Re: [Boost-users] mpl::c_str<mpl::string<>> compile error

Subject: Re: [Boost-users] mpl::c_str<mpl::string<>> compile error
From: Dan Searles (dansearles_at_[hidden])
Date: 2013-05-09 10:11:55

>Hello,
>
>the code here doesn't compile:
>
>#include <boost/mpl/string.hpp>
>#include <iostream>
>
>namespace mpl = boost::mpl;
>
>typedef mpl::string<'h','e','l','l','o',' ','w','o','r','l','d'>::type 
>my_hello;
>
>int main()
>{
>    std::cout << mpl::c_str<my_hello>::value << '\n';
>    std::cout << BOOST_MPL_LIMIT_STRING_SIZE << '\n';
>}
>
>As usual, the example from docs works. The error is:
>
>$ LANG=en g++ t.cpp -o t && ./t
>t.cpp:6:64: error: wrong number of template arguments (11, should be 8)
>/usr/local/include/boost/mpl/string.hpp:102:12: error: provided for 
>'template<int C0, int C1, int C2, int C3, int C4, int C5, int C6, int 
>C7> struct boost::mpl::string'
>t.cpp:6:72: error: expected initializer before 'my_hello'
>t.cpp: In function 'int main()':
>t.cpp:10:26: error: 'my_hello' was not declared in this scope
>t.cpp:10:34: error: template argument 1 is invalid
>
>why does it complain about wrong number of template arguments? 
>BOOST_MPL_LIMIT_STRING_SIZE is 32?
>
>$ svn info
>...
>Path: .
>URL: http://svn.boost.org/svn/boost/trunk
>Repository Root: http://svn.boost.org/svn/boost
>Repository UUID: b8fc166d-592f-0410-95f2-cb63ce0dd405
>Revision: 83988
>...
>$ LANG=en g++ --version
>g++ (GCC) 4.6.3 20120306 (Red Hat 4.6.3-2)
>...
>
>Thanks,
>Olaf

I'm surprised multi-character character constants are being used at all in Boost, they
are not supposed to be portable:

http://stackoverflow.com/questions/6944730/multiple-characters-in-a-character-constant

If you only want to use single character constants, you could place the following #define before any includes:

#define BOOST_MPL_LIMIT_STRING_SIZE (32*4)

That would allow up to 32, provided your compiler supports it (yet another portability issue).

Since in C++, the type of 'x' is char, it should be possible to create another overload of
mpl::string<> that takes BOOST_MPL_LIMIT_STRING_SIZE 'char's, which would work
the way most people would expect.


Dan Searles