pop_back Operation causing SIGSEGV

[Sh0es]

I have this code, which I'm testing to serve as a test bed for my homemade data structures.

#include <cstdlib>
#include <cmath>
#include <iostream>
#include <string>
#include <vector>

using std::cin;
using std::cout;
using std::endl;
using std::flush;
using std::string;
using std::vector;

template <class T>
std::ostream& operator << (std::ostream& o, const std::vector<T>& v)
{
    o << v[0];
    for(int i = 1; i < v.size(); i++)
    { o << ' ' << v[i]; }

    return o;
}

string hello()
{
	string hello = "Hello World.";
	return hello;
}

vector<int> sort(vector<int> ary) //based on Shell Sort from geeksforgeeks.org
{
    int n = ary.size();

    for(int gap = n/2; gap > 0; gap /= 2)
    {
        for (int i = gap; i < n; i++)
        {
            int temp = ary[i];
            int j;
            for (j = i; j >= gap && ary[j - gap] > temp; j -= gap)
            { ary[j] = ary[j - gap]; }

            ary[j] = temp;
        }
    }
    return ary;
}

vector<int> primes(int b) //based on code from algolist.net
{
    vector<int> bucket = vector<int>(b);
    vector<bool> sieve = vector<bool>(b);

    for(int i = 0; i < b; i++)
    { sieve.push_back(false); }

    for(int p = 2; p < sqrt(b); p++)
    {
        if(sieve.at(p) == false)
        {
            for(int k = p * p; k < b; k += p)
            {
				sieve.insert(sieve.begin() + k, true);
            }
        }
    }

	for(int i = 0; i < bucket.capacity(); i++)
	{
		if(sieve.at(i) == false)
		{ bucket.push_back(i); }
	}
	return bucket;
}

string rot(string ins)
{
	string ous = string();
	char ltrsFst[13] = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm'};
	char ltrsFstC[13] ={'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M'};
	char ltrsLst[13] = {'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
	char ltrsLstC[13] ={'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'};

	for(int i = 0; i < ous.size(); i++)
	{
		for(int j = 0; j < 13; j++)
		{
			if(ins[i] == ltrsFst[j])
			{ ous[i] = ltrsLst[j]; }
			else if(ins[i] == ltrsFstC[j])
			{ ous[i] = ltrsLstC[j]; }
			else if(ins[i] == ltrsLst[j])
			{ ous[i] = ltrsFst[j]; }
			else if(ins[i] == ltrsLstC[j])
			{ous[i] = ltrsFstC[j]; }
		}
	}
	return ous;
}

void showArr(vector<int> a)
{
	for(int i = 0; i < a.size(); i++)
	{
		cout << '[' << a[i] << ']';
	}
	for(int i = 0; i < a.capacity(); i++)
	{
		cout << "[ ]";
	}
	cout << endl;
}

int test1()
{
	int se;
	string inp;
	vector<int> ray;

	cout << "Welcome to the Interactive ArrayList." << endl;
	cout << "What size of array do you want?" << endl;

	cin >> se;
	ray = vector<int>(se);

	for(bool close = false; close != true;)
	{ try {
		cout << "Commands are: " << endl
			 << "append `x`, insert `x` `p`, popend, remove `p`, sort, swap `p1` `p2`" << endl;
		showArr(ray);

		cin >> inp;
		if(inp == "X")
		{
			std::cout << "Are you sure?" << endl
				<< "Y/n" << endl;
			cin >> inp;

			if(inp == "Y")
			{
				cout << "Testing Complete" << endl;
				close = true;
			}
			else
			{
				cout << "Resuming Test" << endl;
			}
		}
		else if(inp == "append")
		{
			int num;
			cin >> num;
			ray.push_back(num);
		}
		else if(inp == "insert")
		{
			int num;
			int pos;
			cin >> num >> pos;
			ray.insert(ray.begin() + pos, num);
		}
		else if(inp == "popend")
		{
			ray.pop_back();
		}
		else if(inp == "remove")
		{
			int pos;
			cin >> pos;
			ray.erase(ray.begin() + pos);
		}
		else if(inp == "swap")
		{
			int p1;
			int p2;
			cin >> p1 >> p2;
			std::swap(p1, p2);
		}
		else
		{ cout << "Command not recognized. Try again." << endl; }

		cout << flush;
	}
	catch(std::exception& e)
	{
		cout << "ERROR: " << e.what() << endl;
	}
	}
	return 0;
}

int test2()
{
	string inp;
	vector<int> bucket = vector<int>();
	int num = 0;

	cout << "Welcome to Array Test Auto" << endl;
	cout << endl;
	cout << "--- Printing Test ---" << endl;
	cout << hello() << endl;
	cout << endl;
	cout << "--- Sort Test ---" << endl;

	while(cin.get() != '\n')
	{
		cin >> num;
		bucket.push_back(num);
	}

	cout <<	sort(bucket) << endl;
	cout << endl;
	cout << "--- Primes Test ---" << endl
		 << "Enter a number to count to..." << endl;
	cin >> num;
	cout << "Primes from one to " << num;
	cout << primes(num) << endl;
	cout << endl;
	cout << "--- Rot13 Test ---" << endl
		 << "Enter a string." << endl;
	cin >> inp;
	cout << rot(inp) << endl;
	cout << endl;
	cout << "Testing Complete" << endl;
	return 0;
}

int main()
{
	char inp;
	vector<int> bucket;
	int num = 0;
	int rtn = 0;
	cout << "How do you want to test ArrayList:" << endl
		 << "\t 1: Manipulate an ArrayList of integers." << endl
		 << "\t 2: Print from an ArrayList" << endl
		 << "\t 3: Sort an ArrayList of Integers" << endl
		 << "\t 4: Generate a sequence of prime numbers" << endl
		 << "\t 5: Run Rot13 on a string" << endl
		 << "\t 6: Run all four automatic tests" << endl
		 << endl
		 << "Enter X to quit.";

	cin >> inp;

	for(bool close = false; close != true;)
	{
		if(inp == 'X')
		{
			cout << "Are you sure?" << endl
				 << "Y/n" << endl;
			cin >> inp;

			if(inp == 'Y')
			{
				cout << "Testing Complete" << endl
					 << "Press ENTER to continue...";
				close = true;
				rtn = 0;
				cin.ignore();
			}
			else
			{ cout << "Resuming Test" << endl; }
		}
		else if(inp == '1')
		{
			cout.clear();
			rtn = test1();
			cin.clear();
		}
		else if(inp == '2')
		{
			cout.clear();
			cout << hello() << endl;
			cin.clear();
		}
		else if(inp == '3')
		{
			cout.clear();
			bucket = vector<int>();
			cout << "Enter a list of integers:" << endl
				 << endl;
			while(cin.get() != '\n')
			{
				cin >> num;
				bucket.push_back(num);
			}
			cout << "This is your list:" << endl
				 << bucket << endl
				 << endl;
			bucket = sort(bucket);
			cout << "This is your list sorted:" << endl
				 << bucket << endl;
			cin.clear();
		}
		else if(inp == '4')
		{
			cout.clear();
			cout << "What number do you want to count to?" << endl;
			cin >> num;
			cout << primes(num) << endl;
			cin.clear();
		}
/* ignore this for now it's not done
		else if(inp == '5')
		{
			cout.clear();
			cout << "Enter a string" << endl;
			cin.clear();
		}
*/
		else if(inp == '6')
		{
			cout.clear();
			rtn = test2();
			cin.clear();
		}
		else
		{ cout << "Command not recognized. Try again." << endl; }
	}
	return rtn;
}

I'm running with the MSVC++ compiler on Windows 10 (64-bit) and very time the code tries to call vector::pop_back, I get a segfault. Any idea what's wrong?

[Thomas1965]

Could be that your vector is empty.
Normally VS will break into the debugger so you can see what's wrong.
https://www.codeproject.com/Articles/469416/More-Visual-Studio-Debugging-Tips-for-Native-De
https://www.codeproject.com/Articles/518159/Even-More-Visual-Studio-Debugging-Tips-for-Nati

[keskiverto]

All of these have the same fundamental problem:

		else if(inp == "insert")
		{
			int num;
			int pos;
			cin >> num >> pos;
			ray.insert(ray.begin() + pos, num);
		}
		else if(inp == "popend")
		{
			ray.pop_back();
		}
		else if(inp == "remove")
		{
			int pos;
			cin >> pos;
			ray.erase(ray.begin() + pos);
		}

insert & erase:
What if pos == -42?

pop_back:

If the container is not empty, the function never throws exceptions (no-throw guarantee).
Otherwise, it causes undefined behavior.

What is common to all three is that you don't check whether the actions that you attempt are actually valid. Do not trust the user input. Validate, before using.