I have not been able to find any documentation about what happens if I pass a null pointer to std::make_unique.
Is an exception thrown?
2 Answers
std::make_unique forwards all the arguments passed to a matching target constructor.
Therefore, if you pass nullptr, it will search for a constructor that accepts nullptr_t (or any matching constructor). If it is not there, it will fail to compile.
class A
{
public:
A(void*) { ... } // could also be A(int*) or template <typename T> A(T*)
};
...
std::make_unique<A>(nullptr); // constructs an A by calling A(void*), passing nullptr.
answered Jul 11, 2019 at 16:34
Michael Chourdakis
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1 Comment
Vlad from Moscow
It is not necessary that the constructor had the declaration A( void * ). It can be for example A( int * ); or A( T *ptr, Args ...args );
std::make_unique is meant to construct an object wrapped by forwarding the arguments to the constructor of the wrapped object.
Passing nullptr to it doesn't make any sense in the way you mean. If you need to clear the content of a std::unique_ptr just call reset().
Comments
make_unique? How is passing null pointer different from passing, say.42or"Hello World"?make_uniquedoes not take a pointer and "make it unique", it creates ("makes") an object that has unique and transferrable ownership.std::make_unique<int*>(nullptr)creates a nullint*with those properties.std::make_unqiue<Type>creates a dynamicTypeinstance and assigns ownership to the then-returnedstd::unique_ptr<Type>object. This sounds like you thinkstd::make_uniquedoes something else, or your confusingstd::make_uniquewith the constructor forstd::unique_ptr; they're different beasts.