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[C++] How do I check if a std::string is NULL?


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#1 Extrakun   Members   -  Reputation: 150

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Posted 29 October 2006 - 03:48 AM

One of my function's signature looks like this:
void setName(std::string newName)
{
if (newName==NULL) this->name = GenerateAutoName();
else
this->name = newName;
}

The idea is if the function recieves a NULL as a parameter, it will set name to default. However, it seems that std::string has an overriden operator == which doesn't allow me to check if it is null. (If could try void setName(std::string* newName), but thet setName("name") wouldn't work). Any advice would be appreciated!

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#2 Kwizatz   GDNet+   -  Reputation: 1206

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Posted 29 October 2006 - 03:55 AM

if (newName.empty()) this->name = "Default";

That will work of you call setName(""), you can't call setName(NULL), unless you used a pointer as an argument, but what you really should use is a constant reference instead of passing by value.


#3 Anonymous Poster_Anonymous Poster_*   Guests   -  Reputation:

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Posted 29 October 2006 - 03:57 AM

Hey,

You can use if(.Equals(null)) to check that your string is empty.

Hope that helps.

#4 Zahlman   Moderators   -  Reputation: 1682

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Posted 29 October 2006 - 04:16 AM

'NULL' is a name for the integer constant 0 (which is not the same as an integer which currently happens to have a value of 0). In a pointer context, that means "a null pointer" (even if the bit pattern of a null pointer doesn't represent a zero value - which is the case on some real-world architectures, although probably none that you'll ever have to deal with). The name NULL, however, is properly used to describe such pointers, and *not* integer constants with that value. (It *is* considered good style in C++ to do things the other way around; i.e. use the integer constant 0 - i.e. the integer literal 0 - when checking for null pointers.)

std::string does have an overloaded operator==. But more to the point, it *is an object* (which allows it to have such a thing) and not any kind of pointer (it *is* a typedef, but the typedeffed thing is an instantiation of the basic_string template - not any kind of pointer). Thus, NULL isn't a valid value for a std::string. They *can't* really be NULL.

The relevant overloads for std::string are to compare a std::string to a std::string, a char* to a std::string, or a std::string to a char*. (Obviously, the comparison of char* to char* is built into the language, and cannot be affected by including <string>.) The overloads involving char* are equivalent (although they needn't be defined this way, and probably wouldn't be, for efficiency reasons) to creating a temporary std::string from the char* and comparing the two std::strings directly (which compares the "string data" in a manner similar to strcmp(), rather than any pointer values).

Now, creating a std::string from a char* pointer works by copying the string data pointed at by the pointer into the std::string's internal buffer for string data. What would you expect to happen if you tried to create one from a null pointer? I hope you aren't expecting anything well-defined to happen [smile] since that would mean dereferencing a null pointer, at which point World Is Over™.




Summary: Compare to "" (edit: or better yet, use .empty() as suggested by Kwizatz) instead. Be aware that 'NULL' isn't a sensical value for a std::string object, and you basically don't have to worry about null pointers - that's one of the reasons you're using std::string in the first place - except possibly when you *create* a std::string from a char* (and you don't know that the char* is guaranteed non-NULL).

But also, you should pass the string in by const reference, you don't need to specify this-> explicitly in normal situations, and you don't need an if/else construct for something this simple:


// I assume this is inside the class body, since there's no Thing:: preceding
// setName
void setName(const std::string& newName) {
name = (newName == "") ? "Default" : newName;
}


And finally, consider also if you can get rid of such an ugly thing entirely by just accepting a constructor parameter and setting the name when you create the object. After all, why would the 'name' of an object change over its lifetime? (Note that the use of the ternary operator here, in turn, allows you to set the name via the initializer list, because it converts what would be an if-else statement block into a single *expression* - even neater.)

Edit: AP, you have no idea what you're talking about. The std::string belongs to C++. lowercase 'null' and titlecase .Equals() for strings belong to C# and Java, which spell 'string' in titlecase as well (i.e. String, in whichever package for the two different languages). And anyway, the C++ operator== is functionally the same for std::string as .Equals() is for C# and Java Strings.

#5 MrAccident   Members   -  Reputation: 232

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Posted 29 October 2006 - 06:19 PM

Just to provide a brief example following Zahlman's excellent (if verbose) post:
   std::string* ptrToString = NULL; // this is a null pointer
std::string aString = ""; // this is an empty string

if(ptrToString == NULL) // check if pointer is null
{ ... }

if(aString == "") // check if string is empty
{ ... }

if(aString == NULL) // this doesn't even make sense; you're
{ ... } // comparing a string value to an integer

Semantically, a std::string is an actual object; a string. Objects cannot be NULL. Pointers to objects can. If you have a pointer, you can check it against NULL, but if you have an actual object, that kind of comparison doesn't apply. (Unless the object's class includes a comparison on integers, which std::string doesn't.)

Anyway, just read Zahlman's post. He knows what he's talking about. [smile]




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