If I write code like this:
int a = 123456; cout << setw(20) << setiosflags(ios::right) << a << endl; cout << setiosflags(ios::left) << setw(20) << a << '*' << endl; On the 3rd line, I set the alignment as left align, so my expected output is
123456 123456 *
but the REAL output is
123456 123456*
Why did that happen?
The IDE I use is DevCpp.
std::setiosflags() sets new flags without clearing any existing flags. So on the 3rd line, you are enabling the ios::left flag without disabling the ios::right flag. It does not make sense to have both flags enabled at the same time, and it seems the stream prefers the ios::right flag if it is enabled.
Use std::left and std::right instead. They reset the ios::internal, ios::left, and ios::right flags before setting the new alignment.
int a = 123456; cout << setw(20) << right << a << endl; cout << left << setw(20) << a << '*' << endl; 
ios::leftto cover ios rightIf you remove the line with setiosflags(ios::right), it works as expected, so it seems all common compilers evaluate right before left and short circuit the program flow. Try either manually unsetting ios::right, or better yet, just use std::left like so:
cout << left << setw(20) << a << '\n'; This, as so many standard library functions, takes care of pesky details.
int a = 123456; cout.setf(ios::right, ios::adjustfield); cout << setw(20) << a << endl; cout.setf(ios::left, ios::adjustfield); cout << setw(20) << a << '*' << endl; If I recall, you need to reset the alignment.
The setiosflags sets format flag for the output stream (here it's cout), not for this sentence only. Since the ios::right has priority over ios::left, the second line will be aligned right. So you need to clear the previous format flag and then set the new one.
int a = 123456; cout << setw(20) << setiosflags(ios::right) << a << endl; cout << resetiosflags(ios::right) << setiosflags(ios::left) << setw(20) << a << '*' << endl; But the simplest way is to use std::left and std::right
int a = 123456; cout << setw(20) << right << a << endl; cout << left << setw(20) << a << '*' << endl; 
setiosflags(ios::setw(20))sets the width for the whole program?