C++ for Scientists - Technische Universität Dresden

24 CHAPTER 2. C++ BASICS
int main ()
{
int a= 5; // define #1
{
a= 3; // assign #1, #2 is not defined yet
int a; // define #2
a= 8; // assign #2, #1 is hidden
{
a= 7; // #2
}
} // end of #2’s scope
a= 11; // #1, #2 is now out of scope
return 0;
}
Defining the same variable name twice in the same scope is an error.
The advantage of scopes is that you do not need to worry whether a variable (or something
else) is already defined outside the scope. It is just hidden but does not create a conflict. 4
Unfortunately, the hiding makes the homonymous variables in the outer scope inaccessible.
Best thing you can do is to rename the variable in the inner scope (and eventually in the nextouter
scope(s) to access more of those variables). Renaming the outermost variable also solves
the problem of accessibility but tends to be more work because it is probably more often used
due to its longer life time. A better solution to manage nesting and accessibility are namespaces,
see next section.
Scopes also have the advantage to reuse memory, e.g.:
int main ()
{
int x, y;
float z;
cin ≫x;
if (x < 4) {
y= x ∗ x;
// something with y
} else {
z= 2.5 ∗ float(x);
// something with z
}
}
The example uses three variables. However, they are never used at the same time. y is only
used in the first branch and z only in the second one.
Thus, we rewrite the program as follows
int main ()
{
int x;
cin ≫x;
4 As opposed to macros, an obsolete and reckless legacy feature from C that should be avoided at any price
because it undermines all structure and reliability of the language.