数据结构开发(13)：字符串类的创建

0.目录

1.字符串类的创建

• 1.1 基本字符串类的创建
• 1.2 字符串类的功能扩展

2.小结

1.字符串类的创建

1.1 基本字符串类的创建

历史遗留问题：

• C语言不支持真正意义上的字符串
• C语言用字符数组和一组函数实现字符串操作
• C语言不支持自定义类型，因此无法获得字符串类型

从C到C++的进化过程引入了自定义类型
在C++中可以通过类完成字符串类型的定义
问题：

• C++中的原生类型系统是否包含字符串类型？

StLib中字符串类的设计：

StLib中字符串类的实现：

实现时的注意事项：

• 无缝实现 String 对象与 char* 字符串的互操作
• 操作符重载函数需要考虑是否支持 const 版本
• 通过C语言中的字符串函数实现 String 的成员函数

在StLib中实现自定义 String 类：
StString.h

#ifndef STSTRING_H
#define STSTRING_H

#include "Object.h"

namespace StLib
{

class String : public Object
{
protected:
    char* m_str;
    int m_length;

    void init(const char* s);
public:
    String();
    String(char c);
    String(const char* s);
    String(const String& s);

    int length() const;
    const char* str() const;

    bool operator == (const String& s) const;
    bool operator == (const char* s) const;
    bool operator != (const String& s) const;
    bool operator != (const char* s) const;
    bool operator > (const String& s) const;
    bool operator > (const char* s) const;
    bool operator < (const String& s) const;
    bool operator < (const char* s) const;
    bool operator >= (const String& s) const;
    bool operator >= (const char* s) const;
    bool operator <= (const String& s) const;
    bool operator <= (const char* s) const;

    String operator + (const String& s) const;
    String operator + (const char* s) const;
    String& operator += (const String& s);
    String& operator += (const char* s);

    String& operator = (const String& s);
    String& operator = (const char* s);
    String& operator = (char c);

    ~String();
};

}

#endif // STSTRING_H

StString.cpp

#include <cstring>
#include <cstdlib>
#include "StString.h"
#include "Exception.h"

using namespace std;

namespace StLib
{

void String::init(const char *s)
{
    m_str = strdup(s);

    if( m_str )
    {
        m_length = strlen(m_str);
    }
    else
    {
        THROW_EXCEPTION(NoEnoughMemoryException, "No memory to create String object ...");
    }
}

String::String()
{
    init("");
}

String::String(char c)
{
    char s[] = {c, ''};

    init(s);
}

String::String(const char *s)
{
    init(s ? s : "");
}

String::String(const String &s)
{
    init(s.m_str);
}

int String::length() const
{
    return m_length;
}

const char *String::str() const
{
    return m_str;
}

bool String::operator == (const String& s) const
{
    return (strcmp(m_str, s.m_str) == 0);
}

bool String::operator == (const char* s) const
{
    return (strcmp(m_str, s ? s : "") == 0);
}

bool String::operator != (const String& s) const
{
    return !(*this == s);
}

bool String::operator != (const char* s) const
{
    return !(*this == s);
}

bool String::operator > (const String& s) const
{
    return (strcmp(m_str, s.m_str) > 0);
}

bool String::operator > (const char* s) const
{
    return (strcmp(m_str, s ? s : "") > 0);
}

bool String::operator < (const String& s) const
{
    return (strcmp(m_str, s.m_str) < 0);
}

bool String::operator < (const char* s) const
{
    return (strcmp(m_str, s ? s : "") < 0);
}

bool String::operator >= (const String& s) const
{
    return (strcmp(m_str, s.m_str) >= 0);
}

bool String::operator >= (const char* s) const
{
    return (strcmp(m_str, s ? s : "") >= 0);
}

bool String::operator <= (const String& s) const
{
    return (strcmp(m_str, s.m_str) <= 0);
}

bool String::operator <= (const char* s) const
{
    return (strcmp(m_str, s ? s : "") <= 0);
}

String String::operator + (const String& s) const
{
    return (*this + s.m_str);
}

String String::operator + (const char* s) const
{
    String ret;
    int len = m_length + strlen(s ? s : "");
    char* str = reinterpret_cast<char*>(malloc(len + 1));

    if( str )
    {
        strcpy(str, m_str);
        strcat(str, s ? s : "");

        free(ret.m_str);

        ret.m_str = str;
        ret.m_length = len;
    }
    else
    {
        THROW_EXCEPTION(NoEnoughMemoryException, "No memory to add String values ...");
    }

    return ret;
}

String& String::operator += (const String& s)
{
    return (*this = *this + s.m_str);
}

String& String::operator += (const char* s)
{
    return (*this = *this + s);
}

String& String::operator = (const String& s)
{
    return (*this = s.m_str);
}

String& String::operator = (const char* s)
{
    if( m_str != s )
    {
        char* str = strdup(s ? s : "");

        if( str )
        {
            free(m_str);

            m_str = str;
            m_length = strlen(m_str);
        }
        else
        {
            THROW_EXCEPTION(NoEnoughMemoryException, "No memory to assign new String value ...");
        }
    }

    return *this;
}

String& String::operator = (char c)
{
    char s[] = {c, ''};

    return (*this = s);
}

String::~String()
{
    free(m_str);
}

}

main.cpp测试

#include <iostream>
#include "StString.h"

using namespace std;
using namespace StLib;

void test_1()
{
    cout << "test_1() begin ..." << endl;

    String s;

    s = 'D';

    cout << s.str() << endl;
    cout << s.length() << endl;
    cout << (s == "D") << endl;
    cout << (s > "CCC") << endl;

    s += " Hello World ";

    cout << s.str() << endl;
    cout << s.length() << endl;
    cout << (s == "D Hello World ") << endl;

    cout << "test_1() end ..." << endl;
}

void test_2()
{
    cout << "test_2() begin ..." << endl;

    String a[] = {"E", "D", "C", "B", "A"};
    String min = a[0];

    for(int i=0; i<5; i++)
    {
        if( min > a[i] )
        {
            min = a[i];
        }
    }

    cout << "min = " << min.str() << endl;

    cout << "test_2() end ..." << endl;
}

int main()
{
    test_1();
    test_2();

    return 0;
}

运行结果为：

test_1() begin ...
D
1
1
1
D Hello World 
14
1
test_1() end ...
test_2() begin ...
min = A
test_2() end ...

1.2 字符串类的功能扩展

字符串类中的常用成员函数：

重载数组访问操作符 [ ]：

• char& operator [] (int i);
• char operator [] (int i) const;

注意事项：

• 当 i 的取值不合法时，抛出异常
  1. 合法范围：( 0 <= i ) && ( i < m_length )

重载数组访问操作符 [ ]：

public:
    char& operator [] (int i);
    char operator [] (int i) const;

具体实现：

char& String::operator [] (int i)
{
    if( (0 <= i) && (i < m_length) )
    {
        return m_str[i];
    }
    else
    {
        THROW_EXCEPTION(IndexOutOfBoundsException, "Parameter i is invalid ...");
    }
}

char String::operator [] (int i) const
{
    return (const_cast<String&>(*this))[i];
}

判断是否以指定字符串开始或结束：

• bool startWith(const char* s) const;
• bool startWith(const String& s) const;
• bool endOf(const char* s) const;
• bool endOf(const String& s) const;

判断是否以指定字符串开始或结束：
加入一个equal函数用于比较。

protected:
    bool equal(const char* l, const char* r, int len) const;
public:
    bool startWith(const char* s) const;
    bool startWith(const String& s) const;
    bool endOf(const char* s) const;
    bool endOf(const String& s) const;

具体实现：

bool String::equal(const char* l, const char* r, int len) const
{
    bool ret = true;

    for(int i=0; i<len && ret; i++)
    {
        ret = ret && (l[i] == r[i]);
    }

    return ret;
}

bool String::startWith(const char* s) const
{
    bool ret = (s != NULL);

    if( ret )
    {
        int len = strlen(s);

        ret = (len < m_length) && equal(m_str, s, len);
    }

    return ret;
}

bool String::startWith(const String& s) const
{
    return startWith(s.m_str);
}

bool String::endOf(const char* s) const
{
    bool ret = (s != NULL);

    if( ret )
    {
        int len = strlen(s);
        char* str = m_str + (m_length - len);

        ret = (len < m_length) && equal(str, s, len);
    }

    return ret;
}

bool String::endOf(const String& s) const
{
    return endOf(s.m_str);
}

在指定位置处插入字符串：

• String& insert(int i, const char* s);
• String& insert(int i, const String& s);

在指定位置处插入字符串：

public:
    String& insert(int i, const char* s);
    String& insert(int i, const String& s);

具体实现：

String& String::insert(int i, const char* s)
{
    if( (0 <= i) && (i <= m_length) )
    {
        if( (s != NULL) && (s[0] != '') )
        {
            int len = strlen(s);
            char* str = reinterpret_cast<char*>(malloc(m_length + len + 1));

            if( str != NULL )
            {
                strncpy(str, m_str, i);
                strncpy(str + i, s, len);
                strncpy(str + i + len, m_str + i, m_length - i);

                str[m_length + len] = '';

                free(m_str);

                m_str = str;
                m_length = m_length + len;
            }
            else
            {
                THROW_EXCEPTION(NoEnoughMemoryException, "No memory to insert string value ...");
            }
        }
    }
    else
    {
        THROW_EXCEPTION(IndexOutOfBoundsException, "Parameter i is invalid ...");
    }

    return *this;
}

String& String::insert(int i, const String& s)
{
    return insert(i, s.m_str);
}

去掉字符串两端的空白字符：

• String& trim();

去掉字符串两端的空白字符：

public:
    String& trim();

具体实现：

String& String::trim()
{
    int b = 0;
    int e = m_length - 1;

    while( m_str[b] == ' ' ) b++;
    while( m_str[e] == ' ' ) e--;

    if( b == 0 )
    {
        m_str[e + 1] = '';

        m_length = e + 1;
    }
    else
    {
        for(int i=0, j=b; j<=e; i++, j++)
        {
            m_str[i] = m_str[j];
        }

        m_str[e - b + 1] = '';

        m_length = e - b + 1;
    }

    return *this;
}

main.cpp测试：

#include <iostream>
#include "StString.h"

using namespace std;
using namespace StLib;

int main()
{
    String s = "    ABC    ";

    if( s.trim().insert(0, "Hello").endOf("ABC") && s.startWith("Hello") )
    {
        cout << "[" <<s.str() << "]" << endl;
    }

    return 0;
}

运行结果为;

[HelloABC]

2.小结

• C/C++语言本身不支持字符串类型
• C语言通过字符数组和一组函数支持字符串操作
• C++通过自定义字符串类型支持字符串操作
• 字符串类型通过C语言中的字符串函数实现

最终的自定义字符串类代码：
StString.h

#ifndef STSTRING_H
#define STSTRING_H

#include "Object.h"

namespace StLib
{

class String : public Object
{
protected:
    char* m_str;
    int m_length;

    void init(const char* s);
    bool equal(const char* l, const char* r, int len) const;
public:
    String();
    String(char c);
    String(const char* s);
    String(const String& s);

    int length() const;
    const char* str() const;
    bool startWith(const char* s) const;
    bool startWith(const String& s) const;
    bool endOf(const char* s) const;
    bool endOf(const String& s) const;
    String& insert(int i, const char* s);
    String& insert(int i, const String& s);
    String& trim();

    char& operator [] (int i);
    char operator [] (int i) const;
    bool operator == (const String& s) const;
    bool operator == (const char* s) const;
    bool operator != (const String& s) const;
    bool operator != (const char* s) const;
    bool operator > (const String& s) const;
    bool operator > (const char* s) const;
    bool operator < (const String& s) const;
    bool operator < (const char* s) const;
    bool operator >= (const String& s) const;
    bool operator >= (const char* s) const;
    bool operator <= (const String& s) const;
    bool operator <= (const char* s) const;

    String operator + (const String& s) const;
    String operator + (const char* s) const;
    String& operator += (const String& s);
    String& operator += (const char* s);

    String& operator = (const String& s);
    String& operator = (const char* s);
    String& operator = (char c);

    ~String();
};

}

#endif // STSTRING_H

StString.cpp

#include <cstring>
#include <cstdlib>
#include "StString.h"
#include "Exception.h"

using namespace std;

namespace StLib
{

void String::init(const char *s)
{
    m_str = strdup(s);

    if( m_str )
    {
        m_length = strlen(m_str);
    }
    else
    {
        THROW_EXCEPTION(NoEnoughMemoryException, "No memory to create String object ...");
    }
}

String::String()
{
    init("");
}

String::String(char c)
{
    char s[] = {c, ''};

    init(s);
}

String::String(const char *s)
{
    init(s ? s : "");
}

String::String(const String &s)
{
    init(s.m_str);
}

int String::length() const
{
    return m_length;
}

const char* String::str() const
{
    return m_str;
}

bool String::equal(const char* l, const char* r, int len) const
{
    bool ret = true;

    for(int i=0; i<len && ret; i++)
    {
        ret = ret && (l[i] == r[i]);
    }

    return ret;
}

bool String::startWith(const char* s) const
{
    bool ret = (s != NULL);

    if( ret )
    {
        int len = strlen(s);

        ret = (len < m_length) && equal(m_str, s, len);
    }

    return ret;
}

bool String::startWith(const String& s) const
{
    return startWith(s.m_str);
}

bool String::endOf(const char* s) const
{
    bool ret = (s != NULL);

    if( ret )
    {
        int len = strlen(s);
        char* str = m_str + (m_length - len);

        ret = (len < m_length) && equal(str, s, len);
    }

    return ret;
}

bool String::endOf(const String& s) const
{
    return endOf(s.m_str);
}

String& String::insert(int i, const char* s)
{
    if( (0 <= i) && (i <= m_length) )
    {
        if( (s != NULL) && (s[0] != '') )
        {
            int len = strlen(s);
            char* str = reinterpret_cast<char*>(malloc(m_length + len + 1));

            if( str != NULL )
            {
                strncpy(str, m_str, i);
                strncpy(str + i, s, len);
                strncpy(str + i + len, m_str + i, m_length - i);

                str[m_length + len] = '';

                free(m_str);

                m_str = str;
                m_length = m_length + len;
            }
            else
            {
                THROW_EXCEPTION(NoEnoughMemoryException, "No memory to insert string value ...");
            }
        }
    }
    else
    {
        THROW_EXCEPTION(IndexOutOfBoundsException, "Parameter i is invalid ...");
    }

    return *this;
}

String& String::insert(int i, const String& s)
{
    return insert(i, s.m_str);
}

String& String::trim()
{
    int b = 0;
    int e = m_length - 1;

    while( m_str[b] == ' ' ) b++;
    while( m_str[e] == ' ' ) e--;

    if( b == 0 )
    {
        m_str[e + 1] = '';

        m_length = e + 1;
    }
    else
    {
        for(int i=0, j=b; j<=e; i++, j++)
        {
            m_str[i] = m_str[j];
        }

        m_str[e - b + 1] = '';

        m_length = e - b + 1;
    }

    return *this;
}

char& String::operator [] (int i)
{
    if( (0 <= i) && (i < m_length) )
    {
        return m_str[i];
    }
    else
    {
        THROW_EXCEPTION(IndexOutOfBoundsException, "Parameter i is invalid ...");
    }
}

char String::operator [] (int i) const
{
    return (const_cast<String&>(*this))[i];
}

bool String::operator == (const String& s) const
{
    return (strcmp(m_str, s.m_str) == 0);
}

bool String::operator == (const char* s) const
{
    return (strcmp(m_str, s ? s : "") == 0);
}

bool String::operator != (const String& s) const
{
    return !(*this == s);
}

bool String::operator != (const char* s) const
{
    return !(*this == s);
}

bool String::operator > (const String& s) const
{
    return (strcmp(m_str, s.m_str) > 0);
}

bool String::operator > (const char* s) const
{
    return (strcmp(m_str, s ? s : "") > 0);
}

bool String::operator < (const String& s) const
{
    return (strcmp(m_str, s.m_str) < 0);
}

bool String::operator < (const char* s) const
{
    return (strcmp(m_str, s ? s : "") < 0);
}

bool String::operator >= (const String& s) const
{
    return (strcmp(m_str, s.m_str) >= 0);
}

bool String::operator >= (const char* s) const
{
    return (strcmp(m_str, s ? s : "") >= 0);
}

bool String::operator <= (const String& s) const
{
    return (strcmp(m_str, s.m_str) <= 0);
}

bool String::operator <= (const char* s) const
{
    return (strcmp(m_str, s ? s : "") <= 0);
}

String String::operator + (const String& s) const
{
    return (*this + s.m_str);
}

String String::operator + (const char* s) const
{
    String ret;
    int len = m_length + strlen(s ? s : "");
    char* str = reinterpret_cast<char*>(malloc(len + 1));

    if( str )
    {
        strcpy(str, m_str);
        strcat(str, s ? s : "");

        free(ret.m_str);

        ret.m_str = str;
        ret.m_length = len;
    }
    else
    {
        THROW_EXCEPTION(NoEnoughMemoryException, "No memory to add String values ...");
    }

    return ret;
}

String& String::operator += (const String& s)
{
    return (*this = *this + s.m_str);
}

String& String::operator += (const char* s)
{
    return (*this = *this + s);
}

String& String::operator = (const String& s)
{
    return (*this = s.m_str);
}

String& String::operator = (const char* s)
{
    if( m_str != s )
    {
        char* str = strdup(s ? s : "");

        if( str )
        {
            free(m_str);

            m_str = str;
            m_length = strlen(m_str);
        }
        else
        {
            THROW_EXCEPTION(NoEnoughMemoryException, "No memory to assign new String value ...");
        }
    }

    return *this;
}

String& String::operator = (char c)
{
    char s[] = {c, ''};

    return (*this = s);
}

String::~String()
{
    free(m_str);
}

}