如果不满足条件，我应该在成员函数中返回什么？

Question

For example:

class Foo{
   int data;
   // ...
}

And there is a member function:

Foo func(int a){
  if(a > 10) return Foo(a);
  else {} // problem is here, I do not want return 
             any Foo object, do I throw exception? how?
}

So, I do not want to return a new empty Foo object in else block, how can I handle this situation in an idiomatic way?

Answer 1

Yes, if you've found that there's some scenario in which your function should suddenly not be returning a value, then either:

• throw an exception, or
• your design is broken

Answer 2

If the pre-condition is absolutely needed for the function to continue, then throwing an exception is the proper way of handling it. If the condition isn't required explicitly throughout the rest of the function, then you have a couple options:

• Return a unique pointer that manages a pointer, ie std::unique_ptr . I don't recommend this approach because it has unnecessary dynamic memory allocation.
• Return boost::optional<Foo> or in a future TS std::experimental::optional<Foo> . This expresses intent that the value may or may not be there.
• Re-evaluate your design. Why does the return depend on the condition? What could be done differently? etc.

Answer 3

your best bet is to return a pointer:

Foo* func(int a){
  if(a > 10) return new Foo(a);
  else return 0; // else return plain zerro
}

C++11

Foo* func(int a){
  if(a > 10) return new Foo(a);
  else return nullptr;
}

of course you'll need to check if returned object does exist in your calling function:

Foo* ptr = func(5);
if(ptr)
{
    // use ptr
    *ptr;
}
else
{
    // do not use it
}

// and delete it
if(ptr) delete ptr;

// in c++11 version no need to check just:
delete ptr;

sorry I didn't see you do not want to return 0...

Foo* func(int a){
  if(a > 10) return new Foo(a);
  else throw(a);
}

try
{
    Foo* get = func(4); // this will throw
}

catch(int)
{
     //handle the error here
}