Stream Input & Output Stream I/O Stream I/O
by overloading two functions with names
operator<< and operator>>
#include <iostream>
struct point { int x; int y; };
std::ostream& operator << (std::ostream& os, point const& p) {
return os << '(' << p.x << ',' << p.y << ')';
}
std::istream& operator >> (std::istream& is, point& p) {
return is >> p.x >> p.y;
}
int main () {
point p {1,2};
std::cout << p << '\n'; // prints (1,2)
…
std::cout << "enter two integers: ";
std::cin >> p; // reads 2 ints into p.x and p.y
…
std::cout << "p: " << p << '\n';
}
operator functions for stream input/output
of objects of type T:
std::ostream& operator << (std::ostream& os, T const& x) {
// write to stream …
return os;
}
std::istream& operator >> (std::istream& is, T& x) {
// read from stream …
return is;
}Operators << and >>
return a reference (to their stream parameter) to allow operator chaining:
cin >> x >> y; ⇔ operator>>( operator>>(cin, x), y)cout << x << y; ⇔ operator<<( operator<<(cout,x), y)There are no default stream operations in the standard library for containers like std::vector, because there are too many possible use cases:
- just print values … separated by what?
- format output as plain text / XML / …
- (de-)serialize container
- …
(Some) Standard Library Stream Types (Some) Standard Stream Types Stream Types
istream |
input stream | reference istream&
binds to any other kind of std:: input stream |
ostream |
output stream | reference ostream&
binds to any other kind of std:: output stream |
ifstream |
input file stream | extracted data is read from a file |
ofstream |
output file stream | inserted data is stored in a file |
ostringstream |
output string stream | inserted data is stored in a string buffer |
istringstream |
input string stream | extracted data is read from a string buffer |
Utilities
std::getline (istream&, string&, stopat='\n')reads until the next stopat character
(default = until end of line)
string s;
getline(cin, s);
getline(cin, s, '\t');
getline(cin, s, 'a');std::istream::ignore(n, c)- forwards stream by
ncharacters - until stop character
c
// skip next 8 characters
cin.ignore(8);
// skip by max. 10 characters or until after '='
cin.ignore(10, '=');
// skip until after next newline character
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');#include <iostream>
#include <iomanip>
int main () {
double d = 12.345678;
double e = 2011;
double f = 1e-10;
std::cout << d
<< e << '\n'
<< f << '\n'
<< std::setprecision(4)
<< d << '\n'
<< e << '\n'
<< f << '\n'
<< std::fixed
<< d << '\n'
<< e << '\n'
<< f << '\n'
<< std::scientific
<< d << '\n'
<< e << '\n'
<< f << '\n'
<< true <<' '<< false << '\n'
<< std::boolalpha << '\n'
<< true <<' '<< false << '\n';
}
header
12.345678
2011
1e-010
# of digits
12.35
2011
1e-010
fixed # of decimals
12.3457
2011.0000
1.0000e-010
scientific notation
1.2346e+001
2.0110e+003
1.0000e-010
1 0
booleans as string
true false