哪個自吹自擂的操縱者是“粘性”的？最近我在創建一個stringstream因為我錯誤地假設std::setw()會影響每個插入的字符串流，直到我顯式地更改它。但是，它總是在插入之后被取消設置。// With timestruct with value of 'Oct 7 9:04 AM'std::stringstream ss;ss.fill('0');
ss.setf(ios::right, ios::adjustfield);ss << setw(2) << timestruct.tm_mday;ss << timestruct.tm_hour;
ss << timestruct.tm_min;std::string filingTime = ss.str(); // BAD: '0794'所以，我有幾個問題：為什么setw()這邊請?還有其他操縱者這樣嗎？在行為上有什么區別嗎？std::ios_base::width()和std::setw()?最后，是否有一個在線參考可以清楚地記錄這種行為？我的供應商文檔(MSVisualStudio 2005)似乎沒有清楚地顯示這一點。
3 回答
GCT1015
TA貢獻1827條經驗 獲得超4個贊
@Chareles：那么根據這個要求，所有的機械手都是粘性的。除了SET，它似乎在使用后被重置。
一點兒沒錯!而setw似乎行為不同的唯一原因是，格式化的輸出操作需要顯式地.Width(0)輸出流。
setiosflags resetiosflags setbase setfill setprecision setw
setw
setiosflags: Stickyresetiosflags:Stickysetbase: Stickysetfill: Stickysetprecision: Sticky
[no]boolalpha[no]showbase[no]showpoint[no]showpos[no]skipws[no]unitbuf[no]uppercase dec/ hex/ oct fixed/ scientific internal/ left/ right
ws/ endl/ ends/ flush
#include <iostream>
#include <iomanip>
// Private object constructed by the format object PutSquareBracket
struct SquareBracktAroundNextItem
{
SquareBracktAroundNextItem(std::ostream& str)
:m_str(str)
{}
std::ostream& m_str;
};
// New Format Object
struct PutSquareBracket
{};
// Format object passed to stream.
// All it does is return an object that can maintain state away from the
// stream object (so that it is not STICKY)
SquareBracktAroundNextItem operator<<(std::ostream& str,PutSquareBracket const& data)
{
return SquareBracktAroundNextItem(str);
}
// The Non Sticky formatting.
// Here we temporariy set formating to fixed with a precision of 10.
// After the next value is printed we return the stream to the original state
// Then return the stream for normal processing.
template<typename T>
std::ostream& operator<<(SquareBracktAroundNextItem const& bracket,T const& data)
{
std::ios_base::fmtflags flags = bracket.m_str.flags();
std::streamsize currentPrecision = bracket.m_str.precision();
bracket.m_str << '[' << std::fixed << std::setprecision(10) << data << std::setprecision(currentPrecision) << ']';
bracket.m_str.flags(flags);
return bracket.m_str;
}
int main()
{
std::cout << 5.34 << "\n" // Before
<< PutSquareBracket() << 5.34 << "\n" // Temp change settings.
<< 5.34 << "\n"; // After
}
> ./a.out
5.34
[5.3400000000]
5.34
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