int a = 2;
    int b = 3;
    int *const p1 = &a;//指針常量：被定義的指針變量就變成了一個(gè)指針類型的常變量，指針不可以變，指針指向的內(nèi)容可以變，需要在定義的時(shí)候給定初值
    int const *p2 = &b;//常量指針：指向常量的指針變量，指針可以變，指針指向的內(nèi)容不可以變，等價(jià)于const int *p2 = &b;
    int const *const p3 = &a;//常指針常量: 指針不能改變，指針指向的值也不能改變，等價(jià)于const int *const p3 = &a;
    
    // Error: Cannot assign to variable 'p3' with const-qualified type 'const int *const'
    // p3 = &a;
    
    // Error: Read-only variable is not assignable
    //*p3 = a;
    
    NSLog(@"%d,%d,%d,%d", a, b, *p1, *p2);
    NSLog(@"%p,%p,%p,%p", &a, &b, p1, p2);
    NSLog(@"p3: %d, %p", *p3, p3);
    
    // Error: Cannot assign to variable 'p1' with const-qualified type 'int *const'
    // p1 = &b;
    *p1 = b;
    
    // Error: Read-only variable is not assignable
    // *p2 = a;
    p2 = &a;
    
    NSLog(@"%d,%d,%d,%d", a, b, *p1, *p2);
    NSLog(@"%p,%p,%p,%p", &a, &b, p1, p2);
    NSLog(@"p3: %d, %p", *p3, p3);
    
    /*
     Log:
     2018-07-20 10:55:13.483435+0800 Test[23866:178166] 2,3,2,3
     2018-07-20 10:55:13.483684+0800 Test[23866:178166] 0x7ffeee4ada9c,0x7ffeee4ada98,0x7ffeee4ada9c,0x7ffeee4ada98
     2018-07-20 10:55:13.483834+0800 Test[23866:178166] p3: 2, 0x7ffeee4ada9c
     2018-07-20 10:55:13.484039+0800 Test[23866:178166] 3,3,3,3
     2018-07-20 10:55:13.484224+0800 Test[23866:178166] 0x7ffeee4ada9c,0x7ffeee4ada98,0x7ffeee4ada9c,0x7ffeee4ada9c
     2018-07-20 10:55:13.484377+0800 Test[23866:178166] p3: 3, 0x7ffeee4ada9c
     */

參考：
C++ 指針常量、常量指針和常指針常量