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c++stringstdstringstring-literalsequality-operator

Why is b[2] false?

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?

Solution

  • 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

    As for these two expressions

    s==""

    and

    s==s.c_str()

    then there are compared strings because the standard class std::string has overloaded operator == for the right operand.