Programmatically determine if std::string uses Copy-On-Write (COW) mechanism
Following up on the discussion from this question, I was wondering how does one using native C++ determine programmatically whether or not the std::string implementation they are using utilizes Copy-On-Write (COW)
I have the following function:
#include <iostream>
#include <string>
bool stdstring_supports_cow()
{
//make sure the string is longer than the size of potential
//implementat开发者_Python百科ion 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 is COW." << std::endl;
else
std::cout << "std::string is NOT COW." << std::endl;
return 0;
}
The problem is I can't seem to find a C++ tool chain where it returns true. Is there a flaw in my assumption about how COW is implemented for std::string?
Update: Based on kotlinski comments, I've changed the use of writeble references to data() in the function, it now seems to return "true" for some implementations.
bool stdstring_supports_cow()
{
//make sure the string is longer than the 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 to Basic String", in the upcoming C++0x standard, COW option will be removed from basic_string. thanks to James and Beldaz for bringing that up.
Using &s1[0]
to take the adress is not what you want, [0]
returns a writable reference and will create a copy.
Use data() instead, it returns a const char*, and your tests may pass.
The copy-on-write paradigm is dependent on knowing when you are doing a write. This will occur whenever the object is returning a writable reference.
If you work with const references to the strings, you may be able to compare the addresses if the class was specialized to disable the copy when returning a const reference to the data.
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