c++ - 流 cout 设置标志 std::ios_base::skipws 有什么原因吗？

Question

cout 默认设置了标志 std::ios_base::skipws 和 std::ios_base::dec

您可以使用代码验证这一点:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    ios_base::fmtflags flags = cout.flags();
    string sflags;

    if( flags & ios_base::skipws ) sflags += "skipws";
    if( flags & ios_base::unitbuf ) sflags += sflags.empty() ? "unitbuf" : " unitbuf";
    if( flags & ios_base::uppercase ) sflags += sflags.empty() ? "uppercase" : " uppercase";
    if( flags & ios_base::showbase ) sflags += sflags.empty() ? "showbase" : " showbase";
    if( flags & ios_base::showpoint ) sflags += sflags.empty() ? "showpoint" : " showpoint";
    if( flags & ios_base::showpos ) sflags += sflags.empty() ? "showpos" : " showpos";
    if( flags & ios_base::left ) sflags += sflags.empty() ? "left" : " left";
    if( flags & ios_base::right ) sflags += sflags.empty() ? "right" : " right";
    if( flags & ios_base::internal ) sflags += sflags.empty() ? "internal" : " internal";
    if( flags & ios_base::dec ) sflags += sflags.empty() ? "dec" : " dec";
    if( flags & ios_base::oct ) sflags += sflags.empty() ? "oct" : " oct";
    if( flags & ios_base::hex ) sflags += sflags.empty() ? "hex" : " hex";
    if( flags & ios_base::scientific ) sflags += sflags.empty() ? "scientific" : " scientific";
    if( flags & ios_base::fixed ) sflags += sflags.empty() ? "fixed" : " fixed";
    if( flags & ios_base::hexfloat ) sflags += sflags.empty() ? "hexfloat" : " hexfloat";
    if( flags & ios_base::boolalpha ) sflags += sflags.empty() ? "boolalpha" : " boolalpha";
    if( flags & ios_base::_Stdio ) sflags += sflags.empty() ? "_Stdio" : " _Stdio";

    cout << "Standard flags from cout stream: " << sflags << endl;
}

很明显，标志 std::ios_base::skipws 与 cout 无关。

Answer 1

标志及其默认设置继承自 std::ios_base (好吧，实际上，设置是为 std::basic_ios<cT, Traits> 定义的；默认值在 27.5.5.2 [basic.ios.cons] 中定义)，输入和输出流的公共(public)基类。如果流从输入流和输出流继承，则标志是共享的。还有其他标志对输入或输出流没有多大意义。