为什么我不能从const String＆方法返回NULL？

Question

I have the following method declaration: const String& MyClass::GetAspect(const String& strKey) const

In this method, we've decided to do a null-pointer check before doing some stuff; if the pointer inside this method is null , we want to just return null .

However, I get the following error:

myclass.cpp(222) : error C2440: 'return' : cannot convert from 'int' to 'const String &'
        Reason: cannot convert from 'int' to 'const String'
        No constructor could take the source type, or constructor overload resolution was ambiguous

Could someone help me understand this? Is there some const-correctness concept I don't fully understand here?

Answer 1

NULL is not an object of type const String , so of course you can't return it when a reference to const String is expected. In fact, one of the major advantages of references is that they can't (ever) be NULL .

Re-define the function to return const String * , or return an empty String .

Answer 2

NULL is a pointer (or technically, the integer value zero, which can be converted to/from a pointer, nullptr is a pointer with the value zero).

A std::string& is not a pointer (or an integer), so you can't use NULL for it. You could return "" or "Unknown" or something like that.