Crashing while converting string to wstring

Question

I got the following function that crashes randomly when converting a string to a wstring. It is used to parse HTTP headers ( http://www.codeproject.com/Articles/66625/A-Fully-Featured-Windows-HTTP-Wrapper-in-C ) and always worked fine, but it randomly stopped working when I changed server recently. (This code comes from the classes that comes with the codeproject project)

inline bool ParseRegExpW(const wstring &regExp, bool caseSensitive, int groupCount, const wstring &source, vector<wstring> &result, bool allowDuplicate = false)
{
    const string regEXP = string(regExp.begin(), regExp.end());
    const string Source = string(source.begin(), source.end());
    vector<string> Result;
    if (result.size() != 0){
        for (int i = 0; i < result.size(); i++) {
            Result[i] = string(result[i].begin(), result[i].end());
        }
    }
    bool res = ParseRegExp(regEXP, caseSensitive, groupCount, Source, Result, allowDuplicate);
    if (Result.size() != 0){
        for (int i = 0; i < Result.size(); i++) {
            printf("Result: %i->%s L: %i : R %i r %i\n", i, Result[i].c_str(), Result[i].length(), Result.size(), result.size());//Result[i] contains what it should contain
            if (i < Result.size()){
                printf("Converting...\n");
                result[i] = wstring(Result[i].begin(), Result[i].end());
                printf("Will crash before getting there..\n");
            }
        }
    }
    return res;
}

Why is this happening? It's so weird to me that it was working fine for the last months and stopped working when I changed server.

Answer 1

Assuming result.size() == 0 , when you get into that loop and to ...

result[i] = wstring(Result[i].begin(), Result[i].end());

... then no matter the value of i , you'll get undefined behavior due to accessing a non-existent element of the vector.

If result.size() != 0 , then this code path will be executed, ...

vector<string> Result;
if (result.size() != 0){
    for (int i = 0; i < result.size(); i++) {
        Result[i] = string(result[i].begin(), result[i].end());
    }
}

... which has the same issues in regard to the empty vector Result .

I'm wondering how this could've ever worked.