Segmentation Fault : 11 when popping a vector in C++ - c++

Question

I would ideally like to use one stack in this program but it doesn't let me. I keep getting a segment error 11. I got around this sort of by making a new vector and setting that equal to the first vector and then poping the new vector. But I eventually need to sync the popped vector up with the first vector but it throws a segment error 11.
anyway here is the code, its part of another project but I dont think the context is too important to know what its for. Also this does compile its just when I run this method it throws the error.
void print_well_formed_file(ifstream& ifs) {
vector<string> vec;
vector<string> finalVec;
string line;
string a;
string temp;
string final;
Lexer lexer;
Token tok;
while (getline(ifs, line)) {
lexer.set_input(line);
while (lexer.has_more_token()) {
tok = lexer.next_token();
switch (tok.type) {
case TAG:
if (tok.value[0] != '/'){
a = (tok.value);
if (a == "red") {
a = "\033[31m";
}
if (a == "green") {
a = "\033[32m";
}
if (a == "blue") {
a = "\033[34m";
}
if (a == "yellow") {
a = "\033[33m";
}
if (a == "magenta") {
a = "\033[31m";
}
if (a == "cyan") {
a = "\033[36m";
}
if (a == "dim") {
a = "\033[2m";
}
if (a == "underline") {
a = "\033[4m";
}
if (a == "bright") {
a = "\033[1m";
}
vec.push_back(a);
final = temp + a;
}
else{
string s;
string temp;
string fin;
finalVec = vec;
if(!finalVec.empty()){finalVec.pop_back();}
a = "\033[0m" + finalVec.back();
temp = final;
final = temp + a;
if(!vec.empty()){vec.pop_back();} // throws an error
vec = finalVec; // throws an error
}
break;
case IDENT:
a= (tok.value)+ " ";
temp = final;
final = temp + a;
break;
}
}
}
cout << final << endl;
}

Answer 1

You cannot pop or get an element from an empty vector. It's undefined behaviour, and it will likely crash your app.
You can verify that this is the problem by checking the length before each access in your code (you just "randomly" picked one of the locations in your latest update). Look, it's really simple.. just display an error message if it's empty.

Answer 2

if(!finalVec.empty()){finalVec.pop_back();}
a = "\033[0m" + finalVec.back(); // <-- bug
is obviously buggy: finalVec.back() may not exist: since the last element has just been popped the vector may be empty.