函数模版中的引用

一：函数模版和特化模版的引用

1.首先考虑特化函数模版的参数

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare( T ls, T rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}template<>int compare(const int& ls,const int& rs){std::cout<<"first \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int _tmain(int argc, _TCHAR* argv[]){int first1(1),first2(2);int &second1(first1);int &second2(first2);const int &third1(first1);const int &third2(first2);int r(0);r = compare(first1,first2);std::cout<<r<<'\n';r = compare(second1,second2);std::cout<<r<<'\n';r = compare(third1,third2);std::cout<<r<<'\n';r = compare(3,2);std::cout<<r<<'\n';std::cin>>r;return 0;}

对compare模版定义了 const &int的特化，请预测结果是：

？

实际上结果是

-1

-1

-1

1

也就是四个调用都实例了函数模版，并没有调用特化模版，甚至常量引用third1,third2作为参数仍然么有调用对应的特化模版。

如果我们去掉特化模版形参的const和&如下：

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(  T ls, T rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}template<>int compare( int ls, int rs){std::cout<<"first \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int _tmain(int argc, _TCHAR* argv[]){int first1(1),first2(2);int &second1(first1);int &second2(first2);const int &third1(first1);const int &third2(first2);int r(0);r = compare(first1,first2);std::cout<<r<<'\n';r = compare(second1,second2);std::cout<<r<<'\n';r = compare(third1,third2);std::cout<<r<<'\n';r = compare(3,2);std::cout<<r<<'\n';std::cin>>r;return 0;}

结果为

first

-1

first

-1

first

-1

first

1

全部调用特化函数模版。

当特化模版形参为引用时，如下

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(  T ls, T rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}template<>int compare( int &ls, int &rs){std::cout<<"first \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int _tmain(int argc, _TCHAR* argv[]){int first1(1),first2(2);int &second1(first1);int &second2(first2);const int &third1(first1);const int &third2(first2);int r(0);r = compare(first1,first2);std::cout<<r<<'\n';r = compare(second1,second2);std::cout<<r<<'\n';r = compare(third1,third2);std::cout<<r<<'\n';r = compare(3,2);std::cout<<r<<'\n';std::cin>>r;return 0;}

结果是

-1

-1

-1

1

这次所有都调用的是实例化的函数模版而不是特化函数模版。

2.函数模版的参数变化

接下来对函数模版compare添加引用

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare( T &ls, T &rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}template<>int compare(const int& ls,const int& rs){std::cout<<"first \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int _tmain(int argc, _TCHAR* argv[]){int first1(1),first2(2);int &second1(first1);int &second2(first2);const int &third1(first1);const int &third2(first2);int r(0);r = compare(first1,first2);std::cout<<r<<'\n';r = compare(second1,second2);std::cout<<r<<'\n';r = compare(third1,third2);std::cout<<r<<'\n';r = compare(3,2);std::cout<<r<<'\n';std::cin>>r;return 0;}

调用

r = compare(3,2);

时提示

1>e:\sourcecodes\comparetest\comparetest\comparetest.cpp(67) : error C2664: 'compare' : cannot convert parameter 1 from 'int' to 'int &'

这个错误和

int &a = 1;

这样的代码语句错误是一样的。

以下转自http://developer.51cto.com/art/201002/183476.htm

关于引用的初始化有两点值得注意：

（1）当初始化值是一个左值（可以取得地址）时，没有任何问题；

（2）当初始化值不是一个左值时，则只能对一个const T&（常量引用）赋值。而且这个赋值是有一个过程的：

首先将值隐式转换到类型T，然后将这个转换结果存放在一个临时对象里，最后用这个临时对象来初始化这个引用变量。

const double& cdr = 1; // ok 

相当于

double temp = double(1);  
const double& cdr = temp; 

注释掉语句

r = compare(3,2);std::cout<<r<<'\n';

所得结果为

-1

-1

-1

所以我们再次将函数模版compare的形参加上const

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare( const T &ls, const T &rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}template<>int compare(const int& ls,const int& rs){std::cout<<"first \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int _tmain(int argc, _TCHAR* argv[]){int first1(1),first2(2);int &second1(first1);int &second2(first2);const int &third1(first1);const int &third2(first2);int r(0);r = compare(first1,first2);std::cout<<r<<'\n';r = compare(second1,second2);std::cout<<r<<'\n';r = compare(third1,third2);std::cout<<r<<'\n';r = compare(3,2);std::cout<<r<<'\n';std::cin>>r;return 0;}

结果为

first

-1

first

-1

first

-1

first

1

这次均调用了特化的函数模版！！！！

3.加上重载函数

接下来我们在特化模版后再加入同样形参的函数

int compare(const int& ls,const int& rs)

如下

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare( const T &ls, const T &rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}template<>int compare(const int& ls,const int& rs){std::cout<<"first \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare(const int& ls,const int& rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int _tmain(int argc, _TCHAR* argv[]){int first1(1),first2(2);int &second1(first1);int &second2(first2);const int &third1(first1);const int &third2(first2);int r(0);r = compare(first1,first2);std::cout<<r<<'\n';r = compare(second1,second2);std::cout<<r<<'\n';r = compare(third1,third2);std::cout<<r<<'\n';r = compare(3,2);std::cout<<r<<'\n';std::cin>>r;return 0;}

这次结果为

SECOND

-1

SECOND

-1

SECOND

-1

SECOND1

从结果我们可以看出函数重载了函数模版以及特化模版。

4重载和函数模版

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(  T ls, T rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare(const int& ls,const int& rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}

结果

SECOND

-1

SECOND

-1

SECOND

-1

SECOND

1

重载函数去掉const，只留&

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(  T ls, T rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare( int& ls, int& rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}

结果为

SECOND

-1

SECOND

-1

-1

1

说明int,int &调用的函数，const &,常量数字调用的函数模版。

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(  T ls, T rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare( int ls, int rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}

结果

SECOND

-1

SECOND

-1

SECOND

-1

SECOND

1

均调用的函数非模版。

为模版添加引用

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(  T &ls, T &rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare( int ls, int rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}

结果

SECOND

-1

SECOND

-1

SECOND

-1

SECOND

1

再添加常量

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare( const T &ls, const T &rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare( int ls, int rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}

结果

SECOND

-1

SECOND

-1

SECOND

-1

SECOND

1

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(   T &ls,  T &rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare( int &ls, int &rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}

结果

SECOND

-1

SECOND

-1

r = compare(3,2);

出错

1>e:\sourcecodes\comparetest\comparetest\comparetest.cpp(78) : error C2665: 'compare' : none of the 2 overloads could convert all the argument types1>        e:\sourcecodes\comparetest\comparetest\comparetest.cpp(9): could be 'int compare<int>(T &,T &)' 1>        e:\sourcecodes\comparetest\comparetest\comparetest.cpp(33): or       'int compare(int &,int &)'

#include "stdafx.h"#include<iostream>#include<ostream>#include<istream>template<class T>int compare(   T ls,  T rs){if(ls<rs)return -1;if(rs<ls)return 1;return 0;}int compare( int &ls, int &rs){std::cout<<"SECOND \n";if(ls<rs)return -1;if(rs<ls)return 1;return 0;}

SECOND

-1

SECOND

-1

-1

最后留下的疑问是当函数模版的参数不是引用时，如下，

int compare( T ls, T rs)

为什么特化模版

template<>int compare(const int& ls,const int& rs)

没有起到作用

而

template<>int compare( int ls, int rs)