c++ - One class with a vector data member -

i define class vector data member. class looks follows

class a{ ... private:      std::vector<int> v1; ... }; 

if use operator new allocate memory class a, program ok. however, if memory pre-allocated memory , cast pointer type a*, program crash.

a* = new a; a* b = (a*)pre_allocated_memory_pointer. 

i need 1 vector variable size , hope memory 1 pre-allocated memory. have idea problem?

an std::vector object requires initialization, cannot allocate memory , pretend you've got vector.

if need control memory solution defining operator::new class.

struct myclass {     std::vector<int> x;     ... other stuff ...      void *operator new(size_t sz) {         ... somewhere sz bytes , return pointer them ...     }      void operator delete(void *p) {         ... memory free ...     } }; 

another option instead specify allocate object using placement new:

struct myclass {     std::vector<int> x;     ... other stuff ... };  void foo() {     void * p = ... enough memory sizeof(myclass) ...      myclass *mcp = new (p) myclass();      ... later ...      mcp->~myclass(); // call destructor      ... memory can reused ... } 

note std::vector manages memory contained elements , therefore you'll need use stl "allocators" if want control memory needs coming from.