c++ - Programmatically determine if std::string uses Copy-On-Write (COW) mechanism -

following on discussion question, wondering how 1 using native c++ determine programmatically whether or not std::string implementation using utilizes copy-on-write (cow)

i have following function:

#include <iostream> #include <string>  bool stdstring_supports_cow() {    //make sure string longer size of potential    //implementation of small-string.    std::string s1 = "012345678901234567890123456789"                     "012345678901234567890123456789"                     "012345678901234567890123456789"                     "012345678901234567890123456789"                     "012345678901234567890123456789";    std::string s2 = s1;    std::string s3 = s2;     bool result1 = (&s1[0]) == (&s2[0]);    bool result2 = (&s1[0]) == (&s3[0]);     s2[0] = 'x';     bool result3 = (&s1[0]) != (&s2[0]);    bool result4 = (&s1[0]) == (&s3[0]);     s3[0] = 'x';     bool result5 = (&s1[0]) != (&s3[0]);     return result1 && result2 &&           result3 && result4 &&           result5; }  int main() {   if (stdstring_supports_cow())       std::cout << "std::string cow." << std::endl;    else       std::cout << "std::string not cow." << std::endl;    return 0; } 

the problem can't seem find c++ tool chain returns true. there flaw in assumption how cow implemented std::string?

update: based on kotlinski comments, i've changed use of writeble references data() in function, seems return "true" implementations.

bool stdstring_supports_cow() {    //make sure string longer size of potential    //implementation of small-string.    std::string s1 = "012345678901234567890123456789"                     "012345678901234567890123456789"                     "012345678901234567890123456789"                     "012345678901234567890123456789"                     "012345678901234567890123456789";    std::string s2 = s1;    std::string s3 = s2;     bool result1 = s1.data() == s2.data();    bool result2 = s1.data() == s3.data();     s2[0] = 'x';     bool result3 = s1.data() != s2.data();    bool result4 = s1.data() == s3.data();     s3[0] = 'x';     bool result5 = s1.data() != s3.data();     return result1 && result2 &&           result3 && result4 &&           result5; } 

note: according n2668: "concurrency modifications basic string", in upcoming c++0x standard, cow option removed basic_string. james , beldaz bringing up.

using &s1[0] take adress not want, [0] returns writable reference , create copy.

use data() instead, returns const char*, , tests may pass.