What does passing by reference in C++ allow a function to do?

Passing by reference in C++ allows a function to access the original variable's memory address directly. This means that when a variable is passed by reference to a function, the function operates on the original variable itself rather than a copy of its value. As a result, any changes made to the variable within the function affect the original variable outside of the function. This approach is efficient because it avoids the overhead of copying large amounts of data and allows for direct manipulation of the original data.

In contrast, copying the value of a variable creates a separate instance in memory specific to that function call, which does not allow for modifications to the original variable, nor does it provide access to the original variable's memory address. Therefore, the ability to access the memory address is what sets passing by reference apart in C++. Additionally, while passing by reference may facilitate certain functions to return multiple values through output parameters, it primarily enables direct manipulation of the original variable’s data.