C++ References &

To declare an lvalue reference type, we use an ampersand (&) in the type declaration:

int      // a normal int type
int&     // an lvalue reference to an int object
double&  // an lvalue reference to a double object

For those of you already familiar with pointers, the ampersand in this context does not mean “address of”, it means “lvalue reference to”.

Lvalue reference variables

an lvalue reference variable is a variable that acts as a reference to an lvalue(usually another variable).

int main()
{
    int x = 5;    // x is a normal integer variable
    int& ref = x ; // ref is an lvalue reference variable that can now be used as an alias for variable x

    cout << x << '\n';  // print the value of x (5)
    cout << ref << '\n'; // print the value of x via ref (5)
}

note

From the compiler's perspective, it doesn't matter whether the ampersand is “attached” to the type name (int& ref) or the variable's name (int &ref), and which you choose is a matter of style. Modern C++ programmers tend to prefer attaching the ampersand to the type, as it makes clearer that the reference is part of the type information, not the identifier.

When defining a reference, place the ampersand next to the type (not the reference variable’s name). ✅☑️ int& ref , ❌❎ int &ref

tip

Favor passing by const reference int& xover passing by non-const reference const int& x unless you have a specific reason to do otherwise (e.g. the function needs to change the value of an argument).

Because class types such as string, vector, deque etc can be expensive to copy (sometimes significantly so), class types are usually passed by const reference if that particular variable need not be modified.