C++编程思想 第1卷 第7章 函数重载与默认参数 选择重载还是默认参数

函数重载和默认参数都给函数调用提供了方便

//: C07:Mem.h
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 2000
// Copyright notice in Copyright.txt
#ifndef MEM_H
#define MEM_H
typedef unsigned char byte;

class Mem {
  byte* mem;
  int size;
  void ensureMinSize(int minSize);
public:
  Mem();
  Mem(int sz);
  ~Mem();
  int msize();
  byte* pointer();
  byte* pointer(int minSize);
}; 
#endif // MEM_H ///:~

自动管理内存块

Mem对象包括一个byte块 确保有足够的存储空间
默认的构造函数不分配任何的空间
第二个构造函数确保Mem对象中有sz大小的存储区，析构函数释放空间

//: C07:Mem.cpp {O}
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 2000
// Copyright notice in Copyright.txt
#include "Mem.h"
#include <cstring>
using namespace std;

Mem::Mem() { mem = 0; size = 0; }

Mem::Mem(int sz) {
  mem = 0;
  size = 0;
  ensureMinSize(sz); 
}

Mem::~Mem() { delete []mem; }

int Mem::msize() { return size; }

void Mem::ensureMinSize(int minSize) {
  if(size < minSize) {
    byte* newmem = new byte[minSize];
    memset(newmem + size, 0, minSize - size);
    memcpy(newmem, mem, size);
    delete []mem;
    mem = newmem;
    size = minSize;
  }
}

byte* Mem::pointer() { return mem; }

byte* Mem::pointer(int minSize) {
  ensureMinSize(minSize);
  return mem; 
} ///:~

使用标准C语言库函数memset()把新分配的内存置0
标准C语言库函数memcpy(),存在mem中拷贝到newmem中，最后删除旧的内存，
把新的内存和大小赋给适当的成员

//: C07:MemTest.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 2000
// Copyright notice in Copyright.txt
// Testing the Mem class
//{L} Mem
#include "Mem.h"
#include <cstring>
#include <iostream>
using namespace std;

class MyString {
  Mem* buf;
public:
  MyString();
  MyString(char* str);
  ~MyString();
  void concat(char* str);
  void print(ostream& os);
};

MyString::MyString() {  buf = 0; }

MyString::MyString(char* str) {
  buf = new Mem(strlen(str) + 1);
  strcpy((char*)buf->pointer(), str);
}

void MyString::concat(char* str) {
  if(!buf) buf = new Mem;
  strcat((char*)buf->pointer(
    buf->msize() + strlen(str) + 1), str);
}

void MyString::print(ostream& os) {
  if(!buf) return;
  os << buf->pointer() << endl;
}

MyString::~MyString() { delete buf; }

int main() {
  MyString s("My test string");
  s.print(cout);
  s.concat(" some additional stuff");
  s.print(cout);
  MyString s2;
  s2.concat("Using default constructor");
  s2.print(cout);
  getchar();
} ///:~

简化内存管理
创建了一个简单的string类
创建一个MyString,连接文本，打印输出到一个ostream中

输出
My test string
My test string some additional stuff
Using default constructor