Author: Tianle Yuan

Pointer and Reference⚓︎

This material is focusing on describing the difference when passing parameters

Definition difference⚓︎

Definition diff?

DefinitionVariabilityConstNULLsizeof++SafetyLayers

A pointer is a variable, except that the variable stores an address that points to a storage unit in memory.
A reference is essentially the same thing as the original variable, just an alias for the original variable

The value of the pointer is changeable after initialization
A reference can only be initialized once in a definition. Then it cannot be changed.

Const pointer: int* const a
No const reference: no int& const a
We can have a const referred value： const int& a

Pointer's value can be NULL.
Reference's value cannot be NULL. References must be initialized when they are defined

Pointer: the size of pointer.
Reference': the size of referred variable.

C++

int a[2] = {0,10}
int *pa =a；pa++   //now pa == a[1] == 10;
int &pa =a；pa++   //now pa == a[0] == 1;

Pointer is not type safe
Reference is type safe since it has type checking

Pointer can have several layers such as int** p
Reference can not have several layer such as int&& p

Parameter passing diff⚓︎

Pass by value⚓︎

What is pass by value?

When calling function passes parameter in called function, stack will open up a new space for storing a memory copy of the value of actual parameter from the calling function.
Any operation in the called function on a formal parameter is performed as a local variable since you are using the copied value. It does not affect the value of the actual parameter of the main calling function.

value.cpp

#include<iostream>
using namespace std;

void Rfun(int p) // Called function: copy the value from "a"
{
    cout<<"local p's address:"<<&p<<endl;         //0x6ffdf0
    cout<<"local p's value:"<<p<<endl;            //16
    p=0Xff;
}

int main()
{
    int a=0x10;
    cout<<"a's address:"<<&a<<endl;               //0x6ffe1c
    cout<<"a's value:"<<a<<endl;                  //16
    Rfun(a);     // Calling function: passing a's value
    cout<<"a's now address:"<<a<<endl;            //16
}

>>>
a's address:0x7ffd8a777d04
a's value:16
local p's address:0x7ffd8a777cec
local p's value:16
a's now address:16

Pointer passing⚓︎

Pointer passing

Pointer parameter passing is essentially Pass by value. It passes an address as value.

pointer.cpp

#include<iostream>
using namespace std;

void Pfun(int* p) // Called function: copy the address value from "a"
{
    //new address for storing address value passed by "a"
    cout<<"local p's address:"<<&p<<endl;         //0x6ffdf0
    cout<<"local p's value:"<<p<<endl;            //0x6ffe1c

    //test if p points to a's value
    cout<<"local p's pointing value:"<<*p<<endl;  //16
    *p=0Xff;

    //test if p will affect "a"'s address value
    p++;
    cout<<"local p's updated value:"<<p<<endl;    //0x6ffe1d
}

int main() 
{
    int a=0x10;
    cout<<"a's address:"<<&a<<endl;               //0X6ffe1c
    cout<<"a's value:"<<a<<endl;                  //16
    Pfun(&a);   // Calling function: passing a's address
    cout<<"a's now address:"<<&a<<endl;           //0X6ffe1c
    cout<<"a's now value:"<<a<<endl;              //255
}

<<<
a's address:0x7ffd233a9854
a's value:16
local p's address:0x7ffd233a9838
local p's value:0x7ffd233a9854
local p's pointing value:16
local p's updated value:0x7ffd233a9858
a's now address:0x7ffd233a9854
a's now value:255

Reference passing⚓︎

Reference passing

When calling function passes parameter in called function, stack will open up a new space for storing a memory copy of the address of actual parameter from the calling function.
Any operation in the called function on a formal parameter will affect the value of the actual parameter since you are tracking and editing the content in the same address.

reference.cpp

#include<iostream>
using namespace std;

void Rfun(int &p)
{
    cout<<"local p's address:"<<&p<<endl;         //0x6ffdf0
    cout<<"local p's value:"<<p<<endl;            //16
    p=0Xff;
}

int main()
{
    int a=0x10;
    cout<<"a's address:"<<&a<<endl;               //0x6ffe1c
    cout<<"a's value:"<<a<<endl;                  //16
    Rfun(a);
    cout<<"a's now address:"<<a<<endl;            //255
}

>>>
a's address:0x7ffebbcabd44
a's value:16
local p's address:0x7ffebbcabd44
local p's value:16
a's now address:255

Reference⚓︎

Picture source 1