Are there any issues with allocating memory within constructor initialization lists?

Question

I have used initialization lists a great deal in my C++ programs but wasn't aware that you could allocate memory within them.

So you can do something (as a contrived example) like this:

class Test { private:     int* i;     int* j;     int count;     int* k;  public:     Test(void) : i(new int), j(new int[10]), count(10), k(new int[count])     {     }      ~Test(void)     {         delete i;         delete [] j;         delete [] k;     } }; 

Are there any issues in doing memory allocation in this way? Regarding the order of initialization here is it safe to have a parameter initialized by one initialized in the same list? i.e. as I allocate count before I use it is it safe to use or is there some special initialization order I could fall foul of?

Answer 1

It's not exception-safe. If the new for j throws an exception then the destructor for Test is not called, and so the memory for i is not freed.

The destructor of i is called if the initializer for j throws, it's just that a raw pointer has no destructor. So you could make it exception-safe by replacing i with a suitable smart pointer. In this case, unique_ptr<int> for i and unique_ptr<int[]> for j would do.

You can rely on the initializers to be executed in their correct order (the order the members are defined, not necessarily the order in the list). They can safely use data members that have already been initialized, so there's no problem with using count in the initializer for k.

Answer 2

This code could leak memory in the presence of an initializer that throws an exception.

Note that this could be made to work correctly if the members of Test were smart -- rather than raw -- pointers.