Warning: C++ Is Not Java
As you saw in the previous example, references give us Java-like functionality.
Importantly, however, they are not the same as Java references!
A regular variable specifies a particular location in memory. The location holds a value and that value can change over time, but the location itself is fixed for the life of the variable.
Likewise, a reference refers to a specific location in memory. The location holds a value and that value can change over time, but the location itself is fixed for the life of the reference.
So, while Java references
- Can be changed to point to different objects over time, and
- Can be
null(not point to anything).
C++ references
- Always refer to the same location for their entire lifetime, and
- Always refer to a real location in memory.
If you think you're changing a C++ reference, you're actually changing the value at the location it refers to. This is exactly the same as what happens when you change a regular variable, you don't change where in memory it is, you change the value at the location it refers to.
Hay, wait, so you're making out like references and regular variables are alike because both refer to a location in memory and can't move to refer to a different location, but you're saying that references are also different from regular variables. What gives?
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The difference is that a regular variable has storage that is allocated, then initialized, then used, then destroyed, then deallocated. In contrast, a reference just piggybacks on the storage (and initialization) of something else. It's not allocated, initialized, destroyed, or deallocated in its own right. It's just a name for something that already exists. An alias.
Okay, so a reference is like a nickname? But doesn't it require some memory to store the nickname?
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Actually, no. The nickname is just a name for something that already exists. It doesn't need any memory of its own. It's just a name for something that already exists.
In essence, yes. References do not consume any memory that you are allowed to access.
In practice, behind the scenes, sometimes the system may need to use some memory to make references work, but it's an under-the-hood how-C++-works-inside detail.
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Shhhhh... we don't need to worry about those kinds of details—even the C++ standard doesn't specify how references should be implemented. So we won't draw any extra memory for any that under-the-hood data in our diagram because we can't actually access or interact with it.
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