Crash on vector pop_back() of only remaining entry - c++

Question

Code:
#include <vector>
#include <algorithm>
#include <iostream>
#include <ctime>
struct myClass {
bool bIsDead;
myClass(bool bDead) {
this->bIsDead = bDead;
}
};
void PrintVector(std::vector<myClass*> vec) {
std::cout << "The vector contains: ";
for(int i = 0; i < vec.size(); i++) {
std::cout << vec[i]->bIsDead << " ";
}
std::cout << std::endl;
}
int main() {
std::srand(std::time(0)); // use current time as seed for rng
std::vector<myClass*> myVector;
for(int i = 0; i < 10; i++) {
int tempRand = std::rand() % 2;
// int tempRand = 1;
if(tempRand == 1) {
myVector.push_back(new myClass(true));
}
else {
myVector.push_back(new myClass(false));
}
}
std::cout << "Unsorted: " << std::endl;
PrintVector(myVector);
std::sort(myVector.begin(), myVector.end(), [ ]( const myClass *lhs, const myClass *rhs )
{
return lhs->bIsDead < rhs->bIsDead;
});
std::cout << "Sorted: " << std::endl;
PrintVector(myVector);
while(myVector.back()->bIsDead) {
delete myVector.back();
myVector.pop_back();
}
std::cout << "Removed Dead Ones: " << std::endl;
PrintVector(myVector);
return 0;
}
Output for random inputs:
Unsorted:
The vector contains: 0 0 1 0 0 0 1 1 0 1
Sorted:
The vector contains: 0 0 0 0 0 0 1 1 1 1
Removed Dead Ones:
The vector contains: 0 0 0 0 0 0
Hit ENTER to continue...
Output for all 1's (i.e. pop_back removes all):
Unsorted:
The vector contains: 1 1 1 1 1 1 1 1 1 1
Sorted:
The vector contains: 1 1 1 1 1 1 1 1 1 1
Hit ENTER to continue...
For this console application I'm not given any errors or crash warnings, but it isn't outputting the two cout statements after the pop_back call for the all 1's case.
Any ideas on why popping a vector of pointers back to empty causes the program to crash?

Answer 1

You need to add a check for size of vector. It shouldn't be 0 if you want to access the vector's last element using back().
Change this:
while(myVector.back()->bIsDead) {
to:
while(myVector.size() > 0 && myVector.back()->bIsDead) {
Here's the correct code:
#include <vector>
#include <algorithm>
#include <iostream>
#include <ctime>
struct myClass {
bool bIsDead;
myClass(bool bDead) {
this->bIsDead = bDead;
}
};
void PrintVector(std::vector<myClass*> vec) {
std::cout << "The vector contains: ";
for(int i = 0; i < vec.size(); i++) {
std::cout << vec[i]->bIsDead << " ";
}
std::cout << std::endl;
}
int main() {
std::srand(std::time(0)); // use current time as seed for rng
std::vector<myClass*> myVector;
for(int i = 0; i < 10; i++) {
int tempRand = std::rand() % 2;
// int tempRand = 1;
if(tempRand == 1) {
myVector.push_back(new myClass(true));
}
else {
myVector.push_back(new myClass(false));
}
}
std::cout << "Unsorted: " << std::endl;
PrintVector(myVector);
std::sort(myVector.begin(), myVector.end(), [ ]( const myClass *lhs, const myClass *rhs )
{
return lhs->bIsDead < rhs->bIsDead;
});
std::cout << "Sorted: " << std::endl;
PrintVector(myVector);
while(myVector.size() > 0 && myVector.back()->bIsDead) {
delete myVector.back();
myVector.pop_back();
}
std::cout << "Removed Dead Ones: " << std::endl;
PrintVector(myVector);
return 0;
}
Quoting cppreference
Calling this function(back()) on an empty container causes undefined
behavior.
PS: You don't need to make the question look so intimidating to grab our attention.

Answer 2

As with most problems I have, it was something simple. Thanks to billz and juanchopanza's comments it was pointed out to me that it wasn't the pop_back() that was crashing, but rather the subsequent back() called in the while loop.
This is easily fixable in several ways, 2 examples being:
adding if(myVector.empty) {break;} into the delete's while loop
or like user3286661 said, adding a conditional to the while loop first checking if it has an element.

Answer 3

The problem is that you try to access myVector.back() even when the vector is already empty.
The least intrusive solution (minimal modification to your code) I can think of would this:
while (!myVector.empty() && myVector.back()->bIsDead) {
delete myVector.back();
myVector.pop_back();
}
The "right" thing to do would be to use either std::vector<myClass> or std::vector<std::unique_ptr<myClass>> (no need for manual delete) and use the erase-remove-idiom:
myVector.erase(
std::remove_if(myVector.begin(), myVector.end(),
[](const auto& p) {
return p->bIsDead; //p.bIsDead for std::vector<myClass>
}
),
myVector.end()
);
This might look more complicated, but it is safer (no accidental memory leaks), probably more efficient and makes the call to sort unnecessary.