Consider following program:
#include <iostream>
int main()
{
std::cout<<std::ios::showbase<<123<<", "<<std::hex<<123<<", "<<std::oct<<123<<'\n';
}
Expected Output: 123, 0x7b, 0173
Acquired Output: 512123, 7b, 173 (see live demo here: http://ideone.com/Khzj5j )
But If I modify the above program slightly as following:
#include <iostream>
using namespace std;
int main()
{
cout<<showbase<<123<<", "<<hex<<123<<", "<<oct<<123<<'\n';
}
Now I got desired output. (see live demo here http://ideone.com/gcuHbm ).
Why first program gave incorrect output but 2nd program doesn't? What's going wrong in first program?
std::ios::showbase
is a format flag. std::showbase
is a function that takes a std::ios_base
and calls ios_base::setf(std::ios::showbase)
on it to set the showbase
flag.
You use the prior in your first example and the latter in your second example.
#include <iostream>
int main()
{
std::cout<<std::ios::showbase<<123<<", "<<std::hex<<123<<", "<<std::oct<<123<<'\n';
}
This uses std::ios::showbase
( http://www.cplusplus.com/reference/ios/ios_base/fmtflags/ )
While your other program
#include <iostream>
using namespace std;
int main()
{
cout<<showbase<<123<<", "<<hex<<123<<", "<<oct<<123<<'\n';
}
Uses std::showbase
( http://en.cppreference.com/w/cpp/io/manip/showbase ) which is why you're getting different results.
Changing the first program to use std::showbase
gives you your expected output:
#include <iostream>
int main()
{
std::cout<<std::showbase<<123<<", "<<std::hex<<123<<", "<<std::oct<<123<<'\n';
}
std::ios::showbase
is a format flag that has some implementation defined value. When you call std::cout << std::ios::showbase
you are displaying that value and not setting the stream format flag.
In you second example you are using std::showbase
which sets the format flag of the stream.
In std::ios_base
, showbase
is a fmtflag
that has an implementation-defined value. In this case, it appears to be 512
. On the other hand, there is a stream manipulator (aka, a function) also called showbase
, which calls setf(std::ios_base::showbase)
. This is defined as a free function in namespace std
whereas the fmtflag
is a member of std::ios_base
.
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