Thinking in C++ Vol 2 - Practical Programming |
Prev |
Home |
Next |
Perhaps the most notable difference in typical numeric
computer output from country to country is the punctuator used to separate the
integer and fractional parts of a real number. In the United States, a period
denotes a decimal point, but in much of the world, a comma is expected instead.
It would be quite inconvenient to do all your own formatting for
locale-dependent displays. Once again, creating an abstraction that handles
these differences solves the problem.
That abstraction is the locale. All streams have an
associated locale object that they use for guidance on how to display certain
quantities for different cultural environments. A locale manages the categories
of culture-dependent display rules, which are defined as follows:
Category
|
Effect
|
collate
|
Allows comparing strings according to different, supported
collating sequences.
|
ctype
|
Abstracts the character classification and conversion
facilities found in <cctype>.
|
monetary
|
Supports different displays of monetary quantities.
|
numeric
|
Supports different display formats of real numbers,
including radix (decimal point) and grouping (thousands) separators.
|
time
|
Supports various international formats for display of date
and time.
|
messages
|
Scaffolding to implement context-dependent message
catalogs (such as for error messages in different languages).
|
The following program illustrates basic locale behavior:
//: C04:Locale.cpp {-g++}{-bor}{-edg} {RunByHand}
// Illustrates effects of locales.
#include <iostream>
#include <locale>
using namespace std;
int main() {
locale def;
cout << def.name() << endl;
locale current = cout.getloc();
cout << current.name() << endl;
float val = 1234.56;
cout << val << endl;
// Change to French/France
cout.imbue(locale("french"));
current = cout.getloc();
cout << current.name() << endl;
cout << val << endl;
cout << "Enter the literal 7890,12:
";
cin.imbue(cout.getloc());
cin >> val;
cout << val << endl;
cout.imbue(def);
cout << val << endl;
} ///:~
Here s the output:
C
C
1234.56
French_France.1252
1234,56
Enter the literal 7890,12: 7890,12
7890,12
7890.12
The default locale is the C locale, which is what C and
C++ programmers have been used to all these years (basically, English language
and American culture). All streams are initially imbued with the C locale.
The imbue( ) member function changes the locale that a stream uses.
Notice that the full ISO name for the French locale is displayed (that is,
French used in France vs. French used in another country). This example shows
that this locale uses a comma for a radix point in numeric display. We have to
change cin to the same locale if we want to do input according to the
rules of this locale.
Each locale category is divided into number of facets, which are classes encapsulating the functionality that pertains to
that category. For example, the time category has the facets time_put and time_get, which contain functions for doing time and date input
and output respectively. The monetary category has facets money_get, money_put, and moneypunct. (The latter facet determines the currency symbol.) The following program illustrates the moneypunct facet.
(The time facet requires a sophisticated use of iterators which is
beyond the scope of this chapter.)
//: C04:Facets.cpp {-bor}{-g++}{-mwcc}{-edg}
#include <iostream>
#include <locale>
#include <string>
using namespace std;
int main() {
// Change to French/France
locale loc("french");
cout.imbue(loc);
string currency =
use_facet<moneypunct<char>
>(loc).curr_symbol();
char point =
use_facet<moneypunct<char>
>(loc).decimal_point();
cout << "I made " << currency
<< 12.34 << " today!"
<< endl;
} ///:~
The output shows the French
currency symbol and decimal separator:
You can also define your own facets to construct customized
locales. Be aware that
the overhead for locales is considerable. In fact, some library vendors provide
different flavors of the Standard C++ library to accommodate environments
that have limited space.
Thinking in C++ Vol 2 - Practical Programming |
Prev |
Home |
Next |