自定义流操纵器用于流在任何基地的整数 [英] Custom stream manipulator for streaming integers in any base
问题描述
我可以使用十六进制形式的 std :: ostream
对象输出整数,例如
std :: cout<< std :: hex<< 0xabc; //打印`abc`,而不是base-10表示
所有基地?像
std :: cout< std :: base(4)<< 20; //我想要输出110
如果有一个,那么我没有进一步的问题。
如果没有一个,那么我可以写一个吗?是否需要我访问 std :: ostream
?
的私有实现细节注意,我知道我可以写一个函数,它接受一个数字,并将其转换为一个字符串,它是任何基数中该数字的表示。或者我可以使用已经存在的。
提前感谢
您可以执行以下操作。我已经评论了代码来解释每个部分正在做什么,但基本上它的:
这是代码...
编辑:请注意,我不知道我在这里正确处理 std :: ios_base :: internal
标记 - 知道它的用途。
编辑2:我发现了 std :: ios_base :: internal $
的调用 std :: locacle :: global
以显示如何使所有标准流类默认支持新的流操纵器,而不是 imbue
#include< algorithm>
#include< cassert>
#include< climits>
#include< iomanip>
#include< iostream>
#include< locale>
命名空间StreamManip {
//定义一个基本操纵器类型,它是什么内置的流操纵器
//当他们采取参数,只有他们返回不透明型。
struct BaseManip
{
int mBase;
BaseManip(int base):mBase(base)
{
assert(base> = 2);
assert(base< = 36);
}
static int getIWord()
{
//调用xalloc一次以获得一个索引,我们可以为此存储数据
//操纵器。
static int iw = std :: ios_base :: xalloc();
return iw;
}
void apply(std :: ostream& os)const
{
//存储操作器中的基本值。
os.iword(getIWord())= mBase
}
};
//我们需要这个,所以我们可以应用我们的自定义流操纵器到流。
std :: ostream& operator<<<(std :: ostream& os,const BaseManip& bm)
{
bm.apply(os);
return os;
}
// convience函数,所以我们可以做std :: cout<<基底(16) 100;
BaseManip base(int b)
{
return BaseManip(b);
}
//自定义输出构面。这些由
// streams中的std :: locale代码使用。 num_put构面将流中的数字值的输出处理为字符
//。在这里我们创建一个知道我们的自定义操纵器。
struct BaseNumPut:std :: num_put< char>
{
//这些absVal函数需要std :: abs不支持
// unsigned类型,但是模板化的doPutHelper适用于signed和
// unsigned类型。
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
{
//读取存储在我们的xalloc位置的值。
const int base = str.iword(BaseManip :: getIWord());
//我们只希望这个操纵器影响下一个数值,所以
//重置它的值。
str.iword(BaseManip :: getIWord())= 0;
//正常数字输出,使用内置推杆。
if(base == 0 || base == 10)
{
return std :: num_put< char> :: do_put(out,str,fill,val);
}
//我们想要收敛基数,所以做它和输出。
//从Nawaz的回答中提取的基本转换代码
int digits [CHAR_BIT * sizeof(NumType)];
int i = 0;
NumType tempVal = absVal(val);
while(tempVal!= 0)
{
digits [i ++] = tempVal%base;
tempVal / = base;
}
//获取格式标志。
const std :: ios_base :: fmtflags flags = str.flags();
//如果需要,添加填充(即它们使用std :: setw)。
//仅适用于我们是右对齐,或没有指定。
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 ++ =' - ';
}
//处理内部调整标志。
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是一个迭代器,接受字符
* out ++ = digitChar [digits [ - i]];
}
//如果需要,添加填充(即它们使用std :: setw)。
//仅适用于左对齐。
if(str.flags()& std :: ios_base :: left)
{
std :: fill_n(out,str.width() - i,fill);
}
//清除宽度
str.width(0);
return out;
}
//覆盖虚拟do_put成员函数。
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,填充,val);
}
};
} //命名空间StreamManip
int main()
{
//创建本地使用我们的自定义num_put
std: :locale myLocale(std :: locale(),new StreamManip :: BaseNumPut());
//将我们的locacle设置为默认情况下在创建的所有流中使用的全局变量
//从这里开始。在此应用程序中创建的任何流都将支持
/ / StreamManip :: base修饰符。
std :: locale :: global(myLocale);
// imbue std :: cout,所以它使用的是自定义本地。
std :: cout.imbue(myLocale);
std :: cerr.imbue(myLocale);
//输出一些东西。
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;
}
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 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?
Thanks in advance
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;
}
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