函数返回一个指向对象的指针，在这种情况下我该返回什么？

Question

If you have a function that returns a pointer to an object by looking for an object with a specific attribute in an array, what should I return if I don't find a corresponding object in the array? And, if done badly, could this represent a risk to the security or stability of the program?

Answer 1

If you have a function that returns a pointer to an object by looking for an specific attribute into an array of those objects, what should i return if i don't find that attribute in the array? and if done badly could this represent a risk to the security or stability of the program?

You have three basic possibilities:

• Simply return a null pointer. This is the easiest way and very probably the best.
• Throw an exception. Preferably of a special type, but std::out_of_range might do too.
• Return a pointer to a default object. Only reasonable if the return value must to point to a valid object.

Whatever you choose, it must be documented and as consistent to other cases as possible.

Answer 2

You could return nullptr with the convention that the caller should test that case. Or you could throw an exception.

You might use some smart pointers.

With C++11 you could take a different approach: passing a lambda function to deal with the found object.

Answer 3

If you have a function that returns a pointer to an object by looking for an specific attribute into an array of those objects, what should i return if i don't find that attribute in the array? and if done badly could this represent a risk to the security or stability of the program?

As Columbo said:

You have three basic possibilities:

• Simply return a null pointer. This is the easiest way and very probably the best.
• Throw an exception. Preferably of a special type, but std::out_of_range might do too.
• Return a pointer to a default object. Only reasonable if the return value must to point to a valid object.

However, I disagree on your choice of options. Only the first two can reasonably be considered, unless you are certain you will always return a valid object.

And, if done badly, could this represent a risk to the security or stability of the program?

Yes. Imagine you select possibility #3 and document it in your API. The caller is expecting a valid object each time. And let's say your app is a critical component of a server and an attacker finds an exploit that results in overwriting data in the table. This will most probably lead to a crash of your app: you get instant denial of service attack. And this really isn't a far fetched scenario…

Without even getting that far, if done badly, you could return invalid pointers, which may lead to app crash. Then again, anything done badly leads to the dark side...