refcounttrace.cpp

Think in C++ 第二版源码

//: C12:RefcountTrace.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 1999
// Copyright notice in Copyright.txt
// Refcount.cpp w/ trace info
#include "../require.h"
#include <cstring>
#include <fstream>
using namespace std;

ofstream out("rctrace.out");

class Counted {
  class MemBlock {
    static const int size = 100;
    char c[size];
    int refcount;
    static int blockcount;
    int blocknum;
  public:
    MemBlock() {
      memset(c, 1, size);
      refcount = 1;
      blocknum = blockcount++;
    }
    MemBlock(const MemBlock& rv) {
      memcpy(c, rv.c, size);
      refcount = 1;
      blocknum = blockcount++;
      print("copied block");
      out << endl;
      rv.print("from block");
    }
    ~MemBlock() {
      out << "\tdestroying block "
          << blocknum << endl;
    }
    void print(const char* msg = "") const {
      if(*msg) out << msg << ", ";
      out << "blocknum:" << blocknum;
      out << ", refcount:" << refcount;
    }
    void attach() { ++refcount; }
    void detach() {
      require(refcount != 0);
      // Destroy object if no one is using it:
      if(--refcount == 0) delete this;
    }
    int count() const { return refcount; }
    void set(char x) { memset(c, x, size); }
    // Conditionally copy this MemBlock.
    // Call before modifying the block; assign
    // resulting pointer to your block;
    MemBlock* unalias() {
      // Don't duplicate if not aliased:
      if(refcount == 1) return this;
      --refcount;
      // Use copy-constructor to duplicate:
      return new MemBlock(*this);
    }
  }* block;
  static const int sz = 30;
  char ident[sz];
public:
  Counted(const char* id = "tmp") {
    block = new MemBlock; // Sneak preview
    strncpy(ident, id, sz);
  }
  Counted(const Counted& rv) {
    block = rv.block; // Pointer assignment
    block->attach();
    strncpy(ident, rv.ident, sz);
    strncat(ident, " copy", sz - strlen(ident));
  }
  void unalias() { block = block->unalias(); }
  void addname(const char* nm) {
    strncat(ident, nm, sz - strlen(ident));
  }
  Counted& operator=(const Counted& rv) {
    print("inside operator=\n\t");
    if(&rv == this) {
      out << "self-assignment" << endl;
      return *this;
    }
    // Clean up what you're using first:
    block->detach();
    block = rv.block; // Like copy-constructor
    block->attach();
    return *this;
  }
  // Decrement refcount, conditionally destroy
  ~Counted() {
    out << "preparing to destroy: " << ident
        << "\n\tdecrementing refcount ";
    block->print();
    out << endl;
    block->detach();
  }
  // Copy-on-write:
  void write(char value) {
    unalias();
    block->set(value);
  }
  void print(const char* msg = "") {
    if(*msg) out << msg << " ";
    out << "object " << ident << ": ";
    block->print();
    out << endl;
  }
};

int Counted::MemBlock::blockcount = 0;

int main() {
  Counted A("A"), B("B");
  Counted C(A);
  C.addname(" (C) ");
  A.print();
  B.print();
  C.print();
  B = A;
  A.print("after assignment\n\t");
  B.print();
  out << "Assigning C = C" << endl;
  C = C;
  C.print("calling C.write('x')\n\t");
  C.write('x');
  out << "\n exiting main()" << endl;
} ///:~