pointers_note.cpp

#include<iostream> 
#include<cstdio> 

struct a{ 
    int val; 
}; 

int main(){ 

    /* Pointer's calculation: 
     *
     *  指標的運算是記憶體位址的位移, 根據"指向的型態"大小位移量而有所不同, 
     *  int為4 bytes, char為1 byte, pointer為8 bytes. 
     *
     */ 

    // a1, a2, a3皆指向struct, 這裡為4bytes  
    a *a1 = new a; 
    a1->val = 5; 
    a *a2 = new a; 
    a2->val = 3; 
    a *a3 = new a; 
    a3->val = 4; 
    
    // arr存放指向整數的pointer
    a *arr[3] = { a1, a2, a3 }; 
    // pa指向pointer 
    a **pa = arr; 

    std::cout << "Using arr:\n"; 
    std::printf( "1st val: %d\n", arr[0]->val );
    std::printf( "1st pos: %d\n", arr[0] ); 
    std::printf( "2nd val: %d\n", arr[1]->val ); 
    std::printf( "2nd pos: %d\n", arr[1] ); 

    std::cout << std::endl << std::endl; 

    std::cout << "Using pa:\n"; 
    std::printf( "1st val: %d\n", (*pa)->val ); 
    std::printf( "1st pos: %p\n", (*pa) ); // pa的內容物為指向struct的pointer a1的值, 此值是指向heap的一個位址 
    std::printf( "2nd val: %d\n", (*(pa+1))->val ); // 指向arr2的內容物 
    std::printf( "2nd pos: %p\n", *(pa+1) ); // *(pa+1)表示pa位移其所指向的型態arr, 因此位移一個arr的大小後取出內容物a2的值 
    std::printf( "2nd pos: %p\n", *(pa)+1 ); // *(pa)+1表示取出pa指向的位址arr[0]之內容物a1, a1是指向struct型態, 並未一個向右位移4 bytes. 


    std::cout << "size of (pa) = " << sizeof( pa ) << "\n"; 
    std::cout << "size of *(pa) = " << sizeof( *(pa) ) << "\n\n"; 

    std::cout << std::endl << std::endl; 

    a *pa1 = *(pa); // pa1為pa指向arr位址的內容物a1 
    std::cout << "Using pa1:\n"; 
    std::printf( "1st val: %d\n", pa1->val ); 
    std::printf( "1st pos: %p\n", pa1 ); // 實際上就是指向a1
    std::printf( "2nd val: %d\n", (pa1+1)->val ); 
    std::printf( "2nd pos: %p\n", pa1+1 ); // a1巷右邊移動一個struct大小的位址, 

    std::cout << "size of pa1 = " << sizeof( pa1 ) << "\n\n"; 

    // any type of pointer has size of 8 bytes. 
    std::cout << sizeof( a1 ) << std::endl;  // 8
    std::cout << sizeof( arr ) << std::endl;  // 8 * 3 = 24
    std::cout << sizeof( *(arr+1) ) << std::endl; // 8 
    std::cout << sizeof( pa ) << std::endl; // 8 
    std::cout << sizeof( pa1 ) << std::endl; // 8 
    std::cout << sizeof( a ) << std::endl; // 4 
    std::cout << sizeof( int * ) << std::endl; // 8 
    std::cout << sizeof( int ) << std::endl; // 4 

    std::cout << std::endl << std::endl; 

    delete a1, a2, a3; 

    int b1 = 1, b2 = 2, b3 = 3; 
    int *barr[3] = { &b1, &b2, &b3 }; 
    int **pbarr = barr; 
    int *ppbarr = *pbarr; 
    std::cout << sizeof( b1 ) << std::endl;  // 4 
    std::cout << sizeof( barr ) << std::endl;  // 24 
    std::printf( "&b1 = %d\n", &b1 ); 
    std::printf( "barr[0] = %d\n", barr[0] ); 
    std::printf( "*pbarr = %d\n", *pbarr ); 
    std::printf( "*(pbarr+1) = %d\n", *(pbarr+1) ); 
    std::printf( "barr[1] = %d\n", barr[1] ); 
    std::printf( "(*pbarr)+1 = %d\n", (*pbarr)+1 ); 
    std::printf( "*(ppbarr+1) = %d\n", *(ppbarr+1) ); 
    std::printf( "ppbarr+1 = %d\n", ppbarr+1 ); 
    std::printf( "(pbarr) = %d\n", (pbarr) ); 
    std::printf( "(pbarr+1) = %d\n", (pbarr+1) ); // 移動8個bytes 


    return 0; 
}

/* 
    Using arr:
    1st val: 5
    1st pos: 0x559bbf0bee70
    2nd val: 3
    2nd pos: 0x559bbf0bee90
    Using pa:
    1st val: 5
    1st pos: 0x559bbf0bee70
    2nd val: 3
    2nd pos: 0x559bbf0bee90
    size of (pa) = 8
    size of *(pa) = 8
    Using pa1:
    1st val: 5
    1st pos: 0x559bbf0bee70
    2nd val: 0
    2nd pos: 0x559bbf0bee74
    size of pa1 = 8
*/