c++为字符串类 String 增加下面的成员函数: 1)bool is substring(const char *sub str);//判断sub str是否为当前字符串的子2)bool is substring(const string& sub_str);//判断sub_str是否为当前字符串的子串3)String substring(int start,int len);//取从位置start开始、长度为len的子4)int find replace str(const char *find str, const char *replace_str);;需写3个文件，类的定义在.h中，类的实现在 .cpp中，主函数在main.cpp中 • 所有的数据成员私有 //查找所有子串find str并替换成replace str，返回替换的次数 5)void remove spaces();//删除字符串中的空格 6)int to int();//把由数字构成的字符串转成int类型的值 7)void to_lower_case();//把字符串中的大写字母转成小写字母

好的，以下是对应的代码实现：

String.h 文件

#ifndef STRING_H
#define STRING_H

class String {
public:
    String();
    String(const char* str);
    String(const String& str);
    ~String();
    String& operator=(const char* str);
    String& operator=(const String& str);
    bool operator==(const char* str) const;
    bool operator==(const String& str) const;
    bool isSubstring(const char* sub_str) const;
    bool isSubstring(const String& sub_str) const;
    String substring(int start, int len) const;
    int findReplaceStr(const char* find_str, const char* replace_str);
    void removeSpaces();
    int toInt() const;
    void toLowerCase();
private:
    char* _str;
    int _len;
};

#endif // STRING_H

String.cpp 文件

#include "String.h"
#include <cstring>
#include <cctype>

String::String() {
    _str = nullptr;
    _len = 0;
}

String::String(const char* str) {
    _len = strlen(str);
    _str = new char[_len + 1];
    strcpy(_str, str);
}

String::String(const String& str) {
    _len = str._len;
    _str = new char[_len + 1];
    strcpy(_str, str._str);
}

String::~String() {
    delete[] _str;
}

String& String::operator=(const char* str) {
    if (_str != nullptr) {
        delete[] _str;
    }
    _len = strlen(str);
    _str = new char[_len + 1];
    strcpy(_str, str);
    return *this;
}

String& String::operator=(const String& str) {
    if (_str != nullptr) {
        delete[] _str;
    }
    _len = str._len;
    _str = new char[_len + 1];
    strcpy(_str, str._str);
    return *this;
}

bool String::operator==(const char* str) const {
    return strcmp(_str, str) == 0;
}

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

bool String::isSubstring(const char* sub_str) const {
    return strstr(_str, sub_str) != nullptr;
}

bool String::isSubstring(const String& sub_str) const {
    return strstr(_str, sub_str._str) != nullptr;
}

String String::substring(int start, int len) const {
    if (start + len > _len) {
        len = _len - start;
    }
    char* substr = new char[len + 1];
    strncpy(substr, _str + start, len);
    substr[len] = '\0';
    return String(substr);
}

int String::findReplaceStr(const char* find_str, const char* replace_str) {
    int count = 0;
    int find_len = strlen(find_str);
    int replace_len = strlen(replace_str);
    char* p = _str;
    while ((p = strstr(p, find_str)) != nullptr) {
        int rest_len = _len - (p - _str) - find_len;
        memmove(p + replace_len, p + find_len, rest_len + 1);
        memcpy(p, replace_str, replace_len);
        p += replace_len;
        _len += replace_len - find_len;
        count++;
    }
    return count;
}

void String::removeSpaces() {
    char* p = _str;
    int i = 0, j = 0;
    while (p[i] != '\0') {
        if (p[i] != ' ') {
            p[j++] = p[i];
        }
        i++;
    }
    p[j] = '\0';
    _len = j;
}

int String::toInt() const {
    int num = 0;
    char* p = _str;
    while (*p != '\0' && isdigit(*p)) {
        num = num * 10 + (*p - '0');
        p++;
    }
    return num;
}

void String::toLowerCase() {
    char* p = _str;
    while (*p != '\0') {
        *p = tolower(*p);
        p++;
    }
}

main.cpp 文件

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

using namespace std;

int main() {
    String str1 = "Hello, World!";
    cout << "isSubstring(\"World\") = " << str1.isSubstring("World") << endl;
    cout << "isSubstring(\"world\") = " << str1.isSubstring("world") << endl;
    cout << "isSubstring(\"Hello\") = " << str1.isSubstring("Hello") << endl;
    cout << "substring(7, 5) = " << str1.substring(7, 5) << endl;
    cout << "before replace: " << str1 << endl;
    cout << "replace count: " << str1.findReplaceStr("World", "C++") << endl;
    cout << "after replace: " << str1 << endl;
    String str2 = "   This is a string with spaces.   ";
    cout << "before removeSpaces: " << str2 << endl;
    str2.removeSpaces();
    cout << "after removeSpaces: " << str2 << endl;
    String str3 = "12345";
    cout << "toInt() = " << str3.toInt() << endl;
    String str4 = "AbCdEf";
    cout << "before toLowerCase: " << str4 << endl;
    str4.toLowerCase();
    cout << "after toLowerCase: " << str4 << endl;
    return 0;
}

注意，为了简便起见，这里的 String 类并没有考虑到异常情况，比如构造函数中传入空指针，substring 方法中 start 或 len 参数非法等等，需要使用时请酌情添加异常处理。