Returns NULL in a function that should return a vector

Question

I have a function with the following heading:

std::vector<NxU32> MySewer::createCuttingArray(MyCloth** cloth, NxU32 startPoint, NxU32 endPoint) 

The function should return a vector containing some integer values. But there is one problem: if the string is incorrectly created (there is a way that this can happen), I want to do something like what I did in Java and C # to return NULL. But from what I see in C ++, NULL is defined as an integer value. How can I return a valid NULL for a vector?

Answer 1

I do not think there is a need for boost::optional and std::exception . What you have to return is an empty vector. I'm thinking about it. The function returns a list of integers, and the caller function works in a list of integers. If the list is empty, then the operator does not work. For example (in psuedo)

 std::vector<T> data = createData(args); for(int i = 0; i < data.size(); ++i){ calculate(data[i]); } 

That the loop will not execute if the data is empty. No need for null checks or exception handling.

Answer 2

The “right” way to handle this really depends on what the code that receives the vector actually does. Is this the cause of the error, or is it just “we ignore it”?

If this is an error, use throw some_exception_goes_here; because it’s a lot easier than going down a line with a NULL value.

If you just want to ignore it, return an empty vector, and then make sure that the code below will not have problems with an empty vector.

Answer 3

According to your description, you want boost::optional<std::vector<NxU32>> .

Hope this name makes it obvious.

http://www.boost.org/doc/libs/release/libs/optional/doc/html/index.html

Answer 4

boost::optional<T> solves this problem by extending the type T as a type that can either "have a value of type T " or "have no value". In your case, you return boost::optional<std::vector<NxU32>> , because in some cases you want to return "nothing."

The boost documentation has some examples here .

This functionality seems simple enough that you can expect it to become part of the standard library. Unfortunately, this is not all. See Also: boost :: additional alternative in the C ++ standard library

Answer 5

You can not. To emulate the behavior of reference types in .NET, of which variables can either be bound to a real object or be null, you need to use pointers in C ++. Pointers can be either NULL or point to a valid object.

Answer 6

All in all, I think I would change the template. Create a vector in the caller, pass as a reference (or pointer) to your function and get bool return true / false, indicating success or failure.

Doing this also means that you are not calling the constructor std :: vector <> copy for the return value.

If you already have formatting and start and start installed, I will be tempted to return boost :: optional, as other answers indicate. But note that improving work efficiency can be a headache (it has its own build environment, which is a learning curve), especially if you use StlPort.

Answer 7

You just can't do it in C ++.

You are returning a vector to be copied (and created between other things). Another thing is that you cannot overlay something that is not a vector in NULL.