C++Primer第四版习题16.12答案调试出错的解决

习题16.12原答案g++下编译出错的解决

习题16.12：

Exercise 16.12:编写一个函数模板，接受表示未知类型迭代器的一对值，找出在序列中出现得最频繁的值。

在做C++Primer 16.12的时候原答案在g++上编译老是出错（我改了原答案的部分变量函数名）：

16_12.cpp: In function ‘typename Iter::value_type findMost(Iter, Iter)’:16_12.cpp:18: error: expected ‘;’ before ‘newFirst’16_12.cpp:19: error: expected ‘;’ before ‘newLast’16_12.cpp:21: error: ‘newFirst’ was not declared in this scope16_12.cpp:22: error: ‘sort’ is not a member of ‘std’16_12.cpp:22: error: ‘newLast’ was not declared in this scope16_12.cpp:26: error: expected ‘;’ before ‘preIter’16_12.cpp:27: error: expected ‘;’ before ‘maxOccuElemIt’16_12.cpp:30: error: ‘preIter’ was not declared in this scope16_12.cpp:35: error: ‘maxOccuElemIt’ was not declared in this scope16_12.cpp:40: error: ‘preIter’ was not declared in this scope16_12.cpp:46: error: ‘maxOccuElemIt’ was not declared in this scope16_12.cpp:46: error: ‘preIter’ was not declared in this scope16_12.cpp:48: error: ‘maxOccuElemIt’ was not declared in this scope16_12.cpp: In function ‘typename Iter::value_type findMost(Iter, Iter) [with Iter = __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >]’:16_12.cpp:57:   instantiated from here16_12.cpp:18: error: dependent-name ‘std::VecType::iterator’ is parsed as a non-type, but instantiation yields a type16_12.cpp:18: note: say ‘typename std::VecType::iterator’ if a type is meant16_12.cpp:19: error: dependent-name ‘std::VecType::iterator’ is parsed as a non-type, but instantiation yields a type16_12.cpp:19: note: say ‘typename std::VecType::iterator’ if a type is meant16_12.cpp:26: error: dependent-name ‘std::VecType::iterator’ is parsed as a non-type, but instantiation yields a type16_12.cpp:26: note: say ‘typename std::VecType::iterator’ if a type is meant16_12.cpp:27: error: dependent-name ‘std::VecType::iterator’ is parsed as a non-type, but instantiation yields a type16_12.cpp:27: note: say ‘typename std::VecType::iterator’ if a type is meant

后来反复查看code，以及从网上查找，终于找到原因，在定义完新的类型别名后：

typedef std::vector<typename Iter::value_type> VecType;

原答案在使用容器迭代器的时候直接使用双冒号调用了：

VecType::iterator newFirst=vec.begin();VecType::iterator newLast=vec.end();...VecType::iterator preIter=newFirst;VecType::iterator maxOccuElemIt=newFirst;

但是正像这题想要考察的一样，如果要在函数模板内部使用内置定义类型，必须显示地告诉编译器我们正在使用的名字指的是一个类型。因为编译器（以及程序的读者）不能通过检查得知，由类型形参定义的名字何时是一个类型何时是一个值。所以这里应该加上typename,改写为：

typename VecType::iterator newFirst=vec.begin();typename VecType::iterator newLast=vec.end();...typename VecType::iterator preIter=newFirst;typename VecType::iterator maxOccuElemIt=newFirst;

最后在std::sort(newFirst,newLast);这句也有报错，说sort不是std的成员，只需把std去掉，加上头文件#include\<\algorithm>就可以了。最后贴上在g++编译通过的code:

  1 #include <iostream>  2 #include <vector>  3 #include <algorithm>  4 //#include <iterator>  5 using namespace std;  6   7 template <typename Iter> typename Iter::value_type findMost(Iter first,Iter last)  8 {  9     std::size_t amount=0; 10     Iter start=first; 11     while(start != last) 12     { 13         amount++; 14         start++; 15     } 16     typedef std::vector<typename Iter::value_type> VecType; 17  18     VecType vec(amount); 19  20     typename VecType::iterator newFirst=vec.begin(); 21     typename VecType::iterator newLast=vec.end(); 22  23     std::uninitialized_copy(first,last,newFirst); 24     sort(newFirst,newLast); 25  26     size_t maxOccu=0,occu=0; 27  28     typename VecType::iterator preIter=newFirst; 29     typename VecType::iterator maxOccuElemIt=newFirst; 30     while(newFirst != newLast) 31     { 32         if(*newFirst != *preIter) 33         { 34             if(occu>maxOccu) 35             { 36                 maxOccu=occu; 37                 maxOccuElemIt=preIter; 38             } 39             occu=0; 40         } 41         ++occu; 42         preIter=newFirst; 43         ++newFirst; 44     } 45     if(occu>maxOccu) 46     { 47         maxOccu=occu; 48         maxOccuElemIt=preIter; 49     } 50     return *maxOccuElemIt; 51 } 52  53 int main() 54 { 55     int ia[]={1,5,1,2,5,2,7,9,5,6,4,6,8,1,5,6,8,3,9,8,7,5,6,1,2,4,5,6,8}; 56     int cnt=sizeof(ia)/sizeof(int); 57     cout<<"cnt:"<<cnt<<endl; 58     vector<int> ivec(ia,ia+cnt); 59     cout<<"most find element is:"<<findMost(ivec.begin(),ivec.end())<<endl; 60     return 0; 61 }

参考：http://bbs.csdn.net/topics/320250397