Singleton class for moving heap to stack

问题

I have written some class that moves heap allocated stuff to stack (hopefully :) ). This calss is singleton because only this class should be responsible for holding and managing part of stack. My question is: Is my code correct? Code is correct in the sense of programming (no compile errors, no memory errors and leaks (checked by valgrind)). But does the code really moves heap to stack? Here's the code:

stack.hpp:

class CStack{
public:
  void* getAlloc(long);
  static CStack* Instance();

private:
  static bool _data[5*sizeof(double)];
  static CStack* m_pInstance;

  CStack(){};
  CStack(const CStack&);
  CStack& operator=(const CStack&);
};

stack.cpp:

#include <iostream>
#include "stack.hpp"

CStack* CStack::m_pInstance = 0;

bool CStack::_data[ 5*sizeof(double) ] = { 1 };

CStack* CStack::Instance(){
  if (!m_pInstance)
    m_pInstance = new CStack;
  return m_pInstance;
}

void* CStack::getAlloc(long size){
  std::cout << "  CStack::getAlloc, " << _data << std::endl;
  _pos+=size;
  return &_data[0];
}

store.hpp

class CStore{
public:
  CStore();
  double* myAddr();
  void toStack();
  void out();
  ~CStore();
private:
  double *_data;
  bool _stack;
};

store.cpp:

#include <iostream>
#include <cstring>
#include "store.hpp"
#include "stack.hpp"

CStore::CStore(){
  _data = new double[4];
  _data[0] = 0.1;
  _data[1] = 1.1;
  _data[2] = 2.1;
  _data[3] = 3.1;
  _stack = 0;
}

double* CStore::myAddr(){ return _data; }

void CStore::toStack(){
  double *tmp;

  tmp = (double*)CStack::Instance() -> getAlloc(4*sizeof(double));

  memcpy(tmp, _data, 4*sizeof(double));
  delete [] _data;
  _data = tmp;
  _stack = 1;
}

CStore::~CStore(){
  if (!_stack)
    delete [] _data;
}

void CStore::out(){
  std::cout << _data[0] << " " << _data[1] << " " << _data[2] << " " << _data[3] << std::endl;
}

main.cpp:

#include <iostream>

#include "stack.hpp"
#include "store.hpp"

using namespace std;

int main(){
  CStack::Instance();
  CStore a;
  double stack;

  cout << &stack << endl;
  cout << "Adresa a " << a.myAddr() << endl;
  a.out();

  a.toStack();
  cout << "Adresa a " << a.myAddr() << endl;
  a.out();

  return 0;
}

回答1:

No it's utter nonsense.

First of all this code doesn't even use the stack (as long as we are speaking of the execution stack), and you clearly misunderstood the Singleton desing pattern.

You use Singleton, when you only need a single instance in your program, but you want to grant access to all functions to that single instance. Its like a global variable, but you can restrict the access.

Second, if you need to use casting in C++ like:

tmp = (double*)CStack::Instance() -> getAlloc(4*sizeof(double));

You've clearly gone wrong. Don't use void* in C++ for now. You can use it later, when you've gotten better, not for now.

The main question WHY do you wan't to move variables to the stack? if you simply want to allocate dynamicly, why still limiting it to the scope, use smart pointers, like std::auto_ptr.

if you want to create a stack, and use that to store your doubles you can write:

#include <vector>

int foo()
{
  std::vector<double> stack;

  // push values on the stack
  stack.push_back(1.1);
  stack.push_back(1.2);
  stack.push_back(1.3);

  // remove values from the stack
  stack.pop_back();
  stack.pop_back();
  stack.pop_back();
}

来源：https://stackoverflow.com/questions/8279964/singleton-class-for-moving-heap-to-stack