1.定义函数模板
template <typename T>
inline T const& max (T const& a, T const& b)
{
// if a < b then use b else use a
return a < b ? b : a;
}
2.使用模板函数
#include <iostream>
#include <string>
#include "max.hpp"
int main()
{
int i = 42;
std::cout << "max(7,i): " << ::max(7,i) << std::endl;
double f1 = 3.4;
double f2 = -6.7;
std::cout << "max(f1,f2): " << ::max(f1,f2) << std::endl;
std::string s1 = "mathematics";
std::string s2 = "math";
std::cout << "max(s1,s2): " << ::max(s1,s2) << std::endl;
}
输出结果
3.确定返回的参数
::max<double>(1,2); //ok
//::max(1,1.2);//wrong
::max(static_cast<double>(4),4.1);//ok
若两个参数不正确，或者不支持模板定义的特性,编译时则会出错
4.多个模板参数
template <typename T1,typename T2>
inline T1 const& max (T1 const& a, T2 const& b)
{
// if a < b then use b else use a
return a < b ? b : a;
}
示例
double i = 42.1;
std::cout << "max(7,i): " << ::max(7,i) << std::endl;
返回值是T1,所以返回是int类型,结果是42,出错了
定义3个参数,第3个参数用于表示返回值类型
template <typename T1,typename T2,typename T3>
inline T3 const& max (T1 const& a, T2 const& b)
{
// if a < b then use b else use a
return a < b ? b : a;
}
测试
double i = 42.1;
std::cout << "max(7,i): " << ::max<int,double,double>(7,i) << std::endl;
返回正确的42.1
5.模板函数重载
// maximum of two int values
inline int const& max (int const& a, int const& b)
{
return a < b ? b : a;
}
// maximum of two values of any type
template <typename T>
inline T const& max (T const& a, T const& b)
{
return a < b ? b : a;
}
// maximum of three values of any type
template <typename T>
inline T const& max (T const& a, T const& b, T const& c)
{
return ::max (::max(a,b), c);
}
int main()
{
::max(7, 42, 68); // calls the template for three arguments
::max(7.0, 42.0); // calls max<double> (by argument deduction)
::max('a', 'b'); // calls max<char> (by argument deduction)
::max(7, 42); // calls the nontemplate for two ints
::max<>(7, 42); // calls max<int> (by argument deduction)
::max<double>(7, 42); // calls max<double> (no argument deduction)
::max('a', 42.7); // calls the nontemplate for two ints
}
特别注意::max<>(7, 42);这句的写法
注意:自动类型转换
::max('a', 42.7); 只适用于常规函数,'a’会完成自动类型转换
6.字符串重载的例子
#include <iostream>
#include <cstring>
#include <string>
// maximum of two values of any type
template <typename T>
inline T const& max (T const& a, T const& b)
{
return a < b ? b : a;
}
// maximum of two pointers
template <typename T>
inline T* const& max (T* const& a, T* const& b)
{
return *a < *b ? b : a;
}
// maximum of two C-strings
inline char const* const& max (char const* const& a,
char const* const& b)
{
return std::strcmp(a,b) < 0 ? b : a;
}
int main ()
{
int a=7;
int b=42;
::max(a,b); // max() for two values of type int
std::string s="hey";
std::string t="you";
::max(s,t); // max() for two values of type std::string
int* p1 = &b;
int* p2 = &a;
::max(p1,p2); // max() for two pointers
char const* s1 = "David";
char const* s2 = "Nico";
::max(s1,s2); // max() for two C-strings
}
注意每个重载函数必须存在着一定的差异.
7.总是把重载函数定义在调用之前
// maximum of two values of any type
template <typename T>
inline T const& max (T const& a, T const& b)
{
return a < b ? b : a;
}
// maximum of three values of any type
template <typename T>
inline T const& max (T const& a, T const& b, T const& c)
{
return max (max(a,b), c); // uses the template version even for ints
} // because the following declaration comes
// too late:
// maximum of two int values
inline int const& max (int const& a, int const& b)
{
return a < b ? b : a;
}
3参数的max调用的是2参数模板函数,而非最后定义的max非模板函数
8.std:string和char
#include <string>
// note: reference parameters
template <typename T>
inline T const& max (T const& a, T const& b)
{
return a < b ? b : a;
}
int main()
{
std::string s;
::max("apple","peach"); // OK: same type
::max("apple","tomato"); // ERROR: different types
::max("apple",s); // ERROR: different types
}
“apple”会被转成 char[6]数组,所以比较要求数组维度长度相同,第二条”tomato”与”apple”长度不符合,即出错.
s是std:: string类型,而非char数组,所以也出错
9.以非引用参数方式传递
#include <string>
// note: nonreference parameters
template <typename T>
inline T max (T a, T b)
{
return a < b ? b : a;
}
int main()
{
std::string s;
::max("apple","peach"); // OK: same type
::max("apple","tomato"); // OK: decays to same type
::max("apple",s); // ERROR: different types
}
只有最后类型不符合的编译不通过