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Home Forums What is the difference between “typedef” and “]using” in C++? – MindStick
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1 Answers

Aryan Kumar
Aryan Kumar 10 Jul 2023
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The typedef and using keywords in C++ are both used to create aliases for data types. However, there are some key differences between the two keywords.

  • typedef defines a new type, while using creates an alias for an existing type. This means that you can use a typedef to declare variables, while you can only use a using to refer to a variable that has already been declared using a typedef or a using.
  • typedef can be used to create aliases for any type, while using can only be used to create aliases for user-defined types. This means that you cannot use a using to create an alias for a built-in type, such as int or float.
  • typedef declarations are hoisted, while using declarations are not. This means that the typedef declarations are evaluated at the top of the scope in which they are declared, while the using declarations are evaluated at the point where they are encountered.

In general, you should use typedef if you want to define a new type that you will be using frequently. You should use using if you want to refer to an existing type without having to write out the full name of the type every time you want to use it.

Here are some examples of how to use typedef and using:

C++

// Define a new type called `Point` that is a struct
typedef struct Point {
  int x;
  int y;
} Point;

// Declare a variable of type `Point`
Point point;

// Create an alias for the `Point` type called `MyPoint`
using MyPoint = Point;

// Declare a variable of type `MyPoint`
MyPoint my_point;

In both cases, the variable point and the variable my_point will be of type Point. However, the first code defines a new type called Point, while the second code simply creates an alias for the existing type struct Point.

The using declaration in the second code is not hoisted, which means that the MyPoint alias is not evaluated until the point where it is encountered. This can lead to some unexpected behavior, so it is generally recommended to use typedef instead of using if you want to define a new type.

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