Custom stream manipulator for streaming integers in any base
I can make an std::ostream
object output integer numbers in hex, for example
std::cout << std::hex << 0xabc; //prints `abc`, not the base-10 representation
Is there any manipulator that is universal for all bases? Something like
std::cout << std::base(4) << 20; //I want this to output 110
If there is one, then I 开发者_如何转开发have no further question.
If there isn't one, then can I write one? Won't it require me to access private implementation details of std::ostream
?
Note that I know I can write a function that takes a number and converts it to a string which is the representation of that number in any base. Or I can use one that already exists. I am asking about custom stream manipulators - are they possible?
You can do something like the following. I have commented the code to explain what each part is doing, but essentially its this:
- Create a "manipulator" struct which stores some data in the stream using
xalloc
andiword
. - Create a custom
num_put
facet which looks for your manipulator and applies the manipulation.
Here is the code...
Edit: Note that im not sure I handled the std::ios_base::internal
flag correctly here - as I dont actually know what its for.
Edit 2: I found out what std::ios_base::internal
is for, and updated the code to handle it.
Edit 3: Added a call to std::locacle::global
to show how to make all the standard stream classes support the new stream manipulator by default, rather than having to imbue
them.
#include <algorithm>
#include <cassert>
#include <climits>
#include <iomanip>
#include <iostream>
#include <locale>
namespace StreamManip {
// Define a base manipulator type, its what the built in stream manipulators
// do when they take parameters, only they return an opaque type.
struct BaseManip
{
int mBase;
BaseManip(int base) : mBase(base)
{
assert(base >= 2);
assert(base <= 36);
}
static int getIWord()
{
// call xalloc once to get an index at which we can store data for this
// manipulator.
static int iw = std::ios_base::xalloc();
return iw;
}
void apply(std::ostream& os) const
{
// store the base value in the manipulator.
os.iword(getIWord()) = mBase;
}
};
// We need this so we can apply our custom stream manipulator to the stream.
std::ostream& operator<<(std::ostream& os, const BaseManip& bm)
{
bm.apply(os);
return os;
}
// convience function, so we can do std::cout << base(16) << 100;
BaseManip base(int b)
{
return BaseManip(b);
}
// A custom number output facet. These are used by the std::locale code in
// streams. The num_put facet handles the output of numberic values as characters
// in the stream. Here we create one that knows about our custom manipulator.
struct BaseNumPut : std::num_put<char>
{
// These absVal functions are needed as std::abs doesnt support
// unsigned types, but the templated doPutHelper works on signed and
// unsigned types.
unsigned long int absVal(unsigned long int a) const
{
return a;
}
unsigned long long int absVal(unsigned long long int a) const
{
return a;
}
template <class NumType>
NumType absVal(NumType a) const
{
return std::abs(a);
}
template <class NumType>
iter_type doPutHelper(iter_type out, std::ios_base& str, char_type fill, NumType val) const
{
// Read the value stored in our xalloc location.
const int base = str.iword(BaseManip::getIWord());
// we only want this manipulator to affect the next numeric value, so
// reset its value.
str.iword(BaseManip::getIWord()) = 0;
// normal number output, use the built in putter.
if (base == 0 || base == 10)
{
return std::num_put<char>::do_put(out, str, fill, val);
}
// We want to conver the base, so do it and output.
// Base conversion code lifted from Nawaz's answer
int digits[CHAR_BIT * sizeof(NumType)];
int i = 0;
NumType tempVal = absVal(val);
while (tempVal != 0)
{
digits[i++] = tempVal % base;
tempVal /= base;
}
// Get the format flags.
const std::ios_base::fmtflags flags = str.flags();
// Add the padding if needs by (i.e. they have used std::setw).
// Only applies if we are right aligned, or none specified.
if (flags & std::ios_base::right ||
!(flags & std::ios_base::internal || flags & std::ios_base::left))
{
std::fill_n(out, str.width() - i, fill);
}
if (val < 0)
{
*out++ = '-';
}
// Handle the internal adjustment flag.
if (flags & std::ios_base::internal)
{
std::fill_n(out, str.width() - i, fill);
}
char digitCharLc[] = "0123456789abcdefghijklmnopqrstuvwxyz";
char digitCharUc[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const char *digitChar = (str.flags() & std::ios_base::uppercase)
? digitCharUc
: digitCharLc;
while (i)
{
// out is an iterator that accepts characters
*out++ = digitChar[digits[--i]];
}
// Add the padding if needs by (i.e. they have used std::setw).
// Only applies if we are left aligned.
if (str.flags() & std::ios_base::left)
{
std::fill_n(out, str.width() - i, fill);
}
// clear the width
str.width(0);
return out;
}
// Overrides for the virtual do_put member functions.
iter_type do_put(iter_type out, std::ios_base& str, char_type fill, long val) const
{
return doPutHelper(out, str, fill, val);
}
iter_type do_put(iter_type out, std::ios_base& str, char_type fill, unsigned long val) const
{
return doPutHelper(out, str, fill, val);
}
};
} // namespace StreamManip
int main()
{
// Create a local the uses our custom num_put
std::locale myLocale(std::locale(), new StreamManip::BaseNumPut());
// Set our locacle to the global one used by default in all streams created
// from here on in. Any streams created in this app will now support the
// StreamManip::base modifier.
std::locale::global(myLocale);
// imbue std::cout, so it uses are custom local.
std::cout.imbue(myLocale);
std::cerr.imbue(myLocale);
// Output some stuff.
std::cout << std::setw(50) << StreamManip::base(2) << std::internal << -255 << std::endl;
std::cout << StreamManip::base(4) << 255 << std::endl;
std::cout << StreamManip::base(8) << 255 << std::endl;
std::cout << StreamManip::base(10) << 255 << std::endl;
std::cout << std::uppercase << StreamManip::base(16) << 255 << std::endl;
return 0;
}
Custom manipulators are indeed possible. See for example this question. I'm not familiar with any specific one for universal bases.
You really have two separate problems. The one I think you're asking about is entirely solvable. The other, unfortunately, is rather less so.
Allocating and using some space in the stream to hold some stream state is a problem that was foreseen. Streams have a couple of members (xalloc
, iword
, pword
) that let you allocate a spot in an array in the stream, and read/write data there. As such, the stream manipulator itself is entirely possible. You'd basically use xalloc
to allocate a spot in the stream's array to hold the current base, to be used by the insertion operator when it converts a number.
The problem for which I don't see a solution is rather simpler: the standard library already provides an operator<<
to insert an int
into a stream, and it obviously does not know about your hypothetical data to hold the base for a conversion. You can't overload that, because it would need exactly the same signature as the existing one, so your overload would be ambiguous.
The overloads for int
, short
, etc., however, are overloaded member functions. I guess if you wanted to badly enough, you could get by with using a template to overload operator<<
. If I recall correctly, that would be preferred over even an exact match with a non-template function as the library provides. You'd still be breaking the rules, but if you put such a template in namespace std, there's at least some chance that it would work.
I attempted to write a code, and its working with some limitations. Its not stream manipulator as such, as that is simply not possible, as pointed out by others (especially @Jerry).
Here is my code:
struct base
{
mutable std::ostream *_out;
int _value;
base(int value=10) : _value(value) {}
template<typename T>
const base& operator << (const T & data) const
{
*_out << data;
return *this;
}
const base& operator << (const int & data) const
{
switch(_value)
{
case 2:
case 4:
case 8: return print(data);
case 16: *_out << std::hex << data; break;
default: *_out << data;
}
return *this;
}
const base & print(int data) const
{
int digits[CHAR_BIT * sizeof(int)], i = 0;
while(data)
{
digits[i++] = data % _value;
data /= _value;
}
while(i) *_out << digits[--i] ;
return *this;
}
friend const base& operator <<(std::ostream& out, const base& b)
{
b._out = &out;
return b;
}
};
And this is the test code:
int main() {
std::cout << base(2) << 255 <<", " << 54 << ", " << 20<< "\n";
std::cout << base(4) << 255 <<", " << 54 << ", " << 20<< "\n";
std::cout << base(8) << 255 <<", " << 54 << ", " << 20<< "\n";
std::cout << base(16) << 255 <<", " << 54 << ", " << 20<< "\n";
}
Output:
11111111, 110110, 10100
3333, 312, 110
377, 66, 24
ff, 36, 14
Online demo : http://www.ideone.com/BWhW5
Limitations:
The base cannot be changed twice. So this would be an error:
std::cout << base(4) << 879 << base(8) << 9878 ; //error
Other manipulator cannot be used after
base
is used:std::cout << base(4) << 879 << std::hex << 9878 ; //error std::cout << std::hex << 879 << base(8) << 9878 ; //ok
std::endl
cannot be used afterbase
is used:std::cout << base(4) << 879 << std::endl ; //error //that is why I used "\n" in the test code.
I don't think that syntax is possible for arbitrary streams (using a manipulator, @gigantt linked an answer that shows some alternative non-manipulator solutions). The standard manipulators merely set options that are implemented inside the stream.
OTOH, you could certainly make this syntax work:
std::cout << base(4, 20);
Where base
is an object that provides a stream insertion operator (no need to return a temporary string
).
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