本文介绍了C++ 运算符重载的用法，以我们构造的string类为例子，说明重载的用法。
构造我们自己的string类
声明如下
class mystring_
{
public:
mystring_(/* args */);
mystring_(const mystring_ &mstr);
mystring_(const char *m_str);
mystring_(const string);
~mystring_();
friend ostream &operator<<(ostream &os, const mystring_ &mystr1);
mystring_ &operator=(const mystring_ &mystr);
mystring_ &operator=(string str);
mystring_ &operator=(const char *cstr);
friend mystring_ operator+(const mystring_ &str1, const mystring_ &str2);
char operator[](unsigned int index);
friend class mystringOpr_;
private:
char *m_str;
};
在string类里重载了输出运算符<<，赋值运算符=, 加法运算符+, 取下标运算符[], 又声明了友元类mystringOpr。
class mystringOpr_
{
public:
bool operator()(const mystring_ &, const mystring_ &);
};
该类重载了()运算符，这样mystringOpr的实例对象就是可调用对象了，可以当作仿函数使用。
mystring_类的具体实现
我们先实现构造函数和析构函数
mystring_::mystring_(/* args */) : m_str("")
{
}
mystring_::mystring_(const mystring_ &mystr)
{
if (&mystr == this)
{
return;
}
if (mystr.m_str == nullptr)
{
m_str = nullptr;
return;
}
size_t len = strlen(mystr.m_str);
m_str = new char[len + 1];
strcpy(m_str, mystr.m_str);
m_str[len] = '\0';
}
mystring_::mystring_(const char *mstr)
{
cout << "use mystring_ construct , param is const char *" << endl;
size_t len = strlen(mstr);
m_str = new char[len + 1];
strcpy(m_str, mstr);
m_str[len] = '\0';
}
mystring_::mystring_(const string str)
{
cout << "use mystring_ construct string str" << endl;
size_t len = str.length();
m_str = new char[len + 1];
strncpy(m_str, str.c_str(), len);
m_str[len] = '\0';
}
mystring_::~mystring_()
{
if (m_str == "" || m_str == nullptr)
{
cout << "begin destruct , str is null or empty" << endl;
}
else
{
cout << "begin destruct " << m_str << endl;
}
if (m_str == nullptr || m_str == "")
{
return;
}
delete[] m_str;
m_str = nullptr;
}
实现了形参不同的拷贝构造函数，包括const string类型，const char*类型，const mystring_ &类型，以及默认的无参构造函数。
接下来重载+运算符
mystring_ operator+(const mystring_ &str1, const mystring_ &str2)
{
size_t len = strlen(str1.m_str) + strlen(str2.m_str) + 1;
mystring_ strtotal;
strtotal.m_str = new char[len + 1];
memset(strtotal.m_str, 0, len);
memcpy(strtotal.m_str, str1.m_str, strlen(str1.m_str));
strcat(strtotal.m_str, str2.m_str);
return strtotal;
}
重载=运算符
mystring_ &mystring_::operator=(const mystring_ &mystr)
{
if (&mystr == this)
{
return *this;
}
if (this->m_str != nullptr)
{
delete[] m_str;
this->m_str = nullptr;
}
if (mystr.m_str == nullptr)
{
m_str = nullptr;
}
size_t len = strlen(mystr.m_str);
m_str = new char[len + 1];
strcpy(m_str, mystr.m_str);
m_str[len] = '\0';
return *this;
}
mystring_ &mystring_::operator=(string str)
{
cout << "use operator = string str" << endl;
if (this->m_str != nullptr)
{
delete[] m_str;
this->m_str = nullptr;
}
size_t len = str.length();
m_str = new char[len + 1];
strncpy(m_str, str.c_str(), len);
m_str[len] = '\0';
return *this;
}
mystring_ &mystring_::operator=(const char *cstr)
{
cout << "use operator = const char*" << endl;
if (this->m_str != nullptr)
{
delete[] m_str;
this->m_str = nullptr;
}
size_t len = strlen(cstr);
m_str = new char[len + 1];
strncpy(m_str, cstr, len);
m_str[len] = '\0';
return *this;
}
重载输出运算符
ostream &operator<<(ostream &os, const mystring_ &mystr1)
{
if (mystr1.m_str == nullptr)
{
os << "mystring_ data is null" << endl;
return os;
}
os << "mystring_ data is " << mystr1.m_str << endl;
return os;
}
重载取下表运算符[]
char mystring_::operator[](unsigned int index)
{
if (index >= strlen(m_str))
{
throw "index out of range!!!";
}
return m_str[index];
}
我们写一个函数测试重载效果
void use_mystr_1()
{
auto mystr1 = mystring_("hello zack");
auto mystr2(mystr1);
auto mystr3 = mystring_();
cout << mystr1 << mystr2 << mystr3 << endl;
mystring_ mystr4 = ", i love u";
auto mystr5 = mystr1 + mystr4;
cout << "mystr4 is " << mystr4 << endl;
cout << "mystr5 is " << mystr5 << endl;
mystring_ mystr6 = "";
auto mystr7 = mystr5 + mystr6;
cout << "mystr7 is " << mystr7 << endl;
auto ch = mystr1[4];
cout << "index is 4, char is " << ch << endl;
}
测试结果如下
use mystring_ construct , param is const char *
mystring_ data is hello zack
mystring_ data is hello zack
mystring_ data is
use mystring_ construct , param is const char *
mystr4 is mystring_ data is , i love u
mystr5 is mystring_ data is hello zack, i love u
use mystring_ construct , param is const char *
mystr7 is mystring_ data is hello zack, i love u
index is 4, char is o
可以看出调用不同的构造函数会打印不同的日志，重载+运算符实现了字符串的拼接，重载赋值实现了拷贝。
通过仿函数mystringOpr_实现排序
我们先实现仿函数，这样就可以对我们mystring_类对象排序了
bool mystringOpr_::operator()(const mystring_ &str1, const mystring_ &str2)
{
if (strlen(str1.m_str) > strlen(str2.m_str))
{
return true;
}
if (strlen(str1.m_str) < strlen(str2.m_str))
{
return false;
}
for (unsigned int i = 0; i < strlen(str1.m_str); i++)
{
return str1.m_str[i] > str2.m_str[i];
}
}
我们调用sort实现mystring_排序,在原来的基础上补充排序代码
void use_mystr_1()
{
auto mystr1 = mystring_("hello zack");
auto mystr2(mystr1);
auto mystr3 = mystring_();
cout << mystr1 << mystr2 << mystr3 << endl;
mystring_ mystr4 = ", i love u";
auto mystr5 = mystr1 + mystr4;
cout << "mystr4 is " << mystr4 << endl;
cout << "mystr5 is " << mystr5 << endl;
mystring_ mystr6 = "";
auto mystr7 = mystr5 + mystr6;
cout << "mystr7 is " << mystr7 << endl;
auto ch = mystr1[4];
cout << "index is 4, char is " << ch << endl;
std::vector<mystring_> vec_mystring_;
vec_mystring_.push_back(mystr1);
vec_mystring_.push_back(mystr2);
vec_mystring_.push_back(mystr3);
vec_mystring_.push_back(mystr4);
vec_mystring_.push_back(mystr5);
vec_mystring_.push_back(mystr6);
vec_mystring_.push_back(mystr7);
sort(vec_mystring_.begin(), vec_mystring_.end(), mystringOpr_());
cout << "====================after sort ..." << endl;
for_each(vec_mystring_.begin(), vec_mystring_.end(), [](const mystring_ &str)
{ cout << str << endl; });
}
上述代码通过sort排序vector里的mystring_对象，并用lambda表达式输出，可以看到输出
use mystring_ construct , param is const char *
mystring_ data is hello zack
mystring_ data is hello zack
mystring_ data is
use mystring_ construct , param is const char *
mystr4 is mystring_ data is , i love u
mystr5 is mystring_ data is hello zack, i love u
use mystring_ construct , param is const char *
mystr7 is mystring_ data is hello zack, i love u
index is 4, char is o
====================after sort ...
mystring_ data is hello zack, i love u
mystring_ data is hello zack, i love u
mystring_ data is hello zack
mystring_ data is hello zack
mystring_ data is , i love u
mystring_ data is
mystring_ data is
我们通过仿函数和sort实现了mystring_从大到小的排序。
总结
源码链接：https://gitee.com/secondtonone1/cpplearn
视频链接: https://www.bilibili.com/video/BV1zu411e7DW/
网友评论