请问：调用构造函数有关问题

请教：调用构造函数问题

C/C++ code

class B{public:B(){   cout << "constructor" << endl;}~B(){   cout << "deconstructo" << endl;}B(int i) : data(i){   cout << "constructor by parameter " << data;}private:int data;};B Play(B b){return b;}int main(int argc, char *argv[]){B t1 = Play(5);  //这里为什么可以传5B t2 = Play(t1);return 0;}

C/C++ code

#include <iostream>using std::cout;using std::endl;class B {public:    B() {        cout << "constructor" << endl;    }    ~B() {        cout << "deconstructo" << endl;    }    explicit B(int i) :            data(i) {        cout << "constructor by parameter " << data;    }private:    int data;};B Play(B b) {    return b;}int main(int argc, char *argv[]) {    B t1 = Play(5); //这里为什么可以传5    B t2 = Play(t1);    return 0;}
[解决办法]
《C++ Primer》(英文版，第4版) p. 461提到：
A constructor that can be called with a single argument defines an implicit conversion from the parameter type to the class type.

(如果一个类的构造函数只接收一个实参(argument)，那么就定义了一个从形参(parameter)数据类型到类的类型的隐式转换。)  

详情请参考：
隐式类型转换和explicit关键字的作用