CODEWITHSUNDEEP
c++io
ben
ben

Reputation: 6023

Simple I/O manipulator not working as intended

the following code:

#include <iostream>

std::ios_base &my_manip (std::basic_ios<char> &os) {
    os.unsetf(std::ios_base::basefield);
    os.setf(std::ios_base::scientific);
    return os;
}

int main (int argc, char **argv) {
    std::cout << 8.8888888 << std::endl;
    std::cout << my_manip << 8.8888888 << std::endl;
    return 0;
}

prints:

8.88889

18.88889

While the following code:

#include <iostream>

std::ios_base &my_manip (std::basic_ios<char> &os) {
    os.unsetf(std::ios_base::basefield);
    os.setf(std::ios_base::scientific);
    return os;
}

int main (int argc, char **argv) {
    std::cout << 8.8888888 << std::endl;
    my_manip(std::cout);
    std::cout << 8.8888888 << std::endl;
    return 0;
}

prints the expected result:

8.88889

8.888889e+00

Can anybody tell me what is wrong with the first version?

Upvotes: 4

Views: 178

Answers (3)

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 153935

There are three overloads taking a function pointer which can be used for manipulators. The three signatures are

std::ios_base&                  (*)(std::ios_base&)
std::basic_ios<cT, Traits>&     (*)(std::basic_ios<cT, Traits>&);
std::basic_ostream<cT, Traits>& (*)(std::basic_ostream<cT, Traits>&);

The signature you used (std::ios_base&(*)(std::basic_ios<cT, Traits>&)) is none of these.

I assume you are using the actual settings as examples because the std::scientific manipulator already implements the logic your example code tries to implement.

Upvotes: 2

P0W
P0W

Reputation: 47814

The custom manipulator signature is not matching,

You should be doing this :

std::ostream& my_manip (std::ostream &os) {
    os.unsetf(std::ios_base::basefield);
    os.setf(std::ios_base::scientific);
    return os;
}

std::cout << my_manip << 8.8888888 << std::endl;

Upvotes: 5

jrok
jrok

Reputation: 55415

The problem seems to be the signature of your manipulator. If you change it to

std::basic_ostream<char> &my_manip (std::basic_ostream<char> &os)

then it works as you expect it to.

Upvotes: 1

Related Questions