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

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.