在Ubuntu Linux上使用g ++编译入门示例代码时出错

Question

The code is from C++ primer(3 third) . The error is :

*filterString.cpp: In function 'int main()': filterString.cpp:32:68: error: cannot convert '__gnu_cxx::__normal_iterator*, std::vector > >' to 'std::string* {aka std::basic_string }' in initialization

pls help me analyse the error, thanks.

code:

#include <string>
#include <algorithm>
#include <iterator>
#include <vector>
#include <iostream>

using namespace std;

template <class InputIterator>
void filter_string(InputIterator first, InputIterator last, string filt_elems =  string("\",?.")) {
    for (; first != last; first++){
        string:: size_type pos = 0;
        while ((pos = (*first).find_first_of(filt_elems, pos)) != string::npos)
            (*first).erase(pos, 1);
    }
}

bool length_less (string s1, string s2) {
return s1.size() < s2.size();
}

int main() {
    istream_iterator<string> input(cin), eos;
    vector<string> text;

    copy(input, eos, back_inserter(text));

    string filt_elems("\",.?;:");
    filter_string(text.begin(), text.end(), filt_elems);
    int cnt = text.size();

    string *max = max_element(text.begin(), text.end(), length_less);
    int len = max->size();

    cout << "The number of words read is " << cnt << endl;
    cout << "The longest word has a length of " << len << endl;
    cout << "The longest word is " << *max << endl;

    return 0;
}

Answer 1

In line 32 ,

std::max_element(text.begin(), text.end(), length_less);

this function returns a forward iterator addressing the position of the first occurrence of the largest element in the range searched and not a string .

What you can do instead of this line:

string *max = max_element(text.begin(), text.end(), length_less);

you have to do this ,

//First find the index of the max_element , by subtracting the forward iterator you get from calling max_element from the iterator for first element .

       int index=max_element(text.begin(), text.end(), length_less) - text.begin();

//And then find string stored on that index.

       string *max = text.at(index);

Answer 2

This is interesting. Iterators behave much like pointers, but not exactly. In particular, you can't convert an iterator to a pointer.

However, you can change this code to use an iterator as a sort of string* pointer:

vector<string>::iterator max = max_element(text.begin(), text.end(), length_less);

That declares max to be not a pointer to a string, but an iterator into a vector of strings, which is what the max_element algorithm returns when applies to a vector of strings.

You can also use a pointer, but it's a bad habit. Just for testing the idea, you can:

string *max = &*max_element(text.begin(), text.end(), length_less);

The *max_element(...) returns a reference to the string the returned iterator points to (just like dereferencing a real pointer) and & creates a (string*) pointer to that string.

This invites trouble, since a structural modification of the vector could quietly invalidate that pointer. Subsequent use of the pointer would be treating "random" memory as a string object. Even worse, it might work during your testing and not fail until the software was shipped!

A decent implementation of iterators should detect the invalidation and throw an exception. A predictable fail is better than a random meltdown.

Answer 3

OK, so I went overboard. Here is what I think is a more modern solution using lambdas and auto. I leave it to others to decide if it is easier to understand.

#include <algorithm>
#include <iostream>
#include <iterator>
#include <ostream>
#include <string>
#include <vector>

using namespace std;

template <class InputIterator>
void filter_string(InputIterator first, InputIterator last, 
                   const string filt_elems = const string("\",?.")) 
{
    for_each(first, last, 
        [filt_elems](string& s)
        {
            s.erase(
                // Shift valid characters up before erasing the undesirable
                remove_if(s.begin(), s.end(), 
                    [filt_elems](string::value_type c)
                    { return filt_elems.find_first_of(c) != string::npos; }), 
                s.end());
        });
}

int main()
{
    istream_iterator<string> input(cin);
    istream_iterator<string> eos;

    vector<const string> words;
    copy(input, eos, back_inserter(words));

    const string filt_elems("\",.?;:");
    filter_string(words.begin(), words.end(), filt_elems);
    const int count = words.size();

    // Get a reference to the longest word
    const auto& max_word = *max_element(words.cbegin(), words.cend(), 
        [](const string& lhs, const string& rhs)
        { return lhs.size() < rhs.size(); });
    const int length = max_word.size();

    cout << "The number of words read is " << count << endl;
    cout << "The longest word has a length of " << length << endl;
    cout << "The longest word is " << max_word << endl;

    return 0;
}