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Thinking in C++ Vol 2 - Practical Programming
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Input string streams

To read from a string using stream operations, you create an istringstream object initialized with the string. The following program shows how to use an istringstream object:

//: C04:Istring.cpp
// Input string streams.
#include <cassert>
#include <cmath> // For fabs()
#include <iostream>
#include <limits> // For epsilon()
#include <sstream>
#include <string>
using namespace std;
int main() {
istringstream s("47 1.414 This is a test");
int i;
double f;
s >> i >> f; // Whitespace-delimited input
assert(i == 47);
double relerr = (fabs(f) - 1.414) / 1.414;
assert(relerr <= numeric_limits<double>::epsilon());
string buf2;
s >> buf2;
assert(buf2 == "This");
cout << s.rdbuf(); // " is a test"
} ///:~

You can see that this is a more flexible and general approach to transforming character strings to typed values than the standard C library functions such as atof( ) or atoi( ), even though the latter may be more efficient for single conversions.

In the expression s >> i >> f, the first number is extracted into i, and the second into f. This isn t the first whitespace-delimited set of characters because it depends on the data type it s being extracted into. For example, if the string were instead, 1.414 47 This is a test, then i would get the value 1 because the input routine would stop at the decimal point. Then f would get 0.414. This could be useful if you want to break a floating-point number into a whole number and a fraction part. Otherwise it would seem to be an error. The second assert( ) calculates the relative error between what we read and what we expected; it s always better to do this than to compare floating-point numbers for equality. The constant returned by epsilon( ), defined in <limits>, represents the machine epsilon for double-precision numbers, which is the best tolerance you can expect comparisons of doubles to satisfy.[46]

As you may already have guessed, buf2 doesn t get the rest of the string, just the next white-space-delimited word. In general, it s best to use the extractor in iostreams when you know the exact sequence of data in the input stream and you re converting to some type other than a character string. However, if you want to extract the rest of the string all at once and send it to another iostream, you can use rdbuf( ) as shown.

To test the Date extractor at the beginning of this chapter, we used an input string stream with the following test program:

//: C04:DateIOTest.cpp
//{L} ../C02/Date
#include <iostream>
#include <sstream>
#include "../C02/Date.h"
using namespace std;
void testDate(const string& s) {
istringstream os(s);
Date d;
os >> d;
cout << d << endl;
cout << "input error with \"" << s << "\"" << endl;
int main() {
testDate("08 - 10 - 2003");
} ///:~

Each string literal in main( ) is passed by reference to testDate( ), which in turn wraps it in an istringstream so we can test the stream extractor we wrote for Date objects. The function testDate( ) also begins to test the inserter, operator<<( ).

Thinking in C++ Vol 2 - Practical Programming
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire