从模板函数返回迭代器到 STL 容器

Question

我试图将一个迭代器从一个模板函数返回到一个向量（还不是一个模板类成员——我还在写那个）。编译器不断给我错误（复制如下以方便谷歌搜索）。我基本上知道问题出在哪里，但确切的语法难以捉摸。

我阅读了互联网，搜索了 SO，包括 Where and why do I have to put the "template" and "typename" keywords?，但没有找到有效的答案。我想我应该在这里提出问题并自己回答。

（略）原代码如下：

#include <vector>
#include <algorithm>  // std::lower_bound

template<typename T> std::vector<T>::iterator
              insertIntoVector(std::vector<T>& vec, const T& val)
{  itr = [some std::vector<T> iterator];
   return itr;  // return iterator to val in vec.
} // End of insertIntoVector();

编译器错误：

error C2145: syntax error: missing ';' before identifier 'insertIntoVector'
error C2065: 'T' : undeclared identifier
error C2923: 'std::vector' : 'T' is not a valid template type argument for parameter '_Ty'

明智地，我尝试了这个：

template<typename T> typename std::vector<T>::iterator
              insertIntoVector(std::vector<T>& vec, const T& val)

更多编译器错误：

error C1075: end of file found before the left brace '{'

如果这个问题被解锁，我会在下面发布我的答案。否则，请在Where and why do I have to put the "template" and "typename" keywords? 上查看我的回答。

Answer 1

奇怪的是，我必须在返回类型周围添加括号才能编译它。也就是说，

 template<typename T> (typename std::vector<T>::iterator)    // Tested--works as expected.
             insertIntoVector(std::vector<T>& vec, const T& val)

工作始终如一，但是

 template<typename T>  typename std::vector<T>::iterator     // Compile errors
             insertIntoVector(std::vector<T>& vec, const T& val)

在 MSVC 2013 上给我问题，但在 MSVC 2012 上编译。我不知道为什么。有人吗？

下面的代码可以在 MSVC 2013 上正确编译和运行。

// The following is based on Matt Austern's article,
// "Why you shouldn't use set (and what you should use instead)"
// (http://lafstern.org/matt/col1.pdf).
// I modified it slightly, mostly just reformatting and adding comments,
// but I also changed it to return an iterator to the inserted element.
// Also changed sorted_vector to sortedVector (cammelCase)

#include <vector>
#include <algorithm>  // std::lower_bound


 template<typename T> (typename std::vector<T>::iterator)  // Tested--works as expected.
             insertIntoVector(std::vector<T>& vec, const T& val)
{  // Insert t into vector vec such that v remains in sorted order
   // and all elements of vec are unique (like a set).
   // This only makes sense if the vector elements are
   // maintained in sorted order.
   // A sorted vector might perform better than a set if
   // there are many more access operations than insertions
   // (smaller storage, fast access via [], ... at the cost
   // of much slower insertions).
   // Note: Type T must have a defined < operator.
   /// Todo: overload this function to allow passing a Compare()
   /// function pointer.

   // std::lower_bound() gives log2(N) + O(1) performance.
   typename std::vector<T>::iterator itr
             = lower_bound(vec.begin(), vec.end(), val);
   if ( (vec.end() == itr) || (val < *itr) )
   {  itr = vec.insert(itr, val);
   }
   return itr;  // return iterator to val in vec.
} // End of insertIntoVector();