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Although already available in the bf(C) programming language,
i(local variables) should only be defined once they're needed. Although
doing so requires a little getting used to, eventually it tends to produce
more readable, maintainable and often more efficient code than defining
variables at the beginning of compound statements. We suggest to apply the
following rules hi(rule of thumb) of thumb when defining local variables:
itemization(
it() Local variables should be created at `intuitively right' places, such
as in the example below. This does not only entail the tt(for)-statement, but
also all situations where a variable is only needed, say, half-way through the
function.
it() More in general, variables should be defined in such a way that their
scope is as em(limited) and em(localized) as possible. When avoidable local
variables are not defined at the beginning of functions but rather where
they're first used.
it() It is considered good practice to hi(global variable: avoid)
emi(avoid global variables). It is fairly easy to lose track of which
global variable is used for what purpose. In bf(C++) global variables are
seldom required, and by localizing variables the risk of using the same
variable for multiple purposes (thereby invalidating the separate purposes of
the variable), can easily be avoided.
)
If considered appropriate, emi(nested blocks) can be used to localize
auxiliary variables. However, situations exist where local variables are
considered appropriate inside nested statements. The just mentioned tt(for)
statement is of course a case in point, but local variables can also be
defined within the condition clauses of tt(if-else) statements, within
selection clauses of tt(switch) statements and condition clauses of tt(while)
statements. Variables thus defined are available to the full
statement, including its nested statements. For example, consider the
following tt(switch) statement:
verbinclude(-a examples/switch.cc)
Note the location of the definition of the character `tt(c)': it is
defined in the expression part of the tt(switch) statement. This implies
that `tt(c)' is available em(only) to the tt(switch) statement itself,
including its nested (sub)statements, but not outside the scope of the
tt(switch).
The same approach can be used with tt(if) and tt(while) statements: a
variable that is defined in the condition clause of an tt(if) and tt(while)
statement is available in their nested statements. There are some caveats,
though:
itemization(
it() The variable that is defined in the condition clause must be a
variable which is initialized to a numeric or logical value;
it() The variable definition cannot be nested (e.g., using parentheses)
within a more complex expression.
)
The former point of attention should come as no big surprise: in order to
be able to evaluate the logical condition of an tt(if) or tt(while) statement,
the value of the variable must be interpretable as either zero (false) or
non-zero (true). Usually this is no problem, but in bf(C++) em(objects) (like
objects of the type tt(std::string) (cf. chapter ref(String))) are often
returned by functions. Such objects may or may not be interpretable as numeric
values. If not (as is the case with tt(std::string) objects), then such
variables can em(not) be defined at the condition or expression clauses of
condition- or repetition statements. The following example will therefore
em(not) compile:
verb( if (std::string myString = getString()) // assume getString returns
{ // a std::string value
// process myString
})
The above example requires additional clarification. Often a variable can
profitably be given local scope, but an extra check is required immediately
following its initialization. The initialization em(and) the test cannot
em(both) be combined in one expression. Instead em(two) nested statements are
required. Consequently, the following example won't compile either:
verb( if ((int c = getchar()) && strchr("aeiou", c))
printf("Saw a vowel\n");)
If such a situation occurs, either use two nested tt(if) statements, or
localize the definition of tt(int c) using a nested compound statement:
verb( if (int c = getchar()) // nested if-statements
if (strchr("aeiou", c))
printf("Saw a vowel\n");
{ // nested compound statement
int c = getchar();
if (c && strchr("aeiou", c))
printf("Saw a vowel\n");
})
The bf(C++26) standard introduced emi(name-independent declarations). They're
covered in section ref(20.3.1).
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