Purpose of struct, typedef struct, in C++

Question

In C++ it is possible to create a struct:

struct MyStruct
{
    ...
}

And also possible to do the following:

typedef struct         


        

Answer 1

There are many answers that consider both approaches as equivalent, but they are not.

The typedef keyword is used to create a type alias, that is, it provides a new way of referring to another type. When you use typedef struct {} XXX; you are actually creating an unnamed type and then creating an alias XXX to that unnamed type. On the other hand, when you type struct XXX {}.

Please, read the answer by Michael Burr here (which is a better answer than the one accepted for that question.

Answer 2

Here are the differences between the two declarations/definitions:

---

1) You cannot use a typedef name to identify a constructor or a destructor

struct MyStruct { MyStruct(); ~MyStruct(); }; // ok

typedef struct { MyStructTD(); ~MyStructTD(); } MyStructTD; // not ok

// now consider 
typedef struct MyStruct2 { MyStruct2(); } MyStructTD2; // ok

MyStructTD2::MyStruct2() { } // ok
MyStructTD2::MyStructTD2() { } // not ok

---

2) You cannot hide a typedef name like you can a name introduced via the class-head - or conversely if you already have a function or an object with a certain name, you can still declare a class with that name using the class-head but not via the typedef approach.

struct MyStruct { }; // ok

typedef struct { } MyStructTD; // ok

void MyStruct() { }  // (1) - ok Hides struct MyStruct
void MyStructTD() { }  // (2) - not-ok - ill-formed

//> Or if you flip it around, consider in a new translation unit:

void MyStruct() { }   // ok
void MyStructTD() { }   // ok

struct MyStruct { }; // ok
typedef struct { } MyStructTD; // Not ok

---

3) You cannot use a typedef name in an elaborated type specifier

struct MyStruct {  }; // ok

typedef struct { } MyStructTD; // ok

int main()
{
  void MyStruct(); 
  void MyStructTD(); // ok - new declarative region

  struct MyStruct ms; // ok - names the type
  struct MyStructTD ms2; // not ok - cannot use typedef-name here

}

struct AnotherStruct 
{ 
    friend struct MyStruct;  // ok
    friend struct MyStructTD; // not ok
};

---

4) You cannot use it to define nested structs

struct S { struct M; };

typedef struct { } S::M;  // not ok

struct S::M { }; // ok

As you can see, there is a discernible difference between the two. Some of the quirks of typedefs are a result of C compatibility (which is mainly why both ways exist i believe) - and in most cases, declaring the name in the class-head is more natural C++ - it has its advantages (especially when you need to define constructors and destructors), and is therefore preferable. If you are writing code that needs to be C and C++ compatible, then there is benefit to using both approaches. But if you are writing pure C++, I find specifying the class name in the class-head to be more readable.

Answer 3

The "struct MyStruct { };" construct implicitly defines the equivalent typedef, so typically that would be the preferred modern usage. There are still some uses for a typedef, mainly with pointer types to structures. E.g.

typedef struct MyStruct* MyStructPtr;

Answer 4

You would use a typedef so you do not need to specify the struct keyword when declaring variables of that struct.

Without typedef:

struct MyStruct foo;

With typedef:

MyStruct foo;

Basically you are making MyStruct appear as a new type so it also implements some level of type abstraction as programmers do not need to know explicitly what type it is. You can pass MyStruct into a function, manipulate the data in it and return it and the programmer need never worry about what actually happens.

Much of the C standard library uses typedefs for this reason.

Answer 5

The typedef version is a special case of

typedef foo bar;

which defines a new "type" bar as an alias for foo. In your case, foo happens to be a struct. In C, this was the only way to introduce new "types" (in quotes, because they are not really equivalent to int, float and co). In C++, this is not so useful, because C++ was designed to make definition of new types easier and more complete than C (at least at the beginnings of C++), and the typedef is not even necessary to refer to a previously declared struct (or class).

Answer 6

The latter is there for compatibility with C - use the first in new C++ code.