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CS253 Manipulators

I/O Manipulators

As you recall, there are a number of I/O manipulators:

<ios> or <iostream>	<iomanip>
These don’t take an argument	These do take an argument
[no]boolalpha	resetiosflags
[no]showbase	setiosflags
[no]showpoint	setbase
[no]showpos	setfill
[no]skipws	setprecision
[no]uppercase	setw (non-sticky)
left / right / internal
dec / hex / oct
fixed / scientific

[no]boolalpha

cout                << false << ' ' << true << '\n'
     << boolalpha   << false << ' ' << true << '\n'
     << noboolalpha << false << ' ' << true << '\n';

0 1
false true
0 1

• For boolean output, write true or false instead of 1 or 0.

[no]showbase

cout               << dec << 65 << ' ' << hex << 65 << ' ' << oct << 65 << '\n'
     << showbase   << dec << 65 << ' ' << hex << 65 << ' ' << oct << 65 << '\n'
     << noshowbase << dec << 65 << ' ' << hex << 65 << ' ' << oct << 65 << '\n';

65 41 101
65 0x41 0101
65 41 101

• Label integer output, using the same conventions as integer constants in a C++ program:
  • Prefix hex integer output with 0x.
  • Ensure that octal integer output starts with a 0.
  • There is no binary output.

[no]showpoint

cout                << 123.0 << ' ' << 456.789 << '\n'
     << showpoint   << 123.0 << ' ' << 456.789 << '\n'
     << noshowpoint << 123.0 << ' ' << 456.789 << '\n';

123 456.789
123.000 456.789
123 456.789

• Floating-point numbers get a decimal point, whether or not they’re whole numbers.

[no]showpos

cout              << 123 << ' ' << 0 << ' ' << -456 << '\n'
     << showpos   << 123 << ' ' << 0 << ' ' << -456 << '\n'
     << noshowpos << 123 << ' ' << 0 << ' ' << -456 << '\n';

123 0 -456
+123 +0 -456
123 0 -456

• Non-negative numbers gets a plus sign.

[no]skipws

istringstream iss;
int a;

iss.str("\n\r\v\f 123 ");
iss >> a;
cout << (iss?"good":"bad") << ' ' << a << '\n';

iss.str("\n\r\v\f 456 ");
iss >> skipws >> a;
cout << (iss?"good":"bad") << ' ' << a << '\n';

iss.str("\n\r\v\f 789 ");
iss >> noskipws >> a;
cout << (iss?"good":"bad") << ' ' << a << '\n';

good 123
good 456
bad 0

• Whitespace is normally skipped before formatted input (<<).

[no]uppercase

cout                << hex << 64206 << ' ' << 1e99 << " end\n"
     << uppercase   << hex << 64206 << ' ' << 1e99 << " end\n"
     << nouppercase << hex << 64206 << ' ' << 1e99 << " end\n";

face 1e+99 end
FACE 1E+99 end
face 1e+99 end

• Use UPPERCASE letters in hex output, and in floating-point exponents.
• Strings and characters are unchanged.

left / right / internal

cout                         << 123 << "★\n"
     << setw(10) <<     left << 123 << "★\n"
     << setw(10) <<    right << 123 << "★\n"
     << setw(10) << internal << 123 << "★\n";

123★
123       ★
       123★
       123★

cout                         << -123 << "★\n"
     << setw(10) <<     left << -123 << "★\n"
     << setw(10) <<    right << -123 << "★\n"
     << setw(10) << internal << -123 << "★\n";

-123★
-123      ★
      -123★
-      123★

cout                                         << -123 << "★\n"
     << setw(10) << setfill('x') <<     left << -123 << "★\n"
     << setw(10) << setfill('x') <<    right << -123 << "★\n"
     << setw(10) << setfill('x') << internal << -123 << "★\n";

-123★
-123xxxxxx★
xxxxxx-123★
-xxxxxx123★

• Alignment for numbers and strings. There is no center.

dec / oct / hex

cout        << 10 << ' ' << 30 << ' ' << 50 << '\n'
     << dec << 10 << ' ' << 30 << ' ' << 50 << '\n'
     << oct << 10 << ' ' << 30 << ' ' << 50 << '\n'
     << hex << 10 << ' ' << 30 << ' ' << 50 << '\n';

10 30 50
10 30 50
12 36 62
a 1e 32

• Output in base 10, base 8, and base 16.

cout << bin << 10;

c.cc:1: error: 'bin' was not declared in this scope

• There is no base 2 output.

fixed / scientific / hexfloat / defaultfloat

cout               << 16e5 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n'
   << fixed        << 16e5 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n'
   << scientific   << 16e5 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n'
   << hexfloat     << 16e5 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n'
   << defaultfloat << 16e5 << ' ' << 3.14159 << ' ' << 4.5e-12 << '\n';

1.6e+06 3.14159 4.5e-12
1600000.000000 3.141590 0.000000
1.600000e+06 3.141590e+00 4.500000e-12
0x1.86ap+20 0x1.921f9f01b866ep+1 0x1.3ca8cb153a753p-38
1.6e+06 3.14159 4.5e-12

• Fixed-point notation: no exponent
• Scientific notation: digit . digits e exponent
• Hexadecimal: the bits that represent the number
• Default: choose based on which one makes sense.

setfill

cout                             << 123 << '\n'
                     << setw(10) << 456 << '\n'
     << setfill('*') << setw(10) << 789 << '\n';

123
       456
*******789

• Set the fill (padding) character, which defaults to a space.
• Only relevant if setw() is used.

setprecision

https://cplusplus.com/reference/ios/ios_base/precision/ says:

• Using the default floating-point notation, the precision field specifies the maximum number of meaningful digits to display in total counting both those before and those after the decimal point. Notice that it is not a minimum, and therefore it does not pad the displayed number with trailing zeros if the number can be displayed with less digits than the precision.
• In both the fixed and scientific notations, the precision field specifies exactly how many digits to display after the decimal point, even if this includes trailing decimal zeros. The digits before the decimal point are not relevant for the precision in this case.

setprecision

cout << 123.45 << '\n'
     << setprecision(1) << 123.45 << '\n'
     << setprecision(9) << 123.45 << '\n';

123.45
1e+02
123.45

cout << fixed
     << 123.45 << '\n'
     << setprecision(1) << 123.45 << '\n'
     << setprecision(9) << 123.45 << '\n';

123.450000
123.5
123.450000000

cout << scientific
     << 123.45 << '\n'
     << setprecision(1) << 123.45 << '\n'
     << setprecision(9) << 123.45 << '\n';

1.234500e+02
1.2e+02
1.234500000e+02

setw

cout << setw(5) << 1 << 2 << 3 << 4 << '\n';

    1234

cout << setw(9) <<   6.7 << '\n'
     << setw(9) <<   'x' << '\n'
     << setw(9) << "foo" << '\n';

      6.7
        x
      foo

cout << setfill('*') << setw(4) <<     8.9 << '\n'
                     << setw(4) <<     'y' << '\n'
                     << setw(4) <<   "bar" << '\n'
                     << setw(4) << "alpha" << '\n';

*8.9
***y
*bar
alpha

• setw is different. It’s not sticky—it only applies to the very next formatted output. It’s only good for one conversion.
• It affects more than just numbers.
• It pads with the fill character.

Etiquette

void showhex(int n) {
    cout << hex << setw(8) << setfill('0') << n << '\n';
}

int main() {
    showhex(100);
    cout << 42 << '\n';
}

00000064
2a

It’s rude to change a global resource. showhex()’s job is to display a number, not to display a number and change the state of cout.

main() didn’t expect the base of cout, or its fill character, to get changed. Now, of course, if it’s main() that’s changing those settings, who else would complain?

Etiquette

void showhex(int n) {
    const auto save_flags = cout.flags();
    const auto save_fill = cout.fill();
    cout << hex << setw(8) << setfill('0') << n << '\n';
    cout.flags(save_flags);
    cout.fill(save_fill);
}

int main() {
    showhex(100);
    cout << 42 << '\n';
}

00000064
42

• .flags() saves or restores the flags: hex/oct/dec, fixed/scientific, showbase, etc.
• .fill() saves or restores the fill character.

Defining Your Own

It’s easy to define your own I/O manipulators:

ostream &dog(ostream &os) {
    return os << "Kokopelli";
}

int main() {
    cout << "My dog is " << dog << ".\n";
    return 0;
}

My dog is Kokopelli.

Of course, if that’s all you want:

const string dog = "Kokopelli";
cout << "My dog is " << dog << ".\n";

My dog is Kokopelli.

Better Example

ostream &bucks(ostream &os) {
    return os << fixed << setprecision(2);
}

int main() {
    cout << 34.5 << ' ' << bucks << 78.9 << '\n';
    return 0;
}

34.5 78.90

• This sets output to be two digits after the decimal point, as is traditional for money in the USA.