CODEWITHSUNDEEP
c++c++11
Steven Lu
Steven Lu

Reputation: 43427

Leaving `unique_ptr` unset until I am ready to allocate it: Is this how I re-assign it?

This example is in the context of iOS, but the principle applies elsewhere...

// global variable... 
std::unique_ptr<Foo> global_foo_handle; 

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // use Obj-C to compute a valid path string
    const char * path = [[NSTemporaryDirectory() stringByAppendingPathComponent:[NSString stringWithUTF8String: "file.txt"]] UTF8String];
    // Only now can I call the ctor of my foo. 
    global_foo_handle = std::move(std::unique_ptr<Foo>(new Foo(path)));
}

This doesn't really look all that sane to me. Is there no cleaner way? I see that the only valid unique_ptr operator = are move assignments. I suppose it's clear enough what's being done here. It also seems like I can omit the std::move so long as the ephemeral unique_ptr made for the newly allocated Foo remains a temporary rather than being made into an lvalue.

I guess I'm sort of wondering why there isn't an operator= that takes a pointer to T? That way we skip the creation of the temporary unique_ptr.

Upvotes: 0

Views: 116

Answers (2)

Jody Hagins
Jody Hagins

Reputation: 28349

You do not need the std::move...

global_foo_handle = std::unique_ptr<Foo>(new Foo(path));

Or you can just reset the value...

global_foo_handle.reset(new Foo(path));

However, you must understand that the deleter is part of the unique_ptr type, and any pointer assigned to the unique_ptr via reset will be deleted by the deleter of the original unique_ptr object.

Upvotes: 2

Ed Heal
Ed Heal

Reputation: 59997

Use reset method to achieve this - See http://www.cplusplus.com/reference/memory/unique_ptr/reset/

Upvotes: 1

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