How to allow a std:string parameter to be NULL?

I have a function foo(const std::string& str); that it does crash i开发者_如何学Gof you call it using foo(NULL).

What can I do to prevent it from crashing?

std::string has a constructor that takes a const char* parameter. That's constructor is going to crash when you pass NULL to it, and that constructor is called implicitly when you write foo(NULL).

The only solution I can think of is to overload foo

void foo(const std::string& str)
{
  // your function
}

void foo(const char* cstr)
{
  if (cstr == NULL)
    // do something
  else
     foo(std::string(cstr)); // call regular funciton
}

You could use Boost.Optional.

#include <boost/optional.hpp>
#include <string>

using namespace std;
using namespace boost;

void func(optional<string>& s) {
    if (s) {  // implicitly converts to bool
        // string passed in
        cout << *s << endl; // use * to get to the string
    } else {
        // no string passed in
    }
}

To call it with a string:

string s;
func(optional<string>(s));

and without a string:

func(optional<string>());

Boost.Optional gives you a typesafe way to have nullable values without resorting to pointers and their associated problems.

You have a function that accepts a std::string, so provide it an std::string, not a pointer.

foo(std::string());

This will provide the function with an empty string, which is probably what you would have interpreted your null value as anyhow.