How to assign a unique_ptr value from within a lambda

January 9, 2023

I have a situation where I need to assign a unique ptr value from within a lambda function.

std::unique_ptr<SomeType> unique_ptr_obj;

// Lambda below has fixed return type.
bool var = ()[unique_ptr_obj=std::move(unique_ptr_obj)]-> bool {
 unique_ptr_obj = GetUniqueObject();
 return true 
} ();
 
// Should be able to use unique_ptr_obj
UseUniqueObject(unique_ptr_obj.get());

However, as expected unique_ptr_obj is nullptr as it was moved into lambda. Is there a way I can populate unique_ptr_obj from within a lambda and be able to reuse it later ?

Any suggestions on how to accomplish this ? Should I convert unique_ptr_obj to shared_ptr ?

>Solution :

You should change the declaration of your lambda to capture unique_ptr_obj by reference:

bool var = [&unique_ptr_obj]() -> bool {
    // Whatever the next line does, now it changes that variable by reference.
    // Otherwise you were changing a local copy.
    unique_ptr_obj = GetUniqueObject();
    return true;
} ();