I have declared two variables in following ways:
void main()
{
int a = 20;
int &b = a;
cout<<a<<" "<<b;
}
Output: 20 20
Kindly advise what exactly happens when i write int &b = a and how is it different from the first declaration and also when to use which.
Kindly advise what exactly happens when i write int &b = a and how is it different from the first declaration
In the example below:
int a;
int & b = a;
The b variable refers to the memory address of the a variable. The variable b is just an alias for a.
and also when to use which.
A common usage of references is to avoid copying large objects. Rather than passing (copying) the large object, you pass a reference to it.
const std::string& Get_Tree_Name()
{
static std::string name = "Eucalyptus";
return name;
}
The above function returns a reference to a static string inside the function. So rather than copying the string after the function exits, it is passing a reference. In general, a reference would occupy less space than the std::string object.
Another usage is when a function modifies its parameter:
void Get_Pi(double& pi_value)
{
pi_value = 3.14159264;
}
int main()
{
double my_pi = 0.0;
Get_Pi(my_pi);
}
In the main function of the above example, the code in the Get_Pi function is actually modifying the my_pi variable in the main function, because of the reference mechanism.
Without the references, the Get_Pi function would modify a copy of the my_pi variable and not the original variable, since parameters are passed by copy.
Pointers vs. references
Another alternative for a function to modify its parameter is to pass the parameter by pointer. One issue with using a pointer is that pointer can point to anything and not necessarily a variable.
Passing a reference means that the object must exist. A reference, unlike a pointer, doesn't need to be validated during run-time. The compiler performs the checking. This is important in safety critical systems.
Int a = 0;
This a simple variable of int which can store only -32768 to 32768 (2 bytes)
Int * b;
This also a variable, the * means that this vatiable can store address of some another variable not value.
b=&a;
Now this how you put value in a pointer, writing & before a variable name will give you an address of the variable instead of variable value.
*b;
This how can get value from a pointer, this will give you a value of a
In first variable you are assigning value to the variable a (a=20) In second line you create a variable and give that variable address of a. Now b is also pointing on the adress of a. Now a and b are pointing to the same memory adress which is 20.
