Nested structures in C and C++

Question

Is there any significance behind allowing the visibility of nested structures outside the structure in C but not in C++? I did not find any reference or relevance.

struct a
{
  struct b{
  };
};

int main(){
  struct b var;  // allowed in C not in C++.
}

Wasn't the operator:: introduced only in C++ along with the very notion of different scopes? — Kos, Nov 27 '11 at 07:25

James McNellis · Answer 1 · 2011-11-27T07:25:23.133

36

It is valid in C because C has a single namespace in which all nonlocal types (i.e., types not declared in functions) are defined; there is no scoping of types using namespaces or nesting.

In C++, type b is nested as a member of class a, so its name must be qualified with the scope in which it is declared.

edited Nov 27 '11 at 07:25

answered Nov 27 '11 at 07:16

James McNellis

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Doesn't C have two separate global namespaces for scalar type names and for structs or something like that? – Kerrek SB Nov 27 '11 at 13:37
2

@KerrekSB: Yes, in a sense. The namespace in question here is the tags namespace (in `struct a { };`, the tag is `a`). – James McNellis Nov 27 '11 at 19:02
See ISO C99, 6.2.1 Scopes of identifiers, paragraph 7 on the scope of struct in C99. – Atafar May 22 '15 at 07:04
An example of the usage would be nice. – glibg10b Aug 26 '21 at 12:35

score 9 · Answer 2 · answered Nov 27 '11 at 07:16

9

I believe the ability to reference nested structures outside of the structure was removed in C++ to improve data hiding. If you need to access a nested struct externally, then it probably shouldn't be a nested struct in the first place.

Wikipedia says: "In both C and C++ one can define nested struct types, but the scope is interpreted differently (in C++, a nested struct is defined only within the scope/namespace of the outer struct)." (http://en.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%2B). It doesn't say why, but at least it acknowledges the difference.

You can use the namespace resolution operator to access the struct, however.

answered Nov 27 '11 at 07:16

Chris Parton

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The *why* seems to be avoiding namespace pollution. From http://www2.research.att.com/~bs/sibling_rivalry.pdf: "In C, structure scopes that appear to be nested aren’t, because structure names declared inside are considered to be in the outer scope. This proved to be unmanageable in C++ where nested classes were often used as implementation details. Consequently, C++ adopted nested structure scopes." – UncleBens Nov 27 '11 at 10:54
+1 for "probably shouldn't be a nested struct in the first place". – mskfisher Oct 02 '13 at 19:53

score 4 · Answer 3 · answered Nov 27 '11 at 07:16

4

because b scope is inside a, you have to use struct a::b instead (and unlike in C, the struct keyword is optional).

answered Nov 27 '11 at 07:16

LeleDumbo

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score 4 · Answer 4 · answered Nov 27 '11 at 07:18

You cannot declare anything without a scope in C++ In your example struct b lies inside the struct a, compiler doesn't know where to find struct b

you have to use

struct a :: b var;

In C there is no restriction for scope, but C++ ensures a restriction

Nested structures in C and C++

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