Stream manipulators

Stream manipulators are used to modify streams, so input or output.

All of you have used at least one existing manipulator:

Another existing one is setw(n), a parameterised stream manipulator used to set the width of the stream to n.

But you can write your own as well.

Writing stream manipulators ...

Consider:
cout << endl;

basic_ostream contains the following declaration

basic_ostream<charT,traits> & operator
<< (basic_ostream<charT,traits> & (*f (basic_ostream<charT,traits>&)) {return f(*this)};

where f is a type “pointer to a function with one argument of type basic_ostream& that returns type basic_ostream”.

In practice, f is a pointer to a manipulator.

cout << endl;

... is equivalent to

cout.operator<<(endl);

And the prototype of the endl function is ...

ostream& endl( ostream& os )
{
os << '\n';
return os.flush();
}

#include <iostream>

using namespace std;

ostream& endl( ostream& os)
{
os <<"This is my endl !"<<'\n';
return (os.flush());

}
int main()
{
cout << endl;
}

We could write a manipulator for outputting currencies, using functionality in the header iomanip.

std::ostream& Currency( std::ostream& os ) {
os << '$';
os << std::setprecision(2);
os << std::fixed << std::setfill('*');
os.width(12);
return os;
}

int main()
{
double money=1023.456;
std::cout << Currency << money << std::endl;
}

We will just look at an example.

For whatever reason we want a manipulator, star(n), that prints n stars.

Two steps are needed to implement a manipulator with parameters.

star(10)

operator << ( ostream&, const star& );
class star {
friend ostream& operator<<(ostream &, const star &);

private:
int n;

public:
star( int m ){n=m;}

};

ostream& operator << (ostream& os, const star& r) {
for(int i=0; i<r.n; i++)
os << '*';
return os;
}

int main()
{
cout << star(10) <<endl;
return 0;
}

Stream manipulators can be used to transform data.

void devowel(istream& in, ostream& out)
{
char c;
while (in.get(c)) {
if (isVowe0l(c))
c = '-';
out.put(c);
}
}

int main()
{
devowel(cin, cout);
}

bool inVowel(char x)
{
if(x == 'a' || x == 'e' ...
return 1;
else
return 0;
}