}

Answer 3

*p++

Returns the content, *p , an then increases the pointer's value (postincrement). For example:

int numbers[2];
int *p;
p = &numbers[0];
*p = 4;        //numbers[0] = 4;
*(p + 1) = 8;  //numbers[1] = 8;
int a = *p++;  //a = 4 (the increment takes place after the evaluation)
               //*++p would have returned 8 (a = 8)
int b = *p;    //b = 8 (p is now pointing to the next integer, not the initial one)

And about:

(*p)++

It increases the value of the content, *p = *p + 1; .

(p++); //same as p++

Increases the pointer so it points to the next element (that may not exist) of the size defined when you declared the pointer.

Answer 4

First you have to understand what post increment does;
The post increment, increases the variable by one BUT the expression (p++) returns the original value of the variable to be used in the rest of the expression.

char   data[] = "AX12";
char* p;

p = data;
char* a = p++;

// a -> 'A'  (the original value of p was returned from p++ and assigned to a)
// p -> 'X'

p = data;   // reset;
char  l =  *(p++);

// l =  'A'. The (p++) increments the value of p. But returns the original
             value to be used in the remaining expression. Thus it is the
             original value that gets de-referenced by * so makeing l 'A'
// p -> 'X'

Now because of operator precedence:

*p++ is equivalent to *(p++)

Finally we have the complicated one:

p = data;
char m = (*p)++;

// m is 'A'. First we deference 'p' which gives us a reference to 'A'
//           Then we apply the post increment which applies to the value 'A' and makes it a 'B'
//           But we return the original value ('A') to be used in assignment to 'm'

// Note 1:   The increment was done on the original array
//           data[]  is now "BXYZ";
// Note 2:   Because it was the value that was post incremented p is unchaged.
// p -> 'B' (Not 'X')