C++ std::unique_ptr return from function and test for null

Question

I have a function that needs to return a pointer to an object of class myClass. For this purpose I´m using std::unique_ptr.

If the function succeeds, it shall return a pointer to a object with data. If it fails, it should return null.

This is my code skeleton:

std::unique_ptr<myClass> getData() {    if (dataExists)       ... create a new myClass object, populate and return it ...     // No data found    return std::unique_ptr<myClass> (null); // <--- Possible? } 

on main:

main() {    std::unique_ptr<myClass> returnedData;     returnedData = getData();     if (returnedData != null)  // <-- How to test for null?    {       cout << "No data returned." << endl;       return 0;    }     // Process data } 

So here goes my questions:

a) Is that (returning an object or null) possible to be done using std::unique_ptr?

b) If possible, how to implement is?

c) If not possible, what are there alternatives?

Thanks for helping.

Answer 1

Either of the following should work:

return std::unique_ptr<myClass>{}; return std::unique_ptr<myClass>(nullptr); 

To test whether the returned object points to a valid object or not, simply use:

if ( returnedData ) {    // ... } 

See http://en.cppreference.com/w/cpp/memory/unique_ptr/operator_bool.

Answer 2

Yes it's possible. A default constructed unique_ptr is what you want:

Constructs a std::unique_ptr that owns nothing.

// No data found return std::unique_ptr<myClass>{}; 

That is equivalent to the nullptr_t constructor, so perhaps this is more clear:

// No data found return nullptr;