问个构造函数的问题

[kofwr]

头文件
#include <iostream>
#include <string>

class Sales_item {
friend bool operator==(const Sales_item&, const Sales_item&);
// other members as before
public:
// added constructors to initialize from a string or an istream
Sales_item(const std::string &book):
isbn(book), units_sold(0), revenue(0.0) { }
Sales_item(std::istream &is) { is >> *this; }
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
public:
// operations on Sales_item objects
// member binary operator: left-hand operand bound to implicit this pointer
Sales_item& operator+=(const Sales_item&);
// other members as before

public:
// operations on Sales_item objects
double avg_price() const;
bool same_isbn(const Sales_item &rhs) const
{ return isbn == rhs.isbn; }
// default constructor needed to initialize members of built-in type
Sales_item(): units_sold(0), revenue(0.0) { }
// private members as before
private:
std::string isbn;
unsigned units_sold;
double revenue;

};


// nonmember binary operator: must declare a parameter for each operand
Sales_item operator+(const Sales_item&, const Sales_item&);

inline bool
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
// must be made a friend of Sales_item
return lhs.units_sold == rhs.units_sold &&
lhs.revenue == rhs.revenue &&
lhs.same_isbn(rhs);
}

inline bool
operator!=(const Sales_item &lhs, const Sales_item &rhs)
{
return !(lhs == rhs); // != defined in terms of operator==
}

using std::istream; using std::ostream;

// assumes that both objects refer to the same isbn
inline
Sales_item& Sales_item::operator+=(const Sales_item& rhs)
{
units_sold += rhs.units_sold;
revenue += rhs.revenue;
return *this;
}

// assumes that both objects refer to the same isbn
inline
Sales_item
operator+(const Sales_item& lhs, const Sales_item& rhs)
{
Sales_item ret(lhs); // copy lhs into a local object that we'll return
ret += rhs; // add in the contents of rhs
return ret; // return ret by value
}

inline
istream&
operator>>(istream& in, Sales_item& s)
{
double price;
in >> s.isbn >> s.units_sold >> price;
// check that the inputs succeeded
if (in)
s.revenue = s.units_sold * price;
else
s = Sales_item(); // input failed: reset object to default state
return in;
}

inline
ostream&
operator<<(ostream& out, const Sales_item& s)
{
out << s.isbn << "\t" << s.units_sold << "\t"
<< s.revenue << "\t" << s.avg_price();
return out;
}

inline
double Sales_item::avg_price() const
{
if (units_sold)
return revenue/units_sold;
else
return 0;
}


#endif

执行文件
#include <iostream>
#include "Sales_item.h"

int main()
{
Sales_item item1, item2;

std::cin >> item1 >> item2; //read a pair of transactions
std::cout << item1 + item2 << std::endl; //print their sum

return 0;
}
这里item1 + item2 调用了重载的+操作符，在头文件中
inline
Sales_item
operator+(const Sales_item& lhs, const Sales_item& rhs)
{
Sales_item ret(lhs); // 请问这个调用的是那个构造函数？似乎在头文件的定义中没有参数是lhs引用的构造类型，难道是c++有默认的这种构造方式？
ret += rhs; //
return ret; //
}
我的问题是+重载符实现的注释问题，关于构造这个构造函数的。感谢回答

[caocheng8230]

operator+要使用友元函数

[todototry]

operator+必需重载为名字空间成员（而且一般声明为友元，直接存取私有数据成员，免去调用成员函数的开销），而并不能声明成类成员函数

[kofwr]

我的问题是“Sales_item ret(lhs); // 请问这个调用的是那个构造函数？似乎在头文件的定义中没有参数是lhs引用的构造类型，难道是c++有默认的这种构造方式？
”

[caocheng8230]

Sales_item ret(lhs);
编译首先会找到你的拷备构造函数Sales_item::Sales_item(Sales_item&)
没有则找你的赋值函数Sales_item::operator=(Sales_item&)
没有则自动为你构造一个浅拷备将你的成员变量值进行浅拷备

[caocheng8230]

你可以按照我的思路去在实现函数内部去写一些输出,来验证.

C++Primer当中有我个的详细说明.

[OOPhaisky]

Sales_item ret(lhs); // 请问这个调用的是那个构造函数？似乎在头文件的定义中没有参数是lhs引用的构造类型，难道是c++有默认的这种构造方式？
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这里调用的是copy constructor，如果你没有定义，编译器会为你自动合成一个，执行“按成员拷贝”功能。