How to create pool of contiguous memory of non-POD type?

Question

I have a scenario where I have multiple operations represented in the following way:

struct Op {
  virtual void Run() = 0;
};

struct FooOp : public Op {
  const std::vector<char> v;
  const std::string s;
  FooOp(const std::vector<char> &v, const std::string &s) : v(v), s(s) {}
  void Run() { std::cout << "FooOp::Run" << '\n'; }
};

// (...)

My application works in several passes. In each pass, I want to create many of these operations and at the end of the pass I can discard them all at the same time. So I would like to preallocate some chunk of memory for these operations and allocate new operations from this memory. I came up with the following code:

class FooPool {
public:
  FooPool(int size) {
    foo_pool = new char[size * sizeof(FooOp)]; // what about FooOp alignment?
    cur = 0;
  }

  ~FooPool() { delete foo_pool; }

  FooOp *New(const std::vector<char> &v, const std::string &s) {
    return new (reinterpret_cast<FooOp*>(foo_pool) + cur) FooOp(v,s);
  }

  void Release() {
    for (int i = 0; i < cur; ++i) {
      (reinterpret_cast<FooOp*>(foo_pool)+i)->~FooOp();
    }
    cur = 0;
  }

private:
  char *foo_pool;
  int cur;
};

This seems to work, but I'm pretty sure I need to take care somehow of the alignment of FooOp. Moreover, I'm not even sure this approach is viable since the operations are not PODs.

• Is my approach flawed? (most likely)
• What's a better way of doing this?
• Is there a way to reclaim the existing memory using unique_ptrs?

Thanks!

Answer 1

I think this code will have similar performance characteristics without requiring you to mess around with placement new and aligned storage:

class FooPool {
public:
    FooPool(int size) {
        pool.reserve(size);
    }

    FooOp* New(const std::vector<char>& v, const std::string& s) {
        pool.emplace_back(v, s); // in c++17: return pool.emplace_back etc. etc.
        return &pool.back();
    }

    void Release() {
        pool.clear();
    }

private:
    std::vector<FooOp> pool;
}

The key idea here being that your FooPool is essentially doing what std::vector does.