Question

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array initialization, is referencing a previous element ok?

I wonder if its safe to do such initialization in c/c++ standard:

int a = 5;
int tab[] = { a , tab[0] + 1 , tab[1] };

It successfully compiles and executes with gcc 4.5 and clang 2.9, but will it always be true?

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Printing this table gives 5 6 6. Its initialized in global scope.

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Generally its interesting in both, c and c++, but i want to use it in c++:)

Answer 1

In the C99 standard, it seems that the order of initialization of the members is guaranteed:

§6.7.8/17: Each brace-enclosed initializer list has an associated current object. When no designations are present, subobjects of the current object are initialized in order according to the type of the current object: array elements in increasing subscript order, structure members in declaration order, and the first named member of a union. In contrast, a designation causes the following initializer to begin initialization of the subobject described by the designator. Initialization then continues forward in order, beginning with the next subobject after that described by the designator.

But as @Tomalak mentions in the comment, that does not provide a full guarantee for the operation, as the compiler would first evaluate all of the arguments and then apply the results in the previous order. That is, the previous quote does not impose an order between the initialization of tab[0] and the evaluation of the expression tab[0]+1 that is used to initialize tab[1] (it only imposes an ordering between the initialization of tab[0] and tab[1])

As of the C++ standard, neither in the current standard nor the FDIS of the upcoming C++0x standard seem to have an specific clause defining the order in which the initialization is performed. The only mention of ordering comes from

§8.5.1/2 When an aggregate is initialized the initializer can contain an initializer-clause consisting of a brace-enclosed, comma-separated list of initializer-clauses for the members of the aggregate, written in increasing subscript or member order.

But that only relates to the order by which the entries in the initializer are written, not how it is actually evaluated.