Unit 28: Structures
Learning Objectives
After this unit, students should
- be aware
struct
as a compound data type in C - know how to (i) declare a struct, (ii) assign values to the member of a struct, (iii) pass a struct into a function as value and as reference, (iv) return a struct from a function
- aware of
.
and->
notations for accessing members of a struct - know how to use
typedef
to define a new type
Compound Data Type
We have so far been working with numbers, characters, and strings. Not all real-world objects can be easily abstracted and represented with numbers and characters. It is useful to create our compound data type that represents real-world objects. Each object typically has one or more attributes, which can be of different types: A module has a code, a title, and the number of MCs; A person has a name, height, weight, and age; A phone has a model, price, and brand.
If you look back at the code that we have written, often multiple variables are related to each other and "belong together." When we pass one as an argument into a function, often we need to pass another. It is useful to group them into a compound data type as well. For instance: A 1D array and its length; A 2D array, its width, and its height; A pixel, its row, its column, and its color.
Structure in C
In C, we can define a compound data type using a structure, through the C keyword struct
. The syntax looks like this:
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In the definition above, the structure is given a name, matrix
. The structure contains three members. The first is a pointer to a 2D array, the other two are the number of rows and number of columns of that array.
Note that we need a semicolon ;
after the definition of a struct
.
Here are a few more examples:
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Declaring and Initializing a Structure Variable
Let's see an example of how we can declare and initialize a structure variable:
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Line 1 above declares a variable called cs1010
. Lines 2-4 initialize each member of the structure. Note that we use .
to access each member.
An alternative is to use a compound literal:
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Using compound literal is convenient in certain cases, as uninitialized members are set to 0 (similar to initializers of arrays).
You can read and write to individual members of a structure variable just like any other variable.
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Structure as Parameters
We can pass a structure variable into a function just like a non-array variable. Unlike an array, a struct
is called by value, i.e., it is copied onto the call stack of the function.
Hence, the code below does not actually update the MCs of CS1010:
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To call a structure by reference, we can pass in its pointer.
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The latter example is more common idiom. Since a structure can contain multiple fields and usually occupies more bytes than a pointer, it is less efficient to copy a structure onto the call stack compared to copying its pointer.
Since this is common, C provides another syntax for accessing the member of a structure through its pointer, using the "arrow" notation"
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Returning a Structure
A function can return a structure. Remember in Unit 17 we said that C functions can return only one value and one way to get around this limitation is to use call by reference and the other is to use struct
? Here is how we use struct
to return more than one values:
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When a function returns a struct
, the structure gets copied back to the caller.
Defining a Structure as a Type
To avoid writing the keyword struct
every time we declare a variable or parameter, let's introduce another keyword in C: typedef
.
C allows programmers to define their type based on the existing types. Suppose that I want a type that represents a color component (red, green, or blue) of a pixel. Each pixel can have a value between 0 to 255. So we can represent it with an unsigned char
. I can define:
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Recall that we use the suffix _t
convention to denote user-defined type. You have probably seen these two types elsewhere size_t
and time_t
in the past.
Now that we have defined color_t
as a new type that is equivalent to unsigned char
, we can use it just like another type:
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Using typedef
on struct
frees us from typing the word struct
every time. We can do so with either:
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or
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In either case, we can just use module
like any other type:
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If you use third-party libraries or C libraries, chances are you will come across such type. The use of typedef
on struct
is controversial. There is a school of thoughts that think it makes the code harder to read as it obscured the fact that a variable is a struct. Hidden costs in copying the variable onto the call stack as a parameter or returned value become non-obvious. Interested students can read Linux's Kernel Coding Style for the pros and cons of this approach.