在C ++中从用户定义类型到原始类型的隐式转换 [英] Implicit conversion from user-defined type to primitive type in C++

问题描述

我能够找到很多有关从隐式转换(例如，从int到用户定义类型)的信息.即，如果构造函数将int作为其参数，并且未以显式"作为前缀，则可能会发生隐式转换.

I am able to find plenty of information about implicit conversion from, say, an int to a user defined type. i.e. if a constructor takes an int as its parameter and is not prefaced by "explicit" then implicit conversions can occur.

如果我希望我的班级将隐式转换为一个整数怎么办?

What if I want my class to implicitly convert to an int?

例如，需要在SimpleClass的内部或外部添加什么函数，以便主函数可以编译并向控制台输出"1"?(请参阅评论)

For example, what function needs to be added either inside or outside of SimpleClass such that the main function will compile and output "1" to the console? (see comments)

#include <iostream>

class SimpleClass
{
private:
    int m_int;
public:
    SimpleClass(int value)
    : m_int(value) {}
};

int main(int argc, const char * argv[])
{
    SimpleClass simpleclass(1);
    int i = simpleclass; // does not complile
    std::cout << i << std::endl; // should output "1" to the console
    return 0;
}

推荐答案

隐式转换可以通过两种方式定义:

Implicit conversions can be defined in two ways:

• 非显式单参数构造函数.
• 非显式转换函数(又称转换运算符)，N3337 12.3.2

后面的代码允许定义从类类型到原始类型的转换.只需添加

The later allows defining conversion from class type to primitive type. Just add

class SimpleClass {
   // ...
   operator int() const;
};

SimpleClass::operator int() const
{
   return m_int;
}

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