CODEWITHSUNDEEP
c++c
nikel
nikel

Reputation: 3564

Nested structures in C and C++

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++.
}

Upvotes: 28

Views: 16491

Answers (4)

James McNellis
James McNellis

Reputation: 355069

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.

Upvotes: 35

ammar26
ammar26

Reputation: 1612

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

Upvotes: 4

Chris Parton
Chris Parton

Reputation: 1054

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.

Upvotes: 9

LeleDumbo
LeleDumbo

Reputation: 9340

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

Upvotes: 4

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