c++ template笔记(1)模板函数

 

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