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)

Answer 1

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.