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
#include
#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
#include
#include
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
#include
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;
} ///:~