Comments (3)
Kojoley
commented on August 26, 2024
This is https://svn.boost.org/trac10/ticket/8298.
@fletchjp said there:
The bug here is that this ever worked. The local variables _a do not provide a true variable. This means that _a = 17 followed by _a = 18 amounts to 17 = 18 i.e. assigning to a constant.
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Kojoley
commented on August 26, 2024
I investigated this and found multiple bugs.
-
The
letdocumentation states that_ashould hasinttype, but for C++11 it isint const&because of tr1/decltype result_of differences https://wandbox.org/permlink/27AcBJnXskKPl9uU. If you defineBOOST_RESULT_OF_USE_TR1the example will compile run https://wandbox.org/permlink/Daf4VxdLyETDRs8p. -
Internally Phoenix places locals into a vector and than operates on it through const reference, so actually it cannot modify it and passes
_aasint const&, but mutual operators were working becauseint const&was casted toint&here -
The other bug is that
let(_a = 0)[_a]()returns a reference to temporary (withBOOST_RESULT_OF_USE_TR1even non-const!).
It looks like a disaster, users can use those references to temporaries and do not know this. I have some thoughts about this:
- I think that
let(_a = 0)[--_a]()should work. - The return type of it (and
let(_a = 0)[_a]()) must be a value. - Making things above will require to distinguish between temporaries and references. With C++11 it is not a problem (rvalues will be stored as value types), but for C++98
letwill start copying const references values (it can be avoided by users withref; non-const values will work fine).
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fletchjp
commented on August 26, 2024
Nikita
Thank you for tackling this problem. You will see further back down this thread that I wrote before. Some years ago for some time I was attempting to sort out the problems in Phoenix and I did some but not others. After a while I handed the task to others. I am still around and if you find my name in things do ask me what I was doing.
I made some comments on the error handling systems. I never had the power to change the status so some things may not have been documented as closed although I did. Some of my work is documented in a ChangeLog in Phoenix. I don't think it has been used since my efforts.
Best wishes
John Fletcher
…________________________________
From: Nikita Kniazev [[email protected]]
Sent: 04 November 2018 18:44
To: boostorg/phoenix
Cc: Fletcher, John P; Mention
Subject: Re: [boostorg/phoenix] `phoenix::let` with non-placeholder value can't be modified in c++11 (#69)
I investigated this and found multiple bugs.
1. The let documentation<https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.boost.org%2Fdoc%2Flibs%2F1_68_0%2Flibs%2Fphoenix%2Fdoc%2Fhtml%2Fphoenix%2Fmodules%2Fscope%2Flet.html&data=02%7C01%7CJ.P.Fletcher%40aston.ac.uk%7Cc5aa6ca65f5e48e3a85808d642859b9f%7Ca085950c4c2544d5945ab852fa44a221%7C0%7C0%7C636769538939201706&sdata=9wqFdRWLZvQqGJP%2Bp61BL0fXNV7L%2FbguILk4FojyWv4%3D&reserved=0> states that _a should has int type, but for C++11 it is int const& because of tr1/decltype result_of differences https://wandbox.org/permlink/27AcBJnXskKPl9uU<https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwandbox.org%2Fpermlink%2F27AcBJnXskKPl9uU&data=02%7C01%7CJ.P.Fletcher%40aston.ac.uk%7Cc5aa6ca65f5e48e3a85808d642859b9f%7Ca085950c4c2544d5945ab852fa44a221%7C0%7C0%7C636769538939201706&sdata=bo2FgVnR4OBpSUJH3yF7q2X%2B%2FjyDBI4sRH%2B%2BlbI%2B8j8%3D&reserved=0>. If you define BOOST_RESULT_OF_USE_TR1 the example will compile run https://wandbox.org/permlink/Daf4VxdLyETDRs8p<https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwandbox.org%2Fpermlink%2FDaf4VxdLyETDRs8p&data=02%7C01%7CJ.P.Fletcher%40aston.ac.uk%7Cc5aa6ca65f5e48e3a85808d642859b9f%7Ca085950c4c2544d5945ab852fa44a221%7C0%7C0%7C636769538939211711&sdata=slwkXqqz9Gkz4fsim0KEa8Zfg03jvLiZrf5VDwbQB5A%3D&reserved=0>.
2. Internally Phoenix places locals into a vector and than operates on it through const reference, so actually it cannot modify it and passes _a as int const&, but mutual operators were working because int const& was casted to int& here buthttps://github.com/boostorg/phoenix/blob/5f2061241f528123cc04740294e4811a79138f3a/include/boost/phoenix/scope/detail/local_variable.hpp#L203
3. The other bug that let(_a = 0)[_a]() returns a reference to temporary (with BOOST_RESULT_OF_USE_TR1 even non-const!).
It looks like a disaster, users can use those references to temporaries and do not know this. I have some thoughts about this:
1. I think that let(_a = 0)[--_a]() should work.
2. The return type of it (and let(_a = 0)[_a]()) must be a value.
3. Making things above will require to distinguish between temporaries and references. With C++11 it not be a problem (rvalues will be stored as value types), but for C++98 let will start copying const references values (it can be avoided by users with ref; non-const values will work fine).
—
You are receiving this because you were mentioned.
Reply to this email directly, view it on GitHub<https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fboostorg%2Fphoenix%2Fissues%2F69%23issuecomment-435694950&data=02%7C01%7CJ.P.Fletcher%40aston.ac.uk%7Cc5aa6ca65f5e48e3a85808d642859b9f%7Ca085950c4c2544d5945ab852fa44a221%7C0%7C0%7C636769538939221721&sdata=OW8bSrxtD3NijxeTs4poIJaI4SvlTsbtJ58kJrf48KA%3D&reserved=0>, or mute the thread<https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fnotifications%2Funsubscribe-auth%2FAGG2344Q_BfZ5Loe4mJ1vbQHhnA3Wwiiks5urzWjgaJpZM4VMIV6&data=02%7C01%7CJ.P.Fletcher%40aston.ac.uk%7Cc5aa6ca65f5e48e3a85808d642859b9f%7Ca085950c4c2544d5945ab852fa44a221%7C0%7C0%7C636769538939221721&sdata=94foUisKpwUeHUFbw%2Bu5b2DHMG94HYOHfPE26HmFwSI%3D&reserved=0>.
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Related Issues (20)
- Namespace for Phoenix in the documentation HOT 1
- Request: Adapted std::get and an unpack function HOT 2
- deprecated-copy warnings with GCC 11
- Please merge develop to master HOT 2
- boost 1.8.1 beta1+gcc-12: multiple definition of `boost::phoenix::placeholders::uarg1' HOT 27
- clang-15 warning: definition of implicit copy constructor for 'actor<boost::log::expressions::aux::unary_function_terminal<boost::log::expressions::has_attribute<void>>>' is deprecated because it has a user-declared copy assignment operator HOT 4
- Boost 1.81.0: error: 'arg1' is not a member of 'boost::phoenix::placeholders'; did you mean 'uarg1'? HOT 5
- error: definition of implicit copy constructor for 'member_close_operation<boost::iostreams::detail::linked_streambuf<char>>' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
- Merge to master
- The whole folder of test is unused: bll_compatibility HOT 1
- ASan detects use after scope in `member` test
- trac ticket 9363 can be closed HOT 2
- compile fails when using phoenix construct with qi::_12 placeholder. HOT 1
- Build error on windows (VS 2017) on x86 but not x64 (using boost 1.69.0) HOT 5
- do_ leads to incosistency HOT 1
- List of non BOOST-prefixed macros HOT 5
- VC7.1 errors when including `<boost/phoenix.hpp>` HOT 1
- asan detects stack-buffer-overflow when using try/catch HOT 3
- Merge to master for 1.73 HOT 2
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