如何解决自定义流操纵器，用于以任意基数流式传输整数
| 我可以使一个“ 0”对象以十六进制输出整数std::cout << std::hex << 0xabc; //prints `abc`,not the base-10 representation
std::cout << std::base(4) << 20; //I want this to output 110
解决方法
您可以执行以下操作。我已对代码进行了注释，以解释每个部分的功能，但本质上是这样的：
创建一个\“ manipulator \”结构，使用
xalloc
iword
num_put
std::ios_base::internal
std::locacle::global
imbue
#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,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,fill);
}
// clear the width
str.width(0);
return out;
}
// Overrides for the virtual do_put member functions.
iter_type do_put(iter_type out,long val) const
{
return doPutHelper(out,val);
}
iter_type do_put(iter_type out,unsigned long val) const
{
return doPutHelper(out,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;
}
xalloc
iword
pword
int
short
operator<<
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;
}
};
int main() {
std::cout << base(2) << 255 <<\",\" << 54 << \",\" << 20<< \"\\n\";
std::cout << base(4) << 255 <<\",\" << 20<< \"\\n\";
std::cout << base(8) << 255 <<\",\" << 20<< \"\\n\";
std::cout << base(16) << 255 <<\",\" << 20<< \"\\n\";
}
11111111,110110,10100
3333,312,110
377,66,24
ff,36,14
std::cout << base(4) << 879 << base(8) << 9878 ; //error
std::cout << base(4) << 879 << std::hex << 9878 ; //error
std::cout << std::hex << 879 << base(8) << 9878 ; //ok
base
std::endl
std::cout << base(4) << 879 << std::endl ; //error
//that is why I used \"\\n\" in the test code.
std::cout << base(4,20);
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