1.定義函數模板

template <typename T> inline T const& max (T const& a, T const& b) {// if a < b then use b else use areturn a < b ? b : a; }

2.使用模板函數

#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; }

輸出結果

3.確定返回的參數

::max<double>(1,2); //ok//::max(1,1.2);//wrong::max(static_cast<double>(4),4.1);//ok

若兩個參數不正確，或者不支持模板定義的特性,編譯時則會出錯

4.多個模板參數

template <typename T1,typename T2> inline T1 const& max (T1 const& a, T2 const& b) {// if a < b then use b else use areturn 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類型,結果是42,出錯了

定義3個參數,第3個參數用于表示返回值類型

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 areturn a < b ? b : a; }

測試

double i = 42.1; std::cout << "max(7,i): " << ::max<int,double,double>(7,i) << std::endl;

返回正確的42.1

5.模板函數重載

// 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 }

特別注意::max<>(7, 42);這句的寫法?
注意:自動類型轉換

::max('a', 42.7); 只適用于常規函數,'a’會完成自動類型轉換

6.字符串重載的例子

#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 intstd::string s="hey";std::string t="you";::max(s,t); // max() for two values of type std::stringint* p1 = &b;int* p2 = &a;::max(p1,p2); // max() for two pointerschar const* s1 = "David";char const* s2 = "Nico";::max(s1,s2); // max() for two C-strings }

注意每個重載函數必須存在著一定的差異.

7.總是把重載函數定義在調用之前

// 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; }

3參數的max調用的是2參數模板函數,而非最后定義的max非模板函數

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”會被轉成 char[6]數組,所以比較要求數組維度長度相同,第二條”tomato”與”apple”長度不符合,即出錯.

s是std:: string類型,而非char數組,所以也出錯

9.以非引用參數方式傳遞

#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 }

只有最后類型不符合的編譯不通過

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