boost::iostreams::counter 似乎不起作用

Question

我正在玩 boost::iostreams，用户指南谈到了过滤器“计数器”。所以我用这段代码试试：

#include <iostream>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/device/null.hpp>
#include <boost/iostreams/filter/counter.hpp>
namespace io = boost::iostreams;
int main()
{
    io::counter cnt;
    io::filtering_ostream out(cnt | io::null_sink());
    out << "hello";
    std::cout << cnt.lines() << " " << cnt.characters() << std::endl;
}

它总是给予

0 0

这似乎不是我所期望的。 使用 gdb 进行的初步跟踪表明正在进行计数的计数器对象具有与对象“cnt”不同的地址。我想这是管道中的某种复制？如果是这样，过滤“计数器”有什么用？

Answer 1

查看the documentation，您可以使用任一：

#include <iostream>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/device/null.hpp>
#include <boost/iostreams/filter/counter.hpp>
namespace io = boost::iostreams;
int main()
{
    io::counter cnt;
    io::filtering_ostream out(cnt | io::null_sink());
    out << "hello";
    std::cout << out.component<io::counter>(0)->lines() << " " << out.component<io::counter>(0)->characters() << std::endl;
}

或：

#include <iostream>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/device/null.hpp>
#include <boost/iostreams/filter/counter.hpp>
#include <boost/ref.hpp>
namespace io = boost::iostreams;
int main()
{
    io::counter cnt;
    io::filtering_ostream out;
    out.push(boost::ref(cnt));
    out.push(io::null_sink());
    out << "hello";
    std::cout << cnt.lines() << " " << cnt.characters() << std::endl;
}