Boost.Iostreams 流无法转发 4 个构造函数参数
Boost.Iostreams stream can't forward 4 constructor arguments
我正在尝试使用 Boost 的 stream class 创建自定义 std::istream。它应该比手动创建 std::streambuf 更容易，但我遇到了障碍。
构造一个 stream 需要一个 source 对象。 stream 有一组 forwarding constructors 接受参数列表并将它们转发给源的构造函数——这就是我想要使用的。
// Forwarding constructors
template<typename U>
stream([const] U& u);
template<typename U1, typename U2>
stream([const] U1& u1, const U2& u2);
...
template<typename U1, ..., typename UN>
stream([const] U1& u1, const U2& u2, ..., const UN& uN);
Each of these members constructs an instance of stream and associates it with an instance of the Device T constructed from the given lists of arguments. The T constructors involved must take all arguments by value or const reference.
它在三个参数下效果很好：
#include <memory>
#include <boost/iostreams/stream.hpp>
namespace io = boost::iostreams;
class IoSource: public io::source {
public:
IoSource(int foo, int bar, int baz) { }
std::streamsize read(char *buffer, std::streamsize n) { return -1; }
};
int main() {
auto stream = std::make_unique<io::stream<IoSource>>(1, 2, 3);
}
结果：
$ g++ -Wall -Werror -std=gnu++20 test.cpp -o /dev/null
具有四个参数的同一程序无法编译，并出现大量模板错误：
#include <memory>
#include <boost/iostreams/stream.hpp>
namespace io = boost::iostreams;
class IoSource: public io::source {
public:
IoSource(int foo, int bar, int baz, int quux) { }
std::streamsize read(char *buffer, std::streamsize n) { return -1; }
};
int main() {
auto stream = std::make_unique<io::stream<IoSource>>(1, 2, 3, 4);
}
结果：
$ g++ -Wall -Werror -std=gnu++20 test.cpp -o /dev/null
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h: In instantiation of 'typename std::_MakeUniq<_Tp>::__single_object std::make_unique(_Args&& ...) [with _Tp = boost::iostreams::stream<IoSource>; _Args = {int, int, int, int}; typename std::_MakeUniq<_Tp>::__single_object = std::unique_ptr<boost::iostreams::stream<IoSource>, std::default_delete<boost::iostreams::stream<IoSource> > >]':
test.cpp:14:68: required from here
/usr/include/c++/10/bits/unique_ptr.h:962:30: error: no matching function for call to 'boost::iostreams::stream<IoSource>::stream(int, int, int, int)'
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(U100&, const U0&, const U1&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U100 = int; U0 = int; U1 = int; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; typename boost::disable_if<boost::is_same<U0, T> >::type = void]' (near match)
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: conversion of argument 4 would be ill-formed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: error: invalid conversion from 'int' to 'boost::disable_if_c<false, void>::type*' {aka 'void*'} [-fpermissive]
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| int
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(const U0&, const U1&, const U2&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U0 = int; U1 = int; U2 = int; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; typename boost::disable_if<boost::is_same<U0, T> >::type = void]' (near match)
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: conversion of argument 4 would be ill-formed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: error: invalid conversion from 'int' to 'boost::disable_if_c<false, void>::type*' {aka 'void*'} [-fpermissive]
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| int
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U100, class U0> boost::iostreams::stream<Device, Tr, Alloc>::stream(U100&, const U0&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U100 = U100; U0 = U0; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 3 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U0, class U1> boost::iostreams::stream<Device, Tr, Alloc>::stream(const U0&, const U1&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U0 = U0; U1 = U1; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 3 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U100> boost::iostreams::stream<Device, Tr, Alloc>::stream(U100&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U100 = U100; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 2 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U0> boost::iostreams::stream<Device, Tr, Alloc>::stream(const U0&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U0 = U0; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 2 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(const boost::reference_wrapper<T>&, std::streamsize, std::streamsize) [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; std::streamsize = long int]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate expects 3 arguments, 4 provided
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(Device&, std::streamsize, std::streamsize) [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; std::streamsize = long int]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate expects 3 arguments, 4 provided
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(const Device&, std::streamsize, std::streamsize) [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; std::streamsize = long int]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate expects 3 arguments, 4 provided
In file included from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:143:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream() [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
143 | stream() { }
| ^~~~~~
/usr/include/boost/iostreams/stream.hpp:143:5: note: candidate expects 0 arguments, 4 provided
我做错了什么？我相信我已经将其缩小到核心问题。这似乎纯粹是参数数量的问题；它们是值还是 const 引用并不重要，它们的类型也不重要。
- 提升 1.78
- g++ (Debian 10.2.1-6) 10.2.1 20210110
Boost 不使用 variadic templates 和完美转发来转发参数。它使用预处理器宏来模拟可变参数，具有最大数量的宏：
boost/iostreams/detail/config/limits.hpp
#ifndef BOOST_IOSTREAMS_MAX_FORWARDING_ARITY
# define BOOST_IOSTREAMS_MAX_FORWARDING_ARITY 3
#endif
在包含 Boost 之前定义上面的宏可以增加参数的数量 headers:
#define BOOST_IOSTREAMS_MAX_FORWARDING_ARITY 4
#include <boost/iostreams/stream.hpp>
我正在尝试使用 Boost 的 stream class 创建自定义 std::istream。它应该比手动创建 std::streambuf 更容易，但我遇到了障碍。
构造一个 stream 需要一个 source 对象。 stream 有一组 forwarding constructors 接受参数列表并将它们转发给源的构造函数——这就是我想要使用的。
// Forwarding constructors template<typename U> stream([const] U& u); template<typename U1, typename U2> stream([const] U1& u1, const U2& u2); ... template<typename U1, ..., typename UN> stream([const] U1& u1, const U2& u2, ..., const UN& uN);Each of these members constructs an instance of
streamand associates it with an instance of the DeviceTconstructed from the given lists of arguments. TheTconstructors involved must take all arguments by value orconstreference.
它在三个参数下效果很好：
#include <memory>
#include <boost/iostreams/stream.hpp>
namespace io = boost::iostreams;
class IoSource: public io::source {
public:
IoSource(int foo, int bar, int baz) { }
std::streamsize read(char *buffer, std::streamsize n) { return -1; }
};
int main() {
auto stream = std::make_unique<io::stream<IoSource>>(1, 2, 3);
}
结果：
$ g++ -Wall -Werror -std=gnu++20 test.cpp -o /dev/null
具有四个参数的同一程序无法编译，并出现大量模板错误：
#include <memory>
#include <boost/iostreams/stream.hpp>
namespace io = boost::iostreams;
class IoSource: public io::source {
public:
IoSource(int foo, int bar, int baz, int quux) { }
std::streamsize read(char *buffer, std::streamsize n) { return -1; }
};
int main() {
auto stream = std::make_unique<io::stream<IoSource>>(1, 2, 3, 4);
}
结果：
$ g++ -Wall -Werror -std=gnu++20 test.cpp -o /dev/null
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h: In instantiation of 'typename std::_MakeUniq<_Tp>::__single_object std::make_unique(_Args&& ...) [with _Tp = boost::iostreams::stream<IoSource>; _Args = {int, int, int, int}; typename std::_MakeUniq<_Tp>::__single_object = std::unique_ptr<boost::iostreams::stream<IoSource>, std::default_delete<boost::iostreams::stream<IoSource> > >]':
test.cpp:14:68: required from here
/usr/include/c++/10/bits/unique_ptr.h:962:30: error: no matching function for call to 'boost::iostreams::stream<IoSource>::stream(int, int, int, int)'
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(U100&, const U0&, const U1&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U100 = int; U0 = int; U1 = int; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; typename boost::disable_if<boost::is_same<U0, T> >::type = void]' (near match)
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: conversion of argument 4 would be ill-formed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: error: invalid conversion from 'int' to 'boost::disable_if_c<false, void>::type*' {aka 'void*'} [-fpermissive]
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| int
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(const U0&, const U1&, const U2&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U0 = int; U1 = int; U2 = int; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; typename boost::disable_if<boost::is_same<U0, T> >::type = void]' (near match)
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: conversion of argument 4 would be ill-formed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: error: invalid conversion from 'int' to 'boost::disable_if_c<false, void>::type*' {aka 'void*'} [-fpermissive]
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| int
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U100, class U0> boost::iostreams::stream<Device, Tr, Alloc>::stream(U100&, const U0&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U100 = U100; U0 = U0; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 3 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U0, class U1> boost::iostreams::stream<Device, Tr, Alloc>::stream(const U0&, const U1&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U0 = U0; U1 = U1; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 3 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U100> boost::iostreams::stream<Device, Tr, Alloc>::stream(U100&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U100 = U100; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 2 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/preprocessor/tuple/elem.hpp:23,
from /usr/include/boost/preprocessor/repetition/enum_binary_params.hpp:19,
from /usr/include/boost/iostreams/detail/forward.hpp:22,
from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'template<class U0> boost::iostreams::stream<Device, Tr, Alloc>::stream(const U0&, typename boost::disable_if<boost::is_same<U0, T> >::type*) [with U0 = U0; Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/10/memory:83,
from test.cpp:1:
/usr/include/c++/10/bits/unique_ptr.h:962:30: note: candidate expects 2 arguments, 4 provided
962 | { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/iostreams/stream.hpp:18,
from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(const boost::reference_wrapper<T>&, std::streamsize, std::streamsize) [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; std::streamsize = long int]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate expects 3 arguments, 4 provided
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(Device&, std::streamsize, std::streamsize) [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; std::streamsize = long int]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate expects 3 arguments, 4 provided
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream(const Device&, std::streamsize, std::streamsize) [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>; std::streamsize = long int]'
144 | BOOST_IOSTREAMS_FORWARD( stream, open_impl, Device,
| ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/iostreams/stream.hpp:144:5: note: candidate expects 3 arguments, 4 provided
In file included from test.cpp:3:
/usr/include/boost/iostreams/stream.hpp:143:5: note: candidate: 'boost::iostreams::stream<Device, Tr, Alloc>::stream() [with Device = IoSource; Tr = std::char_traits<char>; Alloc = std::allocator<char>]'
143 | stream() { }
| ^~~~~~
/usr/include/boost/iostreams/stream.hpp:143:5: note: candidate expects 0 arguments, 4 provided
我做错了什么？我相信我已经将其缩小到核心问题。这似乎纯粹是参数数量的问题；它们是值还是 const 引用并不重要，它们的类型也不重要。
- 提升 1.78
- g++ (Debian 10.2.1-6) 10.2.1 20210110
Boost 不使用 variadic templates 和完美转发来转发参数。它使用预处理器宏来模拟可变参数，具有最大数量的宏：
boost/iostreams/detail/config/limits.hpp
#ifndef BOOST_IOSTREAMS_MAX_FORWARDING_ARITY # define BOOST_IOSTREAMS_MAX_FORWARDING_ARITY 3 #endif
在包含 Boost 之前定义上面的宏可以增加参数的数量 headers:
#define BOOST_IOSTREAMS_MAX_FORWARDING_ARITY 4
#include <boost/iostreams/stream.hpp>