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?

Answer 1

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