How to assign a unique_ptr value from within a lambda-CodePudding

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 ?

CodePudding user response：

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;
} ();

CodePudding user response：

You don't want to share ownership. Or maybe you do, but it won't help with the lambda assigning something to unique_ptr_obj, hence using a shared_ptr is not the solution.

You also do not want to move from unique_ptr_obj. Sloppy speaking, moving from something means leaving it in an empty state.

If you want a function to modify its argument then you pass by reference. If you want a lambda to modify something in the outer scope you let it capture it by reference.

This is the same whether its an int or a unique_ptr:

int x = 0;
bool value = [&x]() { x = 42; return true; } ();
         //   ^^ capture x by reference

assert(x == 42);