Why are some local type declarations prefixed with 'struct' in C++

Question

I often see C++ code like this:

void foo()
{
  struct sockaddr* from;
  // ...
}

Why is the struct specifier needed? Does it actually do anything? The compiler can already know that sockaddr is declared as a struct, so I wonder why this is useful or necessary.

I've tried removing it in the past and haven't noticed a difference in behaviour, but I'm not confident it's safe to remove.

Similarly, what's the difference between these two?

sizeof(struct sockaddr)
sizeof(sockaddr)

@Naszta, I have wondered as much. If so, why would it be necessary in C but not in C++? — Drew Noakes, Jun 21 '13 at 21:02
@DrewNoakes: [FYI](http://en.wikipedia.org/wiki/Struct_(C_programming_language)) — Naszta, Jun 21 '13 at 21:04
You might want to read [this](http://stackoverflow.com/a/1675446/36565) answer to a related question. — David Rodríguez - dribeas, Jun 21 '13 at 21:10
See also http://stackoverflow.com/questions/612328/difference-between-struct-and-typedef-struct-in-c — Michael Burr, Jun 21 '13 at 21:11

Andy Prowl · Accepted Answer · 2013-06-21T21:18:12.540

5

As a coding "style", this is most likely a heritage of C, where the keyword is necessary.

In C++ this is not needed in most situations, although it is used sometimes to force the compiler to resolve a name to the name of a type rather than the name of some other entity (like a variable, as in the following example):

struct x
{
};

int main()
{
    int x = 42;
    struct x* pX = nullptr; // Would give a compiler error without struct
}

Personally, I do not consider good style having a variable, or a function with the same name as a type, and I prefer avoiding having to use the struct keyword for this purpose at all, but the language allows it.

edited Jun 21 '13 at 21:18

answered Jun 21 '13 at 21:03

Andy Prowl

124,023
23
387
451

In your code example, could you also clarify the type using the `typename` keyword? – Drew Noakes Jun 21 '13 at 21:19
@DrewNoakes: No, `typename` cannot be used in this context. Basically `typename` can only be used inside templates. – David Rodríguez - dribeas Jun 21 '13 at 21:23

score 4 · Answer 2 · edited Jun 21 '13 at 21:25

This syntax comes from C, where it's obligatory. In C++ there is only one needed usage of this syntax

n3376 9.1/2

A class declaration introduces the class name into the scope where it is declared and hides any class, variable, function, or other declaration of that name in an enclosing scope (3.3). If a class name is declared in a scope where a variable, function, or enumerator of the same name is also declared, then when both declarations are in scope, the class can be referred to only using an elaborated-type-specifier
struct stat {
// ...
};
stat gstat; // use plain stat to
// define variable
int stat(struct stat*); // redeclare stat as function
void f() {
struct stat* ps; // struct prefix needed
// to name struct stat
stat(ps); // call stat()
}

score 1 · Answer 3 · answered Jun 21 '13 at 21:04

1

Code such as this comes directly from C (most likely via copy & paste), where the struct keyword is required before the name of any type that is defined as a struct (unless you add the appropriate typedef statement). Using struct in this way is still supported in C++ for backwards compatibility, but because struct foo { ... }; in C++ automatically adds foo as a type name, it is (usually) safe to refer to the type afterwards as just foo.

answered Jun 21 '13 at 21:04

jwodder

54,758
12
108
124

Yes I usually see this in code that's using C APIs, such as the socket example in my original question. Makes sense, and I'll now feel more confident to strip it out most of the time. – Drew Noakes Jun 21 '13 at 21:31

score 1 · Answer 4 · edited May 23 '17 at 12:20

1

You are largely correct; in C++ the struct keyword is hardly ever relevant in declaring a variable.

More than you ever wanted to know about this: using struct keyword in variable declaration in C++

edited May 23 '17 at 12:20

Community

1
1

answered Jun 21 '13 at 21:04

catfood

4,267
5
29
55

Why are some local type declarations prefixed with 'struct' in C++

4 Answers4