#include <iostream>
#include <string>
#include <cstdlib>
#include <vector>
#include <set>
#include <map>
#include <iterator>
#include <algorithm>
#include <cctype>

using namespace std;

char ToLower (char ch)
{
    return tolower (ch);
}

string & ToLower (string & st);
void Display (const string & s);

int main ()
{
    vector<string> words;
    vector<string> temps;

    string temp;

    cout << "Enter words (enter quit to quit) : " << endl;

    //输入单词，输入quit退出输入，将输入的单词插入到vector容器中；
    while (cin >> temp && temp != "quit")
    {
        words.push_back (temp);
    }

    //使用算法for_each （）来显示vector容器中的内容；
    cout << "Now show the words you just entered : " << endl;
    for_each (words.begin (),words.end (),Display);
    cout << endl;

    //使用transform方法，对每个vector容器中的元素进行ToLower （）函数 并插入到set集合中
    set<string> wordset;
    temps.resize (words.size ());
    transform (words.begin (),words.end (),temps.begin (),ToLower);//代码来自c++ primer plus，此条无法编译通过。
    vector<string>::iterator it;
    for (it = words.begin ();it != words.end ();it ++)
    {
        wordset.insert (*it);
    }

    //使用算法for_each（）输出集合中每个元素；
    cout << "Alphabetic list of words : " << endl;
    for_each (wordset.begin (),wordset.end (),Display);
    cout << endl;

    //将set容器中的元素复制到map关联容器中；
    map<string,int> wordmap;
    set<string>::iterator iter1;
    for (iter1 = wordset.begin ();iter1 != wordset.end ();iter1 ++)
    {
        wordmap.insert (pair <string,int> (*iter1,count (words.begin (),words.end (),*iter1)));
    }

    //统计单词出现次数；
    for (iter1 = wordset.begin ();iter1 != wordset.end ();iter1 ++)
    {
        wordmap[*iter1] = count (words.begin (),words.end (),*iter1);
    }

    //显示各个单词的出现频率；
    for (iter1 = wordset.begin ();iter1 != wordset.end ();iter1 ++)
    {
        cout << *iter1 << " " << wordmap[*iter1] << endl;
    }

    system ("pause");
    return 0;
    
}

string & ToLower (string & st)
{
    transform (st.begin (),st.end (),st.begin (),tolower);
    return st;
}

void Display (const string & s)
{
    cout << s << " " ;
}

求大神解释一下

将
char ToLower (char ch)
{
    return tolower (ch);
}
删除掉

或者transform的最后一个模板参数明确给出用 string & ToLower (string & st) 而不是 char ToLower (char ch)