References

A reference is an alias to an object. That is, it is another name for the object, and so access to the object is the same through a reference as it is through the object's variable name. A reference is declared using a & symbol on the reference name and it is initialized and accessed in exactly the same way as a variable:

    int i = 42; 
    int *pi = &i;  // pointer to an integer 
    int& ri1 = i;  // reference to a variable 
    i = 99;        // change the integer thru the variable 
    *pi = 101;     // change the integer thru the pointer 
    ri1 = -1;      // change the integer thru the reference 
    int& ri2 {i};  // another reference to the variable 
    int j = 1000; 
    pi = &j;       // point to another integer

In this code, a variable is declared and initialized, then a pointer is initialized to point to this data, and a reference is initialized as an alias for the variable. Reference ri1 is initialized with an assignment operator, whereas reference ri2 is initialized using initializer list syntax.

Wherever the variable is used, the reference can be used; whatever you do to the reference is actually the same as performing the same operation on the variable. A pointer points to data, so you can change the data by dereferencing the pointer, but equally so, you can make the pointer point to any data and change that data by dereferencing the pointer (this is illustrated in the last two lines of the preceding code). You can have several aliases for a variable, and each must be initialized to the variable at the declaration. Once declared, you cannot make a reference refer to a different object.

The following code will not compile:

    int& r1;           // error, must refer to a variable 
    int& r2 = nullptr; // error, must refer to a variable

Since a reference is an alias for another variable, it cannot exist without being initialized to a variable. Likewise, you cannot initialize it to anything other than a variable name, so there is no concept of a null reference.

Once initialized, a reference is only ever an alias to the one variable. Indeed, when you use a reference as an operand to any operator, the operation is performed on the variable:

    int x = 1, y = 2;  
    int& rx = x; // declaration, means rx is an alias for x 
    rx = y;      // assignment, changes value of x to the value of y

In this code, rx is an alias to the variable x, so the assignment in the last line simply assigns x with the value of y: the assignment is performed on the aliased variable. Further, if you take the address of a reference, you are returned the address of the variable it references. While you can have a reference to an array, you cannot have an array of references.