Malloc on a struct containing a std::vector

Question

Here is the situation :

I use a malloc to allocate memory for a struct. The struct contains various items such as pointers, string variables and vectors.

The

Answer 1

Your code causes undefined behaviour, because your wMyStructure does not point to an object, so you may not use the accessor operator -> on it.

An object only commences its life after its constructor has completed. Since you don't call any constructor, you do not have an object.

(If your struct were a POD, i.e. just consisting of primitive types and PODs, then this would be OK, because PODs have trivial constructors, which do nothing.)

The concrete problem you're facing is that the string and vector members of your struct didn't get to call their constructors, so those members don't exists, and hence the entire object doesn't.

If you want to decouple memory management from object construction, you can use placement syntax:

// get some memory
char arena[HUGE_VAL];
void * morespace = malloc(HUGE_VAL);

// construct some objects
MyClass   * px = new (arena + 2000) MyClass;  // default constructor
YourClass * py = new (morespace + 5000) YourClass(1, -.5, 'x');  // non-default constructor

(You have to destroy those objects manually, px->~MyClass(); etc., when you're done with them.)