On the other hand, many times, you don't care so much about the underlying representation of data--is it a float or an int or a double--as much as you care about how it will be used. For instance, you might have some variables that always store information about the user's current score in a game or how many points the player can get from performing an action like killing an enemy. You might also have some variables that are intended to keep track of the amount of life left for the player and how many life points he can receive for picking up life-enhancers.
These two concepts are totally different, and there's no time when you'd ever want to store one type of data--for instance, the number of points--in a variable that is intended for the other type--the amount of life points a player has or can gain.
You know from the start that these two concepts are totally different, even if you store both of them as integers. By thinking of the two types of data as different "data types", you give yourself a useful way to add an element of defensive programming to your code by making it clear what type each variable is associated with--for example, you might prefix all variables storing information about life with an l, and about scores with an s.
You'd never expect an assignment from lhigh to shigh--the two concepts are fundamentally incompatible. One way of actually enforcing this constraint is to actually create "wrapper" classes for the different types. This allows your compiler to spot incorrect assignments and prevent you from making them. In C, tags are distinct from all other names for functions, types, variables, and enumeration constants. C compilers maintain tags in a symbol table that's conceptually if not physically separate from the table that holds all other names.
Thus, it is possible for a C program to have both a tag and an another name with the same spelling in the same scope.
For example,. It may not be good practice, but C compilers must accept it. I have never seen a rationale for why C was designed this way. I have always thought it was a mistake, but there it is. Many programmers including yours truly prefer to think of struct names as type names, so they define an alias for the tag using a typedef. For example, defining. A program cannot use S as the name of both a type and a variable or function or enumeration constant :.
The tag name in a struct, union, or enum definition is optional. Many programmers fold the struct definition into the typedef and dispense with the tag altogether, as in:. Using a typedef avoids having to write struct every time you declare a variable of that type:. Notice the typo in EnumDef in the struct Enu u mDef? This compiles without error or warning and is depending on the literal interpretation of the C Standard correct.
The problem is that I just created an new empty enumeration definition within my struct. I am not as intended using the previous definition EnumDef. With a typdef similar kind of typos would have resulted in a compiler errors for using an unknown type:. Linux kernel coding style Chapter 5 gives great pros and cons mostly cons of using typedef. Lots of people think that typedefs "help readability".
Not so. They are useful only for:. Opaqueness and "accessor functions" are not good in themselves. Again - there needs to be a reason for this. If something is "unsigned long", then there's no reason to do. When editing existing code which already uses one or the other set of types, you should conform to the existing choices in that code.
In certain structures which are visible to userspace, we cannot require C99 types and cannot use the 'u32' form above. In general, a pointer, or a struct that has elements that can reasonably be directly accessed should never be a typedef. It turns out that there are pros and cons. A useful source of information is the seminal book "Expert C Programming" Chapter 3. Briefly, in C you have multiple namespaces: tags, types, member names and identifiers. One Tag in the tag namespace and one Type in the type namespace.
In addition, in a declaration like this:. In the above-mentioned book, it mentions that typedefing structs are not very useful as it only saves the programmer from writing the word struct. However, I have an objection, like many other C programmers. Although it sometimes turns to obfuscate some names that's why it is not advisable in large code bases like the kernel when you want to implement polymorphism in C it helps a lot look here for details.
So you can access an outer member flags by the inner struct MyPipe through casting. In these cases brief referencing is a big deal especially if you heavily employ the technique in your code.
Finally, the last aspect with typedef ed types is the inability to extend them, in contrast to macros. If for example, you have:. We do not really care for this for structs because it does not apply to storage specifiers volatile and const. I don't think forward declarations are even possible with typedef.
Use of struct, enum, and union allow for forwarding declarations when dependencies knows about is bidirectional. The typedef helps keep the naming straight. Not so in the C programming language. The use of typedef most often serves no purpose but to obfuscate the data structure usage. Is that data type a union or a struct? Using the straightforward non-typdefed declaration lets you know right away what type it is.
Notice how Linux is written with strict avoidance of this aliasing nonsense typedef brings. The result is a minimalist and clean style. Now, carefully note the last case above; you need to write struct point to declare Structures of that type if you decide to create that type at a later point in your code. Enter typedef. If you intend to create new Structure Structure is a custom data-type at a later time in your program using the same blueprint, using typedef during its definition might be a good idea since you can save some typing moving forward.
To get to the type requires the struct prefix. Turns out in C99 typedef is required. It is outdated, but a lot of tools ala HackRank use c99 as its pure C implementation. And typedef is required there. I'm not saying they should change maybe have two C options if the requirement changed, those of us studing for interviews on the site would be SOL. In 'C' programming language the keyword 'typedef' is used to declare a new name for some object struct, array, function..
For example, I will use a 'struct-s'. In 'C' we often declare a 'struct' outside of the 'main' function. For example:. Each time I decide to use a struct type I will need this keyword 'struct 'something' 'name'.
So our code will be:. If you have some local object struct, array, valuable that will be used in your entire program you can simply give it a name using a 'typedef'. This implementation give also access to compilator type conversion and remove some bugging effects when execution thread leave the application field of initialisation functions. After this declaration, sStudInfo is an alias of struct sStudentsInformations. So instead of using struct sStudentsInformations to declare new structure variables we can use just use sStudInfo.
But it is my recommendation to use the structure tag at the time of the structure declaration. Because if we have not used a structure tag with structure then we will get a compiler error when structure tries to reference itself.
We can also use a typedef with a structure pointer and avoid the use of struct keyword at the time of the structure pointer declaration. We can create different types using the typedef with structure definition. See the below example in which we are creating two types one is structure pointer and the second is structure variable.
We can use a typedef with an array. It increases the readability. Some times in the code we need to create a multi-dimension array. We know that the handling of a multidimensional array is complicated. It becomes more dangerous when we need to pass a multidimensional array in the function.
So let us see an example where we are using a typedef with an array and passing this array in function by the call by reference. In the above example, InitBrickPrice is the function that takes the pointer to the array as arguments and initializes the passed array with a defined price.
Using a typedef, we can make the declaration of function pointer easy and readable. The typedef is very helpful when we create an array of the function pointer or a function returns a function pointer. Now, apfArithmatics is a type of array of a function pointer and we can create a variable using this created type. Some times in the code we need to typecast the address using the function pointer.
It also becomes easy using the typedef. We can use the typedef and enum together in C programming. If we use typedef with enum in C then it increases the code readability and creates a new type for the enum.
Let see an example, where I am creating a list of errors using the enum and defining a new type using the typedef. Important: We have prepared a quiz on typedef mention below. Thanks, Subbu. It was a mistake. I have updated the article. Skip to content.
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