C ++地图 <string,string> :: find似乎返回垃圾迭代器

Question

map<string,string>::find seems to be returning garbage iterator, since i can access neither my_it->first nor second (NB: my_it != my_map.end() is verified). VC2010 reports a debug error, and looking deeper reveals

my_it is (Bad Ptr, Bad Ptr).

The 'offending' map is a class attribute, _match , shown below in context:

class NicePCREMatch
{
private:
    map<string, string, less<string> > _match;

public:
    void addGroup(const string& group_name, const string& value);
    string group(const string& group_name);
};

Here is the code that returns elements by key (the commented-out code works fine):

string NicePCREMatch::group(const string& group_name)
{
    /*for (map<string, string, less<string> >::iterator j = _match.begin(); j != _match.end(); j++)
    {
        if(!strcmp(j->first.c_str(), group_name.c_str()))
        {
            return j->second;
        }
    }

    throw runtime_error("runtime_error: no such group");*/

    map<string, string, less<string> >::iterator i = _match.find(group_name);

    if (i == _match.end())
    {
        throw runtime_error("runtime_error: no such group");
    }

    return i->second;
}

And Here is the code that inserts new elements in the map:

void NicePCREMatch::addGroup(const string& group_name, const string& value)
{
    _match.insert(pair<string, string>(group_name, value));
}

Another class uses NicePCREMatch as follows:

template<class Match_t>
vector<Match_t> NicePCRE<Match_t>::match(const string& buf)
{
[snip]
    Match_t m;
[snip]
    m.addGroup(std::string((const char *)tabptr + 2, name_entry_size - 3), \
                buf.substr(ovector[2*n], ovector[2*n+1] - ovector[2*n]));
[snip]
    addMatch(m);
[snip]
    return _matches;
}

Where,

template<class Match_t>
void NicePCRE<Match_t>::addMatch(const Match_t& m) 
{ 
    _matches.push_back(m);
}

Finally, client code uses NicePCRE class as follows:

void test_NicePCRE_email_match(void)
{
    NicePCRE<> npcre;
    npcre.compile("(?P<username>[a-zA-Z]+?)(?:%40|@)(?P<domain>[a-zA-Z]+\.[a-zA-Z]{2,6})");
    vector<NicePCREMatch> matches = npcre.match("toto@yahoo.com");
    assert(!matches.empty());
    assert(!strcmp(matches.begin()->group("username").c_str(), "toto"));
    cout << matches.begin()->group("domain").c_str() << endl;
    assert(!strcmp(matches.begin()->group("domain").c_str(), "yahoo.com"));
}

BTW, this --is pretty much-- my main (the oddest TDD ever :) ):

int main()
{
    int test_cnt = 0;
    cout << "Running test #" << test_cnt << " .." << endl;
    test_NicePCRE_email_match();
    cout << "OK." << endl << endl;
    test_cnt++;

    SleepEx(5000, 1);

    return 0;
}

What am I doing wrong here?

EDIT: The following modification (compare with the version above) solved my problem. Viz,

void NicePCREMatch::addGroup(const string& group_name, const string& value)
{
    _match.insert(pair<string, string>(group_name.c_str(), value.c_str()));
}

Client code (slightly modified) now looks like this:

void test_NicePCRE_email_match(void)
{
    NicePCRE<> npcre;
    npcre.compile("(?P<username>[a-zA-Z]+?)(?:%40|@)(?P<domain>[a-zA-Z]+\.[a-zA-Z]{2,6})");
    vector<NicePCREMatch> matches = npcre.match("toto@yahoo.com");
    assert(!matches.empty());
    try
    {
        assert(!strcmp(matches.begin()->group("username").c_str(), "toto"));
        assert(!strcmp(matches.begin()->group("domain").c_str(), "yahoo.com"));
        cout << "username = " << matches.begin()->group("username") << endl;
        cout << "domain = " << matches.begin()->group("domain") << endl;
    }
    catch (const runtime_error& e)
    {
        cout << "Caught: " << e.what() << endl;
        assert(0x0);
    }
}

This is quite bizarre. Can someone please explain. However, I consider my problem solved already.

Thanks every one.

Answer 1

This might be caused by three things - either you modify the map in some way after the execution of find or you have a memory coruption somewhere in your program or the debugger is simply not showing the correct values for the iterator.

Try using debug output - if the code crashes when you try to output the values, then probably the iterator is really broken.

Also make sure you do not modify the map after the execution of find. If you do, this may make the iterator invalid and so you need to move the find call immedietly before using the iterator.

If both of the above options don't help you probably have memory corruption somewhere and you need to find it. Maybe use valgrind for that. Please note this should be your last resort only when the two other options are proved impossible.

Answer 2

Your issue is here

if (i == _match.end())
{
    throw runtime_error("runtime_error: no such group");
}

return i->second;

Your find failed for some reason. I can't say why because I don't have the full code. But, after the failure, you are throwing an error, but there is nobody to catch outside. Please add a try catch at the point where you call the method group() and implement the logic if the match is not found. I tried with your sample snippets (+ some changes to get the stuff compiled) and it looks like visual studio continues with the next line in the function even after a throw statement. I don't know the theory behind it. I was bit surprised at seeing such a behavior.

[To make sure that your class structure is not causing the problem, I tried with a simple global method and even the method also gave me the same behavior. If there are somebody who can explain this please feel free.]