C++ C++；Boost:：variant使用Boost:：get对我的用户定义类型出错

我试图通过使用位于（）的Boost基本用法在线教程来学习如何使用Boost:：variant。为了掌握诀窍，我试着从教程（下面）中获取一些代码

#包括“boost/variant.hpp”
#包括
#包括
无效时间（boost:：variant&v）{
if（int*pi=boost:：get（&v））
*pi*=2；
else if（std:：string*pstr=boost:：get（&v））
*pstr+=*pstr；
//不检查复杂-当变量复杂（错误）时将静默返回。
}
int main（）{
boost：：变体v；
v=“你好”；
std:：string&str=boost:：get（v）；
str+=“世界！”；
//v现在包含的std:：string等于“hello world！”。同样，我们可以通过将v流式传输到标准输出来演示这一点：
std:：cout@chris的猜测是正确的，这是因为缺少运算符，可能是您的类型没有重载
运算符或者从未分配用户定义类型“structy”的内容对于我的变量，我当然没有尝试在其上使用流运算符或尝试打印它，因此我不认为为structy重载流运算符对其进行编译是必要的。我怀疑这是因为您输出了变量。请解释它是如何工作的。因此，您必须定义为structy调用的所有函数和运算符n变量，即使该特定类型（structy）中的变量从未显式使用它们（ostream）？如果您给出答案而不是注释，那就好了。我不知道。我从未使用过，也几乎没有读过该特定库。
#include "boost/variant.hpp"
#include <iostream>
#include <complex>

void times_two(boost::variant<int, std::string, std::complex<double> > & v) {
    if (int* pi = boost::get<int>(&v))
        *pi *= 2;
    else if (std::string* pstr = boost::get<std::string>(&v))
        *pstr += *pstr;
    //No check for complex - will silently return when variant is complex (BAD).
}

int main() {
    boost::variant<int, std::string, std::complex<double> > v;
    v = "hello";
    std::string& str = boost::get<std::string>(v);
    str += " world! ";
//the std::string contained by v now is equal to "hello world! ". Again, we can demonstrate this by streaming v to standard output:
    std::cout << v << std::endl;
    times_two(v);
    std::cout << v << std::endl;
    std::complex<double> a(5.0, 6.0);
    v = a;
    times_two(v);
    return 0;
}

#include "boost/variant.hpp"
#include <iostream>
#include <complex>

struct structy {
    int a;
};

void times_two(
        boost::variant<int, std::string, std::complex<double>, structy> & v) {
    if (int* pi = boost::get<int>(&v))
        *pi *= 2;
    else if (std::string* pstr = boost::get<std::string>(&v))
        *pstr += *pstr;
    //No check for complex or structy - should silently returns.
}

int main() {

    boost::variant<int, std::string, std::complex<double>, structy> v;
    v = "hello";
    std::string& str = boost::get<std::string>(v);
    str += " world! ";
//As desired, the std::string contained by v now is equal to "hello world! ". Again, we can demonstrate this by streaming v to standard output:
    std::cout << v << std::endl;
    times_two(v);
    std::cout << v << std::endl;
    std::complex<double> a(5.0, 6.0);
    v = a;
    times_two(v);
    return 0;
}

johnmichaelreed@Ubuntu1204LTS:~/Downloads/boost_1_54_0$ g++ ~/temp.cpp -o temp 
In file included from /usr/local/include/boost/variant/variant.hpp:2416:0,
             from /usr/local/include/boost/variant.hpp:17,
             from /home/johnmichaelreed/temp.cpp:1:
/usr/local/include/boost/variant/detail/variant_io.hpp: In member function ‘void boost::detail::variant::printer<OStream>::operator()(const T&) const [with T = structy, OStream = std::basic_ostream<char>]’:
/usr/local/include/boost/variant/variant.hpp:1017:32:   instantiated from ‘boost::detail::variant::invoke_visitor<Visitor>::result_type boost::detail::variant::invoke_visitor<Visitor>::internal_visit(T&, int) [with T = const structy, Visitor = boost::detail::variant::printer<std::basic_ostream<char> >, boost::detail::variant::invoke_visitor<Visitor>::result_type = void]’
/usr/local/include/boost/variant/detail/visitation_impl.hpp:130:9:   instantiated from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke_impl(int, Visitor&, VoidPtrCV, T*, mpl_::true_) [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >, VoidPtrCV = const void*, T = structy, typename Visitor::result_type = void, mpl_::true_ = mpl_::bool_<true>]’
/usr/local/include/boost/variant/detail/visitation_impl.hpp:173:9:   instantiated from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke(int, Visitor&, VoidPtrCV, T*, NoBackupFlag, int) [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >, VoidPtrCV = const void*, T = structy, NoBackupFlag = boost::variant<int, std::basic_string<char>, std::complex<double>, structy>::has_fallback_type_, typename Visitor::result_type = void]’
/usr/local/include/boost/variant/detail/visitation_impl.hpp:260:1:   instantiated from ‘typename Visitor::result_type boost::detail::variant::visitation_impl(int, int, Visitor&, VoidPtrCV, mpl_::false_, NoBackupFlag, Which*, step0*) [with Which = mpl_::int_<0>, step0 = boost::detail::variant::visitation_impl_step<boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<4l>, int, boost::mpl::l_item<mpl_::long_<3l>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<2l>, std::complex<double>, boost::mpl::l_item<mpl_::long_<1l>, structy, boost::mpl::l_end> > > > >, boost::mpl::l_iter<boost::mpl::l_end> >, Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >, VoidPtrCV = const void*, NoBackupFlag = boost::variant<int, std::basic_string<char>, std::complex<double>, structy>::has_fallback_type_, typename Visitor::result_type = void, mpl_::false_ = mpl_::bool_<false>]’
/usr/local/include/boost/variant/variant.hpp:2326:13:   instantiated from ‘static typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::internal_apply_visitor_impl(int, int, Visitor&, VoidPtrCV) [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >, VoidPtrCV = const void*, T0_ = int, T1 = std::basic_string<char>, T2 = std::complex<double>, T3 = structy, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_, typename Visitor::result_type = void]’
/usr/local/include/boost/variant/variant.hpp:2348:13:   instantiated from ‘typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::internal_apply_visitor(Visitor&) const [with Visitor = boost::detail::variant::invoke_visitor<boost::detail::variant::printer<std::basic_ostream<char> > >, T0_ = int, T1 = std::basic_string<char>, T2 = std::complex<double>, T3 = structy, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_, typename Visitor::result_type = void]’
/usr/local/include/boost/variant/variant.hpp:2370:52:   instantiated from ‘typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::apply_visitor(Visitor&) const [with Visitor = boost::detail::variant::printer<std::basic_ostream<char> >, T0_ = int, T1 = std::basic_string<char>, T2 = std::complex<double>, T3 = structy, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_, typename Visitor::result_type = void]’
/usr/local/include/boost/variant/detail/variant_io.hpp:88:5:   instantiated from ‘std::basic_ostream<E, T>& boost::operator<<(std::basic_ostream<E, T>&, const boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&) [with E = char, T = std::char_traits<char>, U0 = int, U1 = std::basic_string<char>, U2 = std::complex<double>, U3 = structy, U4 = boost::detail::variant::void_, U5 = boost::detail::variant::void_, U6 = boost::detail::variant::void_, U7 = boost::detail::variant::void_, U8 = boost::detail::variant::void_, U9 = boost::detail::variant::void_, U10 = boost::detail::variant::void_, U11 = boost::detail::variant::void_, U12 = boost::detail::variant::void_, U13 = boost::detail::variant::void_, U14 = boost::detail::variant::void_, U15 = boost::detail::variant::void_, U16 = boost::detail::variant::void_, U17 = boost::detail::variant::void_, U18 = boost::detail::variant::void_, U19 = boost::detail::variant::void_]’
/home/johnmichaelreed/temp.cpp:25:16:   instantiated from here
/usr/local/include/boost/variant/detail/variant_io.hpp:64:9: error: no match for ‘operator<<’ in ‘((const boost::detail::variant::printer<std::basic_ostream<char> >*)this)->boost::detail::variant::printer<std::basic_ostream<char> >::out_ << operand’
/usr/local/include/boost/variant/detail/variant_io.hpp:64:9: note: candidates are:
/usr/include/c++/4.6/ostream:110:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:110:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&) {aka std::basic_ostream<char>& (*)(std::basic_ostream<char>&)}’
/usr/include/c++/4.6/ostream:119:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>, std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
/usr/include/c++/4.6/ostream:119:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}’
/usr/include/c++/4.6/ostream:129:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:129:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/4.6/ostream:167:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:167:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘long int’
/usr/include/c++/4.6/ostream:171:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:171:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘long unsigned int’
/usr/include/c++/4.6/ostream:175:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:175:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘bool’
/usr/include/c++/4.6/bits/ostream.tcc:93:5: note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/bits/ostream.tcc:93:5: note:   no known conversion for argument 1 from ‘const structy’ to ‘short int’
/usr/include/c++/4.6/ostream:182:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:182:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘short unsigned int’
/usr/include/c++/4.6/bits/ostream.tcc:107:5: note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/bits/ostream.tcc:107:5: note:   no known conversion for argument 1 from ‘const structy’ to ‘int’
/usr/include/c++/4.6/ostream:193:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:193:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘unsigned int’
/usr/include/c++/4.6/ostream:202:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:202:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘long long int’
/usr/include/c++/4.6/ostream:206:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:206:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘long long unsigned int’
/usr/include/c++/4.6/ostream:211:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:211:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘double’
/usr/include/c++/4.6/ostream:215:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:215:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘float’
/usr/include/c++/4.6/ostream:223:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:223:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘long double’
/usr/include/c++/4.6/ostream:227:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:227:7: note:   no known conversion for argument 1 from ‘const structy’ to ‘const void*’
/usr/include/c++/4.6/bits/ostream.tcc:121:5: note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
/usr/include/c++/4.6/bits/ostream.tcc:121:5: note:   no known conversion for argument 1 from ‘const structy’ to ‘std::basic_ostream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}’
/usr/local/include/boost/blank.hpp:93:46: note: template<class E, class T> std::basic_ostream<_CharT, _Traits>& boost::operator<<(std::basic_ostream<_CharT, _Traits>&, const boost::blank&)
/usr/local/include/boost/variant/detail/variant_io.hpp:79:46: note: template<class E, class T, class U0, class U1, class U2, class U3, class U4, class U5, class U6, class U7, class U8, class U9, class U10, class U11, class U12, class U13, class U14, class U15, class U16, class U17, class U18, class U19> std::basic_ostream<E, T>& boost::operator<<(std::basic_ostream<E, T>&, const boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&)
/usr/include/c++/4.6/complex:521:5: note: template<class _Tp, class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::complex<_Tp>&)
/usr/include/c++/4.6/ostream:528:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)
/usr/include/c++/4.6/ostream:523:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)
/usr/include/c++/4.6/ostream:510:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)
/usr/include/c++/4.6/bits/ostream.tcc:323:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)
/usr/include/c++/4.6/ostream:493:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)
/usr/include/c++/4.6/ostream:473:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)
/usr/include/c++/4.6/ostream:468:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)
/usr/include/c++/4.6/ostream:462:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)
/usr/include/c++/4.6/ostream:456:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)
/usr/include/c++/4.6/ostream:451:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)
/usr/include/c++/4.6/bits/basic_string.h:2693:5: note: template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
johnmichaelreed@Ubuntu1204LTS:~/Downloads/boost_1_54_0$