C++ What is wrong with this Vector Splitting in While Loop - c++

Question

This is my partial code:
if(action=="auth")
{
myfile.open("account.txt");
while(!myfile.eof())
{
getline(myfile,sline);
vector<string> y = split(sline, ':');
logincheck = "";
logincheck = y[0] + ":" + y[3];
if (sline==actionvalue)
{
sendClient = "login done#Successfully Login.";
break;
}
else
{
sendClient = "fail login#Invalid username/password.";
}
y.clear();
}
myfile.close();
}
If i don't have this
logincheck = y[0] + ":" + y[3];
The code will not have any segmentation core dump error, but when I add that line, it will went totally wrong.
My account.txt is as followed:
admin:PeterSmite:hr:password
cktang:TangCK:normal:password
The split function:
std::vector<std::string> split(std::string const& str, std::string const& delimiters = "#") {
std::vector<std::string> tokens;
// Skip delimiters at beginning.
string::size_type lastPos = str.find_first_not_of(delimiters, 0);
// Find first "non-delimiter".
string::size_type pos = str.find_first_of(delimiters, lastPos);
while (string::npos != pos || string::npos != lastPos) {
// Found a token, add it to the vector.
tokens.push_back(str.substr(lastPos, pos - lastPos));
// Skip delimiters. Note the "not_of"
lastPos = str.find_first_not_of(delimiters, pos);
// Find next "non-delimiter"
pos = str.find_first_of(delimiters, lastPos);
}
return tokens;
}
std::vector<std::string> split(std::string const& str, char const delimiter) {
return split(str,std::string(1,delimiter));
}

Answer 1

You should do some basic input checking before you blithely assume that the vector contains at least 4 elements, otherwise y[3] will explode when you parse a line of input without three colons:
if (y.size >= 4) {
// Do login check
} else {
// Invalid input
}
I'd guess that you probably have a blank line in your input.
Wrap the whole section of code that relies on reading a "a:b:c:d" line of input:
if(action=="auth") {
myfile.open("account.txt");
while(getline(myfile,sline))
{
vector<string> y = split(sline, ':');
if (y.size >= 4) {
logincheck = "";
logincheck = y[0] + ":" + y[3];
if (sline==actionvalue) {
sendClient = "login done#Successfully Login.";
break;
} else {
sendClient = "fail login#Invalid username/password.";
}
}
}
myfile.close();
}

Answer 2

The problem is the structure of your loop:
while(!myfile.eof())
{
getline(myfile,sline);
istream::eof() isn't guaranteed to return true until you attempt to read past the end of the stream. So what happens is you read 2 lines and eof() still hasn't return true. Then you enter the loop for the 3rd time. Since you don't check for errors after getline call, you happily access sline when its content is unspecified - it could be empty, it could still carry the content from the previous iteration, it could contain something else.
You always need to check if getline() call is succesful before you attempt to access the string. The idiomatic way is to put it in the condition of the loop:
while (getline(myfile, sline)) { /* do your stuff */ }
This way you only enter the loop body if the read is successful.

Answer 3

The problem is that the call to getline that pulls the last usable line isn't setting EOF, so you do one extra loop iteration after you have gotten the last usable line. That loop operation is running on an empty sline, which causes bad things to happen, namely, split doesn't return a vector with four elements, but then you try to access those elements.
You can just use
while (getline(myfile,sline))
{
// do stuff
}
in place of
while(!myfile.eof())
{
getline(myfile,sline);
// do stuff
}