C++ “没有可行的重载'='”为什么？
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"No viable overloaded '=' " why?
提问by user3377191
I'm making a project for one of my classes and I'm pretty sure I'm almost done with the project. Basically I have to input 2 people's tickets and I have to find the maximum and minimum price. I need to overload the * and / operators to fix that issue for the project. Also, the frienddeclaration is a necessity for this project via teacher's instructions.
我正在为我的一个班级制作一个项目，我很确定我几乎完成了这个项目。基本上我必须输入2人的票，我必须找到最高和最低价格。我需要重载 * 和 / 运算符来解决该项目的问题。此外，friend通过教师的指示，该项目的声明是必要的。
Now, for the issue. I'm trying to store the proper ticket variable (either t1 or t2) into t3 so I can return that to the main. When I use "=" to set either t1 to t3, it says "no viable overloaded '='". The following is my code:
现在，对于这个问题。我正在尝试将正确的票证变量（t1 或 t2）存储到 t3 中，这样我就可以将其返回到 main。当我使用“=”将 t1 设置为 t3 时，它说“没有可行的重载 '='”。以下是我的代码：
#include <iostream>
using namespace std;
class ticket
{
public:
ticket();
double input();
double output();
friend ticket operator *(const ticket &t1, const ticket &t2);
friend ticket operator /(const ticket &t1, const ticket &t2);
private:
void cost();
string name;
double miles, price;
int transfers;
};
int main()
{
ticket customer1, customer2, customer3;
//------------------------------------------------
cout << "*** Customer 1 ***" << endl;
customer1.input();
cout << "--- Entered, thank you ---" << endl;
cout << "*** Customer 2 ***" << endl;
customer2.input();
cout << "--- Enter, thank you ---" << endl;
//------------------------------------------------
//------------------------------------------------
cout << "Testing of the * operator: " << endl;
customer3 = customer1 * customer2;
cout << "*** Database printout: ***" << endl;
customer3.output();
cout << endl;
cout << "--- End of Database ---" << endl;
//------------------------------------------------
//------------------------------------------------
cout << "Testing of the / operator:" << endl;
customer3 = customer1 / customer2;
cout << "*** Database printout: ***" << endl;
customer3.output();
cout << endl;
cout << "--- End of Database ---" << endl;
//------------------------------------------------
return 0;
}
ticket operator *(const ticket &t1, const ticket &t2)
{
ticket t3;
if (t1.price > t2.price)
t3 = t1.price;
else
t3 = t2.price;
return t3;
}
ticket operator /(const ticket &t1, const ticket &t2)
{
ticket t3;
if (t1.price < t2.price)
t3 = t1.price;
else
t3 = t2.price;
return t3;
}
ticket::ticket()
{
}
double ticket::input()
{
cout << "Miles? ";
cin >> miles;
cout << endl << "Transers? ";
cin >> transfers;
cout << endl << "Name? ";
cin >> name;
cost();
cout << endl << "Price is: " << price << endl;
return miles;
}
double ticket::output()
{
cout << name << '\t' << miles << "mi \t " << transfers << " transfers \t" << price;
return miles;
}
void ticket::cost()
{
price = (.5 * miles) - (50 * transfers);
}
回答by sepp2k
You don't define an operator=for ticketthat takes a double as its argument. As a consequence you can't assign doubles to ticketobjects.
您没有定义一个以 double 作为参数的operator=for ticket。因此，您不能为ticket对象分配双精度值。
回答by Yuchen Zhong
This is how I get the same compile error. I have mark one of my getter function as constwhile I still want to modify class member variable. See the following simple example:
这就是我得到相同编译错误的方式。我已经标记了我的 getter 函数之一，因为const我仍然想修改类成员变量。请参阅以下简单示例：
class CPath {
private:
std::string m_extension;
public:
std::string GetExtension() const {
if (m_extension.length()==0) {
m_extension = "txt";
}
return m_extension;
}
}
In this case, we have two solutions:
在这种情况下，我们有两种解决方案：
1) drop constmodifier in the function definition;
2) mark the property m_extensionas mutable.
1)const函数定义中的drop修饰符；
2) 将该属性标记m_extension为mutable。
回答by michaeltang
you might want to have member function
你可能想要成员函数
set_price(const& double price)
set_price(const& double price)
so you can change the error code like this,which would be better
所以你可以像这样更改错误代码，这会更好
if (t1.price > t2.price)
t3.set_price(t1.price);
else
t3.set_price(t2.price);
