詭異的模板執行個體化..

#include "stdafx.h"#include <iostream>#include <conio.h>using namespace std;template <typename T>class C{public:C(int){cout<<"C(int)"<<endl;}};void Func(const C<double>& ) {cout<<"Func(C<double> const& )"<<endl;}int main(){Func(23);getch();}//////輸出:////C(int)////Func(C<double> const& ) 這裡的23 怎麼會由一個int 型轉換成一個類呢???template <typename T>class C{public:C(int){cout<<"C(int)"<<endl;}};void Func(const C<double>& ) {cout<<"Func(C<double> const& )"<<endl;}void Func(int) {cout<<"Func(int)"<<endl;}int main(){Func(23);getch();}// 輸出:// Func(int) 正確//**********************************************************template <typename T>class C{public:C(int){cout<<"C(int)"<<endl;}};void Func(const C<double>& ) {cout<<"Func(C<double> const& )"<<endl;}void Func(int) {cout<<"Func(int)"<<endl;}int main(){Func(23);C<int> tmpC(22);Func(tmpC); // error C2665: 'Func' : none of the 2 overloads could convert all the argument typesgetch();}//下面我把有歧義的地方注釋掉..發現不能轉換..//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++template <typename T>class C{public:C(int){cout<<"C(int)"<<endl;}};void Func(int) {cout<<"Func(int)"<<endl;}int main(){Func(23);C<int> tmpC(22);Func(tmpC);getch();}//error C2664: 'Func' : cannot convert parameter 1 from 'C<T>' to 'int'// 為什麼類不能轉換成int,而前面的int可以轉換成一個類呢 ???//^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^template <typename T>class C{public:C(int){cout<<"C(int)"<<endl;}};void Func(const C<double>& ) {cout<<"Func(C<double> const& )"<<endl;}int main(){C<int> tmpC(22);Func(tmpC);getch();}//再次錯誤error C2664: 'Func' : cannot convert parameter 1 from 'C<T>' to 'const C<T> &'// 不能由C<int> 轉換成C<double>//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%template <typename T>class C{public:C(int){cout<<"C(int)"<<endl;}};void Func(const C<double>& ) {cout<<"Func(C<double> const& )"<<endl;}int main(){C<double> tmpC(22);Func(tmpC);getch();}//正確//C(int)//Func(C<double> const& )