C++: is string.empty() always equivalent to string == “”?

Question

Can I make an assumption that given

std::string str;
... // do something to str

Is the following statement is always true?

Answer 1

Yes (str.empty() == (str == "")) is always* true for std::string. But remember that a string can contain '\0' characters. So even though the expression s == "" may be false, s.c_str() may still return an empty C-string. For example:

#include 
#include 
using namespace std;

void test( const string & s ) {
    bool bempty = s.empty();
    bool beq = std::operator==(s, ""); // avoid global namespace operator==
    const char * res = (bempty == beq ) ? "PASS" : "FAIL";
    const char * isempty = bempty ? "    empty " : "NOT empty ";
    const char * iseq = beq ? "    == \"\"" : "NOT == \"\"";
    cout << res << " size=" << s.size();
    cout << " c_str=\"" << s.c_str() << "\" ";
    cout << isempty << iseq << endl;
}

int main() {
    string s;          test(s); // PASS size=0 c_str=""     empty     == ""
    s.push_back('\0'); test(s); // PASS size=1 c_str="" NOT empty NOT == ""
    s.push_back('x');  test(s); // PASS size=2 c_str="" NOT empty NOT == ""
    s.push_back('\0'); test(s); // PASS size=3 c_str="" NOT empty NOT == ""
    s.push_back('y');  test(s); // PASS size=4 c_str="" NOT empty NOT == ""
    return 0;
}

**barring an overload of operator== in the global namespace, as others have mentioned*