Redefinition allowed in C but not in C++?

Question

Why does this code work in C but not in C++?

int i = 5;
int i; // but if I write int i = 5; again I get error in C also

int main(){

  // using i
}

Answer 1

Tentative definition is allowed in C but not in C++.

A tentative definition is any external data declaration that has no storage class specifier and no initializer.

C99 6.9.2/2

A declaration of an identi?er for an object that has ?le scope without an initializer, and without a storage-class speci?er or with the storage-class speci?er static, constitutes a tentative de?nition. If a translation unit contains one or more tentative de?nitions for an identi?er, and the translation unit contains no external de?nition for that identi?er, then the behavior is exactly as if the translation unit contains a ?le scope declaration of that identi?er, with the composite type as of the end of the translation unit, with an initializer equal to 0.

So int i is a tentative definition. The C compiler will combine all of the tentative definitions into a single definition of i.

In C++ your code is ill-formed due to the One Definition Rule (Section 3.2/1 ISO C++)

No translation unit shall contain more than one definition of any variable, function, class type, enumeration type or template.

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// but if I write int i = 5; again I get error in C also

Because in that case it no longer remains a tentative definition because of the initializer (5).

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Just for the sake of information

J.5.11 Multiple external definitions

There may be more than one external definition for the identifier of an object, with or without the explicit use of the keyword extern; if the definitions disagree, or more than one is initialized, the behavior is undefined (6.9.2).

Also check out this excellent post on external variables.