C++编程思想 第2卷 第3章 深入理解字符串 字符串内部是什么

在C语言中，字符串基本上就是字符型数组，并且总是以二进制零
通常被称为空结束符 null terminator  作为其最末元素

C++标准没有定义字符串类内存布局 memory layout 的确切实现

在C语言中，每个char型数组都占据各自的物理存储区
在C++中，独立的几个string对象可以占据也可以不占据各自特定的
物理存储区，但是，如果采用引用计数避免了保存同一数据的拷贝副本，
那么各个独立的对象 必须看起来并表现得就像独占地拥有各自的存储区
一样

//: C03:StringStorage.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#ifndef STRINGSTORAGE_H
#define STRINGSTORAGE_H
#include <iostream>
#include <string>
#include "../TestSuite/Test.h"
using std::cout;
using std::endl;
using std::string;

class StringStorageTest : public TestSuite::Test {
public:
  void run() {
    string s1("12345");
    // This may copy the first to the second or
    // use reference counting to simulate a copy:
    string s2 = s1;
    test_(s1 == s2);
    // Either way, this statement must ONLY modify s1:
    s1[0] = '6';
    cout << "s1 = " << s1 << endl;  // 62345
    cout << "s2 = " << s2 << endl;  // 12345
    test_(s1 != s2);
  }
};
#endif // STRINGSTORAGE_H ///:~

//: C03:StringStorage.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
//{L} ../TestSuite/Test
#include "StringStorage.h"

int main() {
  StringStorageTest t;
  t.run();
  return t.report();
  getchar();
} ///:~

只有当字符串被修改的时候才创建各自的拷贝，这种实现方式称为
写时复制 copy-on-write 策略
当字符串只是作为值参数 value parameter 或在其他只读情形下使用，
这种方法能够节省时间和空间

不论一个库的实现是不是采用引用计数，它对string类的使用者来说
都应该是透明的

输出 用命令行的
s1 = 62345
s2 = 12345
Test "class StringStorageTest":
        Passed: 2       Failed: 0
        
还要Test.h Test.cpp

//: TestSuite:Test.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#ifndef TEST_H
#define TEST_H
#include <string>
#include <iostream>
#include <cassert>
using std::string;
using std::ostream;
using std::cout;

// fail_() has an underscore to prevent collision with
// ios::fail(). For consistency, test_() and succeed_()
// also have underscores.

#define test_(cond) \
  do_test(cond, #cond, __FILE__, __LINE__)
#define fail_(str) \
  do_fail(str, __FILE__, __LINE__)

namespace TestSuite {

class Test {
  ostream* osptr;
  long nPass;
  long nFail;
  // Disallowed:
  Test(const Test&);
  Test& operator=(const Test&);
protected:
  void do_test(bool cond, const string& lbl,
    const char* fname, long lineno);
  void do_fail(const string& lbl,
    const char* fname, long lineno);
public:
  Test(ostream* osptr = &cout) {
    this->osptr = osptr;
    nPass = nFail = 0;
  }
  virtual ~Test() {}
  virtual void run() = 0;
  long getNumPassed() const { return nPass; }
  long getNumFailed() const { return nFail; }
  const ostream* getStream() const { return osptr; }
  void setStream(ostream* osptr) { this->osptr = osptr; }
  void succeed_() { ++nPass; }
  long report() const;
  virtual void reset() { nPass = nFail = 0; }
};

} // namespace TestSuite
#endif // TEST_H ///:~

//: TestSuite:Test.cpp {O}
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#include "Test.h"
#include <iostream>
#include <typeinfo>
using namespace std;
using namespace TestSuite;

void Test::do_test(bool cond, const std::string& lbl,
  const char* fname, long lineno) {
  if(!cond)
    do_fail(lbl, fname, lineno);
  else
    succeed_();
}

void Test::do_fail(const std::string& lbl,
  const char* fname, long lineno) {
  ++nFail;
  if(osptr) {
    *osptr << typeid(*this).name()
           << "failure: (" << lbl << ") , " << fname
           << " (line " << lineno << ")" << endl;
  }
}

long Test::report() const {
  if(osptr) {
    *osptr << "Test \"" << typeid(*this).name()
           << "\":\n\tPassed: " << nPass
           << "\tFailed: " << nFail
           << endl;
  }
  return nFail;
} ///:~