问题
I have the following code:
int MimeDocument::GetAttachmentId( std::string const& content_id )
{
using namespace boost::lambda;
using boost::lambda::_1;
using boost::bind;
int id = 0;
std::vector<std::string>::iterator it =
std::find_if( attachment_list_.begin(), attachment_list_.end(),
bind( &std::string::find, content_id, _1 ) != std::string::npos
);
if( it != attachment_list_.end() ) {
id = std::distance( attachment_list_.begin(), it );
}
return id;
}
Which when compiled on MSVC9 SP1 results in tons of C2780 compiler errors. Here are just a few from the top of the list:
1>c:\code\work\cmake-mds\server\gmmserver\domino\server\interface\dimime.cpp(210) : error C2780: 'boost::_bi::bind_t<_bi::dm_result<MT::* ,A1>::type,boost::_mfi::dm<M,T>,_bi::list_av_1<A1>::type> boost::bind(M T::* ,A1)' : expects 2 arguments - 3 provided
1> c:\code\work\cmake-mds\build-vc9\third_party\boost\1.48.0\include\boost\bind\bind.hpp(1728) : see declaration of 'boost::bind'
1>c:\code\work\cmake-mds\server\gmmserver\domino\server\interface\dimime.cpp(210) : error C2780: 'boost::_bi::bind_t<Rt2,boost::_mfi::cmf8<R,T,B1,B2,B3,B4,B5,B6,B7,B8>,_bi::list_av_9<A1,A2,A3,A4,A5,A6,A7,A8,A9>::type> boost::bind(boost::type<T>,R (__thiscall T::* )(B1,B2,B3,B4,B5,B6,B7,B8) const,A1,A2,A3,A4,A5,A6,A7,A8,A9)' : expects 11 arguments - 3 provided
1> c:\code\work\cmake-mds\build-vc9\third_party\boost\1.48.0\include\boost\bind\bind_mf2_cc.hpp(223) : see declaration of 'boost::bind'
1>c:\code\work\cmake-mds\server\gmmserver\domino\server\interface\dimime.cpp(210) : error C2780: 'boost::_bi::bind_t<Rt2,boost::_mfi::mf8<R,T,B1,B2,B3,B4,B5,B6,B7,B8>,_bi::list_av_9<A1,A2,A3,A4,A5,A6,A7,A8,A9>::type> boost::bind(boost::type<T>,R (__thiscall T::* )(B1,B2,B3,B4,B5,B6,B7,B8),A1,A2,A3,A4,A5,A6,A7,A8,A9)' : expects 11 arguments - 3 provided
1> c:\code\work\cmake-mds\build-vc9\third_party\boost\1.48.0\include\boost\bind\bind_mf2_cc.hpp(212) : see declaration of 'boost::bind'
Any compiler errors relating to boost are virtually unreadable and unhelpful to me, so I hope someone can help me figure out what is going on. Thanks in advance.
回答1:
There are four overloads of std::string::find:
size_t find(const string& str, size_t pos = 0) const;
size_t find(const char* s, size_t pos, size_t n) const;
size_t find(const char* s, size_t pos = 0) const;
size_t find(char c, size_t pos = 0) const;
Therefore one must help the compiler to pick one (resolve ambiguity) by specifying which particular overloaded function the address is taken of, e.g.:
boost::bind( static_cast<size_t(std::string::*)(const std::string&, size_t) const>(&std::string::find), content_id, _1, 0)
Rather ugly, isn't it?
Note, that std::string::find() returns std::string::npos (which most often is size_t(-1)) on an unsuccessful search. Then it would convert size_t(-1) to bool(true) and result in std::find_if() returning its first argument no matter what the rest of arguments are.
The result of std::string::find() needs to be compared against std::string::npos. Using boost::bind that would look like:
// ...
std::vector<std::string>::iterator it = std::find_if(
attachment_list_.begin()
, attachment_list_.end()
, boost::bind(
std::not_equal_to<std::string::size_type>()
, std::string::npos
, boost::bind(
static_cast<size_t(std::string::*)(const std::string&, size_t) const>(&std::string::find)
, &content_id // pass by pointer, don't copy
, _1
, 0)
)
);
Which doesn't look too readable either.
It may be a tiny little bit more readable with boost::lambda::bind:
#include <boost/lambda/bind.hpp>
#include <boost/lambda/lambda.hpp>
// ...
std::vector<std::string>::iterator it =
std::find_if(
attachment_list_.begin()
, attachment_list_.end()
, boost::lambda::constant(std::string::npos) != boost::lambda::bind(
static_cast<size_t(std::string::*)(const std::string&, size_t) const>(&std::string::find)
, &content_id // pass by pointer, don't copy
, boost::lambda::_1
, 0
)
);
It looks most readable and elegant with C++11 lambda:
std::vector<std::string>::iterator it = std::find_if(
attachment_list_.begin()
, attachment_list_.end()
, [&content_id](std::string const& i) { return std::string::npos != content_id.find(i); }
);
Further I noticed that id returned for unsuccessful search is 0. It is the same value that is returned when the search succeeds on the first element. In other words, the caller of this function won't be able to distinguish between an unsuccessful search and when the first (0th) element matched.
It is most simple and portable to use a plain loop for search here:
std::string* MimeDocument::GetAttachmentId(std::string const& content_id) {
for( std::vector<std::string>::iterator i(attachment_list_.begin()), j(attachment_list_.end())
; i != j
; ++i
) {
if(std::string::npos != content_id.find(*i))
return &*i;
}
return NULL;
}
Using this version the caller can easily tell between a successful and unsuccessful search and find out the matching index if necessary:
MimeDocument doc;
// ... populate doc
if(std::string* found = doc.GetAttachmentId("1")) {
// the search was successful.
size_t found_index = found - &doc.attachment_list_.front();
}
So, pick your poison...
回答2:
The parameter types to bind are not related to each other in any way (orthogonal template types), and only within the body could the compiler determine which overload of find is needed. In fact the compiler is only allowed to look at the function declaration to figure out what to pass in and there are ambiguous possible overloads of find and the compiler can't use the bound argument type to help figure out which one to use.
In this case I think it may work out simpler to just write a 5 line functor to do the nested string searches for you.
来源：https://stackoverflow.com/questions/9724363/using-boostbind-on-stdstringfind-fails-to-compile