showbase和stringstream的前导零

Question

Here is the code:

std::wstringstream wss;    

wss.setf(std::ios_base::hex, std::ios_base::basefield);

wss.setf(std::ios_base::showbase);
// wss << std::showbase;

// wss.width(2);
wss.fill('0');

wss << std::setw(2) << 7;
// wss << std::noshowbase;
wss.unsetf(std::ios_base::showbase);
wss << std::setw(2) << 0;
wss << std::setw(2) << 20;
wss << std::setw(2) << 6;

auto ret = wss.str();

If I set showbase for the stream, I always get below output:
0x7001406 , instead of 0x07001406

How can I get this zero before 7? I don't want to set the prefix 0x manually like wss << "0x" .

Thanks in advance!!

Answer 1

Problem is: the prefix is part of the output width! Try wss << std::setw(4) << 7; for comparison (you now get 00x7 , which still is unwanted...).

Unfortunately, you cannot use precision for integers to get a behaviour equivalent to printf("%#.2x\\n", 7); , which obviously is what you intend...

My personal variant is having my own converter:

template <typename T>
struct Hex
{
    Hex(T value, size_t width) : value(value), width(width) { }
private:
    T value;
    size_t width;

    template <typename Stream>
    friend Stream& operator<<(Stream& s, Hex h)
    {
        auto fill = s.fill();
        auto flags = s.flags();
        s.fill('0');
        s << "0x" << std::noshowbase << std::hex << std::setw(h.width) << h.value;
        s.fill(fill);
        s.flags(flags);
        return s;

    }
};
template <typename T>
auto hex(T t, size_t width = sizeof(T) * 2) { return Hex<T>(t, width); }

You now can use it as:

wss << hex(7, 2);

getting even shorter than having wss << std::setw(2) << 7; and coming with nice default appropriate to size of the type...

One minor drawback yet: you need specialisations or overloads for signed and unsigned char, as for these, the character representation ( 0x0s ) is output instead of numerical value ( 0x73 ).

auto hex(char t, size_t width = sizeof(char) * 2)
{ return Hex<unsigned int>(t, width); }
auto hex(signed char t, size_t width = sizeof(signed char) * 2)
{ return Hex<signed int>(t, width); }
auto hex(unsigned char t, size_t width = sizeof(unsigned char) * 2)
{ return Hex<unsigned int>(t, width); }

You might replace 2 with CHAR_BIT / 4 in the default, depending on your needs/desires, might cover systems having eg CHAR_BIT == 16 better...

Answer 2

Thanks @Aconcagua for your clue!

I think we can use std::ios_base::adjustfield and std::ios_base::internal to do it like this:

wss.setf(std::ios_base::hex, std::ios_base::basefield);
int oldFlag = wss.setf(std::ios_base::internal, std::ios_base::adjustfield);

Then

wss.setf(std::ios_base::showbase);
wss.fill('0');

wss << std::setw(4) << 7;
wss.unsetf(std::ios_base::showbase);
//  wss.setf(oldFlag);

wss << std::setw(2) << 0;
wss << std::setw(2) << 20;
wss << std::setw(2) << 6;

Then I get 0x07001406. Correct me if I cannot do it like this, thanks!