pointers and references practice

This page contains set of challenges that you can go through to test your understanding on the topic. It aditionally contains links to resources and or explanations on the topic.

This page was last updated on 10.11.2019

You can find its source on github.

Pointer is a variable that can contain a memory address.

Recommended reading: The C Programming Language", 2nd edition, Kernighan and Ritchie; Chapter 5

Useful links: cppreference.com, cplusplus.com, Stanford 106, random pointer tutorial, and more pointers.

[+] Show example code


    string* ptrToElement(vector* const pVec, int i){
        //returns address of the string in position
        // i of vector that pVec points to
        return &((*pVec)[i]);
    }
    // declaring some varibles
    int zz = 1;
    int zz2 = 343;
    string szz;

    //declare a pointer
    int* pAPointer;

    //declare and initialize a pointer
    string * pTestpointer = nullptr;

    // assign address of variable/object to the pointer
    pAPointer = &zz;
    pTestpointer = &szz;

    // reassign pointer to new pointee
    pAPointer = &zz2;

    // modify object pointer points too (pointee) by deferencing
    *pTestpointer = "test string\n";

    // print pointer address then object it references
    cout << pAPointer << " | " << *pAPointer << endl;
    cout << pTestpointer << " | " << *pTestpointer;

    /* constant pointers can only point to objects
    they were initialized to point to. like all constants
    they have to be initialized on declaration */
    int* const pConstatnPointer = &zz;

    // pointers to a constant cant be used to change
    // value they point to.
    const int* pZz;

    /* constant pointer to a constant combines the
    restrictions of a constant pointer and a pointer
    to a constant. This means that a constant pointer
    to a constant can only point to the object that it
    was initialized to point to. In addition, it can’t
    be used to change the value of the object to which
    it points*/
    const int* const pCPC = &zz2;

    // when passing pointers to function
    // we have to use address of object
    pointerExample(&zz);

    // ...which is same as using pointer
    pointerExample(pCPC);

    // returning pointers
    vector tempVec(10,"spam");
    cout << *( ptrToElement(&tempVec, 3) ) << endl;

    // Assigning a Returned Pointer to a Pointer
    string* pMagic = ptrToElement(&tempVec, 3);
    *pMagic = "magic";
    cout << tempVec[3] << endl;

    // an array name is a constant pointer to
    // the first element of the array
    string tempArray[3] = {"foo", "bar", "spam"};
    *tempArray="hehexd";
    cout << tempArray[0] << endl;

A reference is a simple reference datatype that is less powerful but safer than the pointer type inherited from C.

Useful links: cppreference.com, isocpp.org.

[+] Show example code


    string& ref2element(vector& barVec, int i){
            return barVec[i]; // returns reference
        }

    void swapNums(int& a, int& b){
        int temp = a;
        a = b;
        b = temp;
    }

    int intD = 10;

    // if we want to prevent reference modifying we use const
    const int& rIntD = intD;

    cout << rIntD << endl;

    int intE = 2; int intF = 3;
    swapNums(intE, intF);
    cout << intE << " " << intF << endl; // 3 2

    // small example of reference to reference
    vector barVec;
    barVec.push_back("foo");
    barVec.push_back("bar");

    // assignees returned reference to reference
    string& rStr = ref2element(barVec,1);
    cout << rStr << endl; // bar
    rStr = "spaz"; // rStr == barVec[1]
    cout << barVec[1] << endl; // spaz

1. Create pointer 'pNum' that points to int 'num'. Set value of num to 10 by deferencing pNum pointer. Initialize new varibale 'num2' by directly acessing 'num' throught pNum pointer. Output num2.

[+] Show solution

                            
int num;
int* pNum = &num;
*pNum = 10;
int num2 = *pNum;
std::cout << num2 << std::endl;
-------------------
output: 10

2. How would you output memory address of num from previous example? How would you output memory address of num by using pointer? How would you ouput memory address of the pointer itself?

[+] Show solution

                                
std::cout << &num << std::endl;
std::cout << pNum << std::endl;
std::cout << &pNum << std::endl;
---------------
output:
0x28ff28 - num
0x28ff28 - num over pointer
0x28ff24 - pointers address

3. What does following code output?


void coutA(int*a){
    cout << &a << " | " << a << " | " << *a << " | in pointer func" << endl;
}
    
void coutA(int&a){
    cout << &a << " | " << a << "\t\t | in reference func" << endl;
}
                    
int main() {
    int a = 42;
    int *pA = &a;
    int **pA2 = &pA;

    coutA(a);
    coutA(&a);
    coutA(pA);
    coutA(*pA);
    coutA(*pA2);
    coutA(**pA2);

    cout << endl;
    cout << &a << endl;
    cout << &pA << endl;
    cout << &pA2 << endl;

[+] Show solution

                        0x28ff28 | 42            | in reference func
                        0x28ff10 | 0x28ff28 | 42 | in pointer func
                        0x28ff10 | 0x28ff28 | 42 | in pointer func
                        0x28ff28 | 42            | in reference func
                        0x28ff10 | 0x28ff28 | 42 | in pointer func
                        0x28ff28 | 42            | in reference func
                                                        
                        42
                        0x28ff28
                        0x28ff28
                        0x28ff24
                        0x28ff20

3. Initialize int and float variables 'foo' and 'bar' both to 0. Declare a new void pointer and use it to change values of foo and bar to 3 and 3.4 respectively. Output values of foo and bar.

[+] Show solution

                    
int foo = 0;
float bar = 0;
void *pointy;

pointy = &foo;
*(int *)pointy = 3;

pointy = &bar;
*(float *)pointy = 3.4;

std::cout << foo << ", " << bar << std::endl;
--------------------------------------------
output: 3, 3.4

4. Define an array of first 5 natural numbers. Output whole array by deferencing a pointer and change 3 to 4 before outputing it.

[+] Show solution

                    
int arr[] = {1,2,3,4,5};
for(int i = 0; i<sizeof(arr)/sizeof(*arr); ++i){
   if(*(arr+i) == 3) std::cout << 4 << " ";
   else std::cout << *(arr+i) << " ";
}
------------------------------------------------
output: 1 2 4 4 5

5. What does following code output?


int a = 1;
int b = 2;
int c = 3;
int arr[] = {a,b,c};

*(arr+2) = 7;

for(int i = 0; i<sizeof(arr)/sizeof(*arr); ++i){
    std::cout << &arr[i] << " | " << arr[i] << std::endl;
    std::cout << arr + i << " | " << *(arr+i) << std::endl;
}

[+] Show solution

                    0x28ff14 | 1
                    0x28ff14 | 1
                    0x28ff18 | 2
                    0x28ff18 | 2
                    0x28ff1c | 7
                    0x28ff1c | 7

6. Define new pancake using struct bellow and assign it values 10 and 7.68 using pointers. Output pancake info using alternative notation.

struct Pancake {
    int i;
    float f;
};

[+] Show solution

                    
Pancake *ptr, pancake;
ptr = &pancake;

(*ptr).i = 10;
(*ptr).f = 7.68;

std::cout << ptr->i << " " << ptr->f;
------------------------------------
output: 10, 7.68