charの配列を、std::stringに代入したときの挙動を調べたので、置いておきます。

サイズ指定が無いcharからstd::stringへの代入は、0が出てきたら文字列が途切れる。
サイズ指定すれば、0が含まれる文字であっても、ちゃんと値を持っている。
std::stringstreamのstr()が返す内容も、0が含まれていても、ちゃんと値を持っている。

b.cpp

# include <string>
# include <iostream>
# include <sstream>

int main(int argc, char **argv) {
        char text[] = {97, 0, 98, 0, 99, 0};

        std::cout << "text output." << std::endl;
        std::cout << text << std::endl;
        std::cout << std::endl;

        std::string s1 = text;

        std::cout << "s1 output." << std::endl;
        std::cout << s1 << "\t" << s1.size() << std::endl;
        std::cout << std::endl;

        std::string s2(text, sizeof(text) / sizeof(char));

        std::cout << "s2 output." << std::endl;
        std::cout << s2 << "\t" << s2.size() << std::endl;
        std::cout << std::endl;

        for (size_t i = 0, ei = s2.size(); i < ei; ++i) {
                std::cout << s2[i]  << "\t" << static_cast<int>(s2[i]) << std::endl;
        }

        std::cout << std::endl;

        char text2[s2.size()];
        std::char_traits<char>::copy(text2, s2.c_str(), s2.size());

        std::cout << "text2 output." << std::endl;
        std::cout << text2 << std::endl;
        std::cout << std::endl;

        for (size_t i = 0, ei = sizeof(text2) / sizeof(char); i < ei; ++i) {
                std::cout << text2[i] << "\t" << static_cast<int>(text2[i]) << std::endl;
        }

        std::cout << std::endl;

        std::stringstream ss;
        ss.write(text, sizeof(text) / sizeof(char));

        std::string s3 = ss.str();

        std::cout << "s3 output." << std::endl;
        std::cout << s3 << "\t" << s3.size() << std::endl;
        std::cout << std::endl;

        for (size_t i = 0, ei = s3.size(); i < ei; ++i) {
                std::cout << s3[i]  << "\t" << static_cast<int>(s3[i]) << std::endl;
        }
}

[todanano@localhost ~]$ g++ --version
g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[todanano@localhost ~]$ g++ b.cpp
[todanano@localhost ~]$ ./a.out
text output.
a

s1 output.
a       1

s2 output.
abc     6

a       97
        0
b       98
        0
c       99
        0

text2 output.
a

a       97
        0
b       98
        0
c       99
        0

s3 output.
abc     6

a       97
        0
b       98
        0
c       99
        0

[todanano@localhost ~]$