Why is b[2] false?

Question

string s;
bool b[] = {s=="",  s==s.c_str(),  s.c_str()==""};

sets

b[] = {true, true, false};

why is b[2] false?

If A==B and A==C, should that not imply B==C?

Answer 1

In this expression

s.c_str()==""

there are compared two pointers (addresses). The first one is the pointer returned by s.c_str() and the second one is the pointer to the first character (terminaring zero character) of the string literal "".

It is evident that the addresses are different (bear also in mind that the string literal has the static storage duration).

To get the expected result you should write instead

std::strcmp( s.c_str(), "" ) == 0

至于这两个表达

s==""

和

s==s.c_str()

然后比较字符串，因为标准类std :: string的右操作数重载了运算符==。