我可以使std::ostream对象以十六进制形式输出整数
std::cout << std::hex << 0xabc; //prints `abc`, not the base-10 representation
是否有适用于所有基地的通用机械手？就像是
std::cout << std::base(4) << 20; //I want this to output 110
如果有一个，那么我没有其他问题了。
如果没有，那我可以写一个吗？是否不需要我访问
std::ostream的私有(private)实现详细信息？请注意，我知道我可以编写一个接受数字并将其转换为字符串的函数，该字符串以任何基数表示该数字。或者，我可以使用已经存在的一种。我问的是自定义流操纵器-它们可能吗？
提前致谢
最佳答案
您可以执行以下操作。我已经对代码进行了注释，以解释每个部分的功能，但本质上是这样的:
xalloc 和 iword 在流中存储一些数据。 num_put 构面，该构面将查找您的操纵器并应用操纵。 这是代码...
编辑:请注意，我不确定我在这里是否正确处理了
std::ios_base::internal标志-因为我实际上不知道其用途。编辑2:我了解了
std::ios_base::internal的用途，并更新了代码以对其进行处理。编辑3:添加了对
std::locacle::global的调用，以显示默认情况下如何使所有标准流类支持新的流操纵器，而不必对它们进行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, 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;
}
关于c++ - 自定义流操纵器，用于以任意基数流式传输整数，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/6478745/