Exception rethrowing

Question

What is the difference between:

foo();

and

try{
    foo();
} catch (...){
    throw;
}

As I understand - if there is no try-catch block in function that calls foo() - the exception will be caught by try-catch block of outside function(if there is any). Am I right in this?

But what if there is a try-catch block in function that calls foo(), but there is no catcher who can handle it - must I write catch (...) { throw; } to let it be caught by someone outside it? Is it necessary?

Answer 1

A throw-expression with no operand rethrows the exception being handled. [§15.1/8]

So there are the same in practice.

 

the exception will be caught by try-catch block of outside function(if there is any). Am I right in this?

Yes.

 

But what if there is a try-catch block in function that calls foo(), but there is no catcher who can handle it - must I write catch (...) { throw; } to let it be caught by someone outside it? Is it necessary?

Handle exceptions which is expected you handle them in the calling point and leave the others. You don't have to re-throw them. If no catcher, catches the exception finally std:terminate will be invoked.

try
{
    foo();
}
catch (YourExpectedException &ex)
{
  // ...
}
catch (...)    \
{               \
                 > // You don't need this kind of re-throwing
    throw;      /
}              /