c_str() return value not matching const char*

Question

c_str() returning a const char*, I would assume the following code to print "equal", but it doesn't.

Could someone explain to me where am I wrong ?

string xx = "hello";
const char* same = "hello";

const char* buf = xx.c_str();

if (buf == same)
{
    cout << "equal" << endl;
}

Answer 1

I would assume the following code to print "equal"

That's a wrong assumption. std::string copies the literal data to its internal buffer, so the pointers will differ.

In case you wanted to compare the data instead of pointers, don't use c_str() at all and compare the const char* to the string directly - the overloaded comparison operator will do what you expect it to.

Answer 2

operator== for const char* does not do a string comparison, it directly compares the pointers. The buffer used by xx is not the same as the same pointer, so they are not equal.

To do string comparison with a std::string and a const char* you can just use the operator provided by std::string :

if (xx == same)
{
    cout << "equal" << endl;
}

Answer 3

Operator == for char* pointers compares the pointers themselves, not the strings they point to.

If you want to second-guess std::string, you need to use comparison routines, like following:

if (strncmp(buf, same, xx.size())
...

Answer 4

buf == same is not comparing the strings but the address of the pointers. Since they are two different variables they will have two different address that will not be equal. If you need the compare char* s or const char* s then you can use std::strcmp

Answer 5

You are comparing the values of two pointers, which can never be equal. same points to read-only memory, and the pointer returned by c_str is the current data buffer of the std::string object with a null terminator guaranteed to be appended.

You could write xx == same : this is because std::string overloads == with the appropriate data types. Alternatively, and perhaps less elegantly, you could use std::strcmp on the two pointers.