Use of variables in global namespace

Question

Why should this be an error?

int a = 0; a = 42; int main() { }

A possibe match for this behavior i could find:

(3.4.1/4) A name used in global scope, outside of any function, class or user-declared namespace, shall be declared before its use in global scope.

Could this be a defect in standard?

No, it's not a defect to the standard. The part you quote means you need to have declarations for all the names you want to use before you use them. It doesn't mean you can do whatever you want at global scope. — Mat
– Mat, Commented Nov 1, 2011 at 11:32
struct A {}; A foo(); The name A is used in the declaration of foo(). — Mat
– Mat, Commented Nov 1, 2011 at 11:42

Sarfaraz Nawaz · Accepted Answer · 2011-11-01 11:59:48Z

int a = 0; //it is a declaration (and definition too) statement a = 42; //it is an assignment statement

The second line is the cause of error, for it is an assignment statement.

At the namespace-level, only declaration and definition statements are allowed. Assignment-statements are not allowed at namespace level.

And by "shall be declared before its use in global scope" (from the spec's quotation) means the following:

int a = 42; int b = 2 * a; //a is being used here int c = a + b; //both a and b are being used here

If you define type instead, then:

struct A {}; //definition of A struct B { A a; }; //a is declared as member of B //(which means, A is being "used") void f(const A&, const B&); //both A and B are used in the declaration of f

Well, it's an expression statement, to be precise. Assignment is an expression, not a statement.
@CatPlusPlus: If one includes ;, then it becomes statement.
@CatPlusPlus, expression statement is not an expression though.
@avakar: Good lord, where did I say it's an expression. I think I named it a statement at least twice.

AndersK · Accepted Answer · 2011-11-01 11:31:18Z

You cannot write an assignment statement like that in the global namespace

it needs to be either in main or in some [member] function

int main() { a=42; return 0; }

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