C++中的explicit constructor

 1 #include <iostream>
 2  using std::cout;
 3 using std::endl;
 4 class complexNumbers {
 5   double real, img;
 6 public:
 7   complexNumbers() : real(0), img(0) { }
 8   complexNumbers(const complexNumbers& c) { real = c.real; img = c.img; }
 9   explicit complexNumbers( double r, double i = 0.0) { real = r; img = i; }
10   friend void display(complexNumbers cx);
11 };
12 void display(complexNumbers cx){
13   cout<<"Real Part: "<<cx.real<<" Imag Part: "<<cx.img<<endl;
14 }
15 int main() {
16   complexNumbers one(1);
17   display(one);
18   complexNumbers two = 2;  //错误！必须显示地进行类型转换
19   display(200);            //error: conversion from ‘int’ to non-scalar type ‘complexNumbers’ requested
20   return 0;
21 }