CODEWITHSUNDEEP
c++
JASON
JASON

Reputation: 7491

Returning a NULL when return type is not a pointer

I have a function defined as

ClassType f(int i) {
  if (...) return NULL;
  ...
  ClassType obj;
  return obj;
}

I am curious why a "NULL" could be legally accepted when a class type is required as return type? I think NULL is usually defined by Macro as a synonym of '0'. Could someone explain the rationale behind this? Thanks!

update:

The class is defined as:

class ClassType {
public:
  ClassType() {};
  ClassType(char* s) {...};
...
}

One more question, Does c++ do implicit conversion when return by value?

Thanks!

Upvotes: 1

Views: 150

Answers (4)

jrok
jrok

Reputation: 55395

class ClassType {
public:
  ClassType() {};
  ClassType(char* s) {...}; // <<-----
}

The constructor that I marked is used when you say return NULL;. The value is implicitly convertible to char* (NULL is usualy defined as 0 and that's a valid pointer value).

One more question, Does c++ do implicit conversion when returning a value?

Yes, as it has to construct an object of suitable type.

Upvotes: 3

Sebastian Mach
Sebastian Mach

Reputation: 39089

Not accurately an answer to your question, but in general, to indicate optional return values, consider the following.

For cases where an optional return value makes sense, you might look into boost::optional, and C++14 will possibly define std::optional (based on the boost-library).

Additional return values in diguise, like bools indicating success, often come at the burden that you cannot initialize an object only in case you can, and if you want to return an optional value for which there is no default construction, you either can't, or you have to pass around reference-pointers, and then have to check pointer at the called site, etc.:

std::optional<Intensity> sample_spectrum (float); // clean and concise

-------------------------------------------------------------------------

bool sample_spectrum (Intensity &out); // either not possible or you
                                       // have a wasted initialization

-------------------------------------------------------------------------

bool sample_spectrum (Intensity *& out) { // now you have two problems
    if (!out) throw std::logic_error(""); // I
    ....
}
....
    try { .... } catch (...) { .... }     // II

-------------------------------------------------------------------------

Intensity *sample_spectrum(float) ; // please no

-------------------------------------------------------------------------

std::shared_ptr<Intensity> 
   sample_spectrum(float); // just use std/boost::optional

Upvotes: 0

Raxvan
Raxvan

Reputation: 6505

Null is a macro that is defined with 0 which is an integer value. If the ClassType has a constructor that accepts integers,or pointers then the compiler will automatically convert the integer 0 to ClassType :

class ClassType 
{
public:
    ClassType(const int);
};

If the class doens't have a constructor that can take 0 then the compiler will throw an error.

Upvotes: 1

Some programmer dude
Some programmer dude

Reputation: 409166

To begin with, in this case you can return NULL, as the compiler will then use the ClassType(char* s) constructor with NULL passed as the constructor argument.

But generally, for cases like these you might want to pass the object as a reference argument to the function, and then return boolean true or false is the function was successful or not.

Upvotes: 1

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