Insight 10.1.1.
When trying to pronounce complex looking data types in C++ it sometimes helps to read the backwards.
int& b; can be read as “b is a reference to an integer”.
Section 10.1 References
In the string chapter we mentioned that the a function that takes a string parameter probably should be written as:
void foo(const string& s);
Instead of:
void foo(string s);
So why is the first version preferable? To answer that question, we will start by reviewing how normal variables work. Each regular variable is a name for a different location in memory (a different box to store things in). Given this code:
int a = 5;
int b = a;
There are two storage locations, each with its own name. The second line of main make a new storage area with the name
b and copies the value a has to it. When reading that code, you should picture a memory diagram that looks like this:
a and b that each hold 5.Changing the value in the storage named by
b does not change the value stored in a. This is how things usually work in C++.
A reference changes this. A reference does not name its own storage, instead a reference always “points” to some other storage. We declare a variable to be a reference by adding
& to the data type, like int& b;. The & change the type of `b —it no longer is an “int”, it is a “reference to an int”.
A memory diagram for
int& b = a; should look like:
a that holds 5 and b that is a reference to a.b is an alias for a - it is another way to name the exact same storage that a names. If we change b, we change that value. Run this Codelens sample to see its animation of the situation:
Note 10.1.2.
The formal name in C++ for variables that name their own storage is an lvalue (location). A reference is an rvalue (reference). It is also possible to have an rvalue reference using the symbol
&&.
Those terms and the concept of an rvalue reference are beyond the scope of this book.
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