Bits, bytes, and memory addressing
In lesson 1.3 -- Introduction to variables , we talked about the fact that variables are names for a piece of memory that can be used to store information. To recap briefly, computers have random access memory (RAM) that is available for programs to use. When a variable is defined, a piece of that memory is set aside for that variable.
The smallest unit of memory is a binary digit (also called a bit), which can hold a value of 0 or 1. You can think of a bit as being like a traditional light switch -- either the light is off (0), or it is on (1). There is no in-between. If you were to look at a random segment of memory, all you would see is …011010100101010… or some combination thereof.
Memory is organized into sequential units called memory addresses (or addresses for short). Similar to how a street address can be used to find a given house on a street, the memory address allows us to find and access the contents of memory at a particular location.
Perhaps surprisingly, in modern computer architectures, each bit does not get its own unique memory address. This is because the number of memory addresses are limited, and the need to access data bit-by-bit is rare. Instead, each memory address holds 1 byte of data. A byte is a group of bits that are operated on as a unit. The modern standard is that a byte is comprised of 8 sequential bits.
In C++, we typically work with “byte-sized” chunks of data.
The following picture shows some sequential memory addresses, along with the corresponding byte of data:
Some older or non-standard machines may have bytes of a different size (from 1 to 48 bits) -- however, we generally need not worry about these, as the modern de-facto standard is that a byte is 8 bits. For these tutorials, we’ll assume a byte is 8 bits.
Because all data on a computer is just a sequence of bits, we use a data type (often called a “type” for short) to tell the compiler how to interpret the contents of memory in some meaningful way. You have already seen one example of a data type: the integer. When we declare a variable as an integer, we are telling the compiler “the piece of memory that this variable uses is going to be interpreted as a non-fractional number”.
When you assign a value to an object, the compiler and CPU take care of encoding your value into the appropriate sequence of bits for that data type. When you ask for your value back, your number is “reconstituted” from the sequence of bits in memory.
As an example, when you give an integer object the value 65, that value is stored in memory as the sequence of bits
0100 0001. When you need that value, those bits are reconstituted back into the integer value 65.
Fortunately, the compiler and CPU take care of converting values into bit sequences (and reconstructing those values from bit sequences again when needed), so you don’t need to worry about that part. All you need to do is pick a data type for your object that best matches your desired use.
Fundamental data types
C++ comes with built-in support for many different data types. These are called fundamental data types, but are often informally called basic types, primitive types, or built-in types.
Here is a list of the fundamental data types, some of which you have already seen:
|Floating Point||a number with a fractional part||3.14159|
|bool||Integral (Boolean)||true or false||true|
|Integral (Character)||a single character of text||‘c’|
long long (C++11)
|Integral (Integer)||positive and negative whole numbers, including 0||64|
|std::nullptr_t (C++11)||Null Pointer||a null pointer||nullptr|
This chapter is dedicated to exploring these fundamental data types in detail (except std::nullptr_t, which we’ll discuss when we talk about pointers). C++ also supports a number of other more complex types, called compound types. We’ll explore compound types in a future chapter.
The terms “integer” and “integral” are similar, but have different meanings. Integers are positive and negative whole numbers, including 0. The term “integral types” (which means “like an integer”) includes all of the boolean, characters, and integer types (and thus is a bit broader in definition). Integral types are named so because they are stored in memory as integers, even though they behave slightly differently.
The _t suffix
Many of the types defined in newer versions of C++ (e.g. std::nullptr_t) use a _t suffix. This suffix means “type”, and it’s a common nomenclature applied to modern types.
If you see something with a _t suffix, it’s probably a type. But many types don’t have a _t suffix, so this isn’t consistently applied.
|4.2 -- Void |
|3.x -- Chapter 3 summary and quiz |