从 function c++ 返回 NULL 字符串

Question

string receiveFromServer();

this function returns a string that was received from some server. If there was an error along the way (including protocol), i want to return a NULL string. However, this doesn't work in c++ (unlike java). i tried:

string response = receiveFromServer();
if (response==NULL) {
    cerr << "error recive response\n";
}

but its not legal. Since an empty string is also legal to return, what can i return that will indicate the error?

thank you!

Answer 1

You can throw an exception to indicate an error.

try {
    std::string response = receiveFromServer();
} catch(std::runtime_error & e) {
    std::cerr << e.what();
}

Then you would need a throw std::runtime_error("Error receive response") somewhere in receiveFromServer() .

It is often considered good practice (though some might disagree) to create your own exception-classes that derive from std::runtime_error to enable clients to catch your errors specifically.

Answer 2

You can either throw an exception (better way), or return boost::optional< string >, but then you have to check if the return value is valid (this is actually worse).

Answer 3

NULL has only a meaning when using pointers. In java strings are pointers so you can do that.

In C++, if you return an std::string, it must exist. So you have some possibilites

• Throw an exception
• Return a pair with a bool indicating the success
• Return an empty string
• Use a pointer (I strongly discourage that option)

Answer 4

Maybe you should try to handle with exceptions

 try
  {
    string response = receiveFromServer();
  }
  catch (...)
  {
    cerr << "error recive response\n";
  }

Answer 5

If you want to return an empty string you could also use the function string::empty() to test if it is empty