boost::container::strings and leading whitespace

[alanbarr]

Hi,

I've noticed some strange behaviour when reading out values into a boost::container::string. Where a value begins with leading spaces it is not present when retrieved with get(). This behaves the same on 1.68.0 and 1.71.0.

For the program below, I expect the output to be the same for both std::string and boost::container::string. However std::string correctly preserves the leading space (" value") where as boost:container::string does not ("value").

Output

### boost::container::string ###
{
    "key": "       value"
}

"value"

### std::string ###
{
    "key": "       value"
}

"       value"

Program

#include <boost/container/string.hpp>
#include <boost/property_tree/json_parser.hpp>
#include <boost/property_tree/ptree.hpp>
#include <iostream>

#define LEADING_WS "       value"

void print_ptree(boost::property_tree::ptree const & pt)
{
    std::stringstream stream;
    boost::property_tree::write_json(stream, pt);
    std::cout << stream.str() << std::endl;
}

void boost_container_string(void)
{
    boost::container::string const value(LEADING_WS);

    boost::property_tree::ptree pt;
    pt.put<boost::container::string>("key", value);

    boost::container::string read_back = pt.get<boost::container::string>("key");

    std::cout << "### boost::container::string ###" << std::endl;
    print_ptree(pt);
    std::cout << "\"" << read_back << "\"" << std::endl;
    std::cout << std::endl;
}

void std_string(void)
{
    std::string const value(LEADING_WS);

    boost::property_tree::ptree pt;
    pt.put<std::string>("key", value);

    std::string read_back = pt.get<std::string>("key");

    std::cout << "### std::string ###" << std::endl;
    print_ptree(pt);
    std::cout << "\"" << read_back << "\"" << std::endl;
    std::cout << std::endl;
}

int main()
{
    boost_container_string();
    std_string();

    return 0;
}

[alanbarr]

Additionally if a string has white space in it an exception is / can be raised. E.g. change the now poorly named macro in the above program to: #define LEADING_WS "va lue"

An exception is raised:

terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::property_tree::ptree_bad_data> >'
  what():  conversion of data to type "N5boost9container12basic_stringIcSt11char_traitsIcENS0_13new_allocatorIcEEEE" failed

GDB

(gdb) where
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ffff70e5801 in __GI_abort () at abort.c:79
#2  0x00007ffff7ad8957 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x00007ffff7adeab6 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4  0x00007ffff7adeaf1 in std::terminate() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#5  0x00007ffff7aded24 in __cxa_throw () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#6  0x0000000000405dd9 in boost::throw_exception<boost::exception_detail::error_info_injector<boost::property_tree::ptree_bad_data> > (e=...)
    at /usr/include/boost/throw_exception.hpp:69
#7  0x000000000040592f in boost::exception_detail::throw_exception_<boost::property_tree::ptree_bad_data> (x=..., 
    current_function=0x417d49 "typename boost::enable_if<detail::is_translator<Translator>, Type>::type boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::less<std::__cxx11::b"..., file=0x4170bd "/usr/include/boost/property_tree/detail/ptree_implementation.hpp", line=672)
    at /usr/include/boost/throw_exception.hpp:86
#8  0x00000000004139d3 in boost::property_tree::basic_ptree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::get_value<boost::container::basic_string<char, std::char_traits<char>, boost::container::new_allocator<char> >, boost::property_tree::stream_translator<char, std::char_traits<char>, std::allocator<char>, boost::container::basic_string<char, std::char_traits<char>, boost::container::new_allocator<char> > > > (this=0x6360a0, tr=...)
    at /usr/include/boost/property_tree/detail/ptree_implementation.hpp:670
#9  0x0000000000413646 in boost::property_tree::basic_ptree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::get_value<boost::container::basic_string<char, std::char_traits<char>, boost::container::new_allocator<char> > > (this=0x6360a0)
    at /usr/include/boost/property_tree/detail/ptree_implementation.hpp:679
#10 0x00000000004041fe in boost::property_tree::basic_ptree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::get<boost::container::basic_string<char, std::char_traits<char>, boost::container::new_allocator<char> > > (this=0x7fffffffd650, path=...)
    at /usr/include/boost/property_tree/detail/ptree_implementation.hpp:751
#11 0x0000000000403870 in boost_container_string () at main.cpp:22
#12 0x0000000000403c44 in main () at main.cpp:47

Compilers

• clang version 9.0.0
• g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0

[ashtum]

The distinction lies in the get function. When using pt.get<std::string>("key"), it employs id_translator, which directly returns the internal std::string as is. However, with pt.get<boost::container::string>("key"), it utilizes stream_translator, leveraging the overloaded stream extraction operator to retrieve the value. Consequently, ignores leading and trailing spaces.

If you use boost::property_tree::basic_ptree<std::string, boost::container::string> as the ptree type, the results will be as expected.