Error when accessing member over multiple classes

I'm new to C++, and I'm writing a simple blockchain program as a sort of exercise. When I run the below code, I seem to get an error of sorts:

Process returned -1073741819 (0xC0000005)

The code is below:

#include <iostream>
#include <time.h>
#include <string>
using namespace std;

class Block
{
    int data, previous_hash;
public:
    string timestamp;
    Block(string a, int b, int c)
    {
        timestamp = a;
        data = b;
        previous_hash = c;
    };
};

string getTime()
{
    time_t now = time(NULL);
    struct tm tstruct;
    char buf[40];
    tstruct = *localtime(&now);
    strftime(buf, sizeof(buf), "%X", &tstruct);
    return buf;  //Simple code to return current time
}

class BlockChain
{
public:
    Block chain[];
    BlockChain()
    {
        chain[0]=createGenesisBlock();
    }
    Block createGenesisBlock()
    {
        return Block(getTime(), 10, 0);
    }
};

int main()
{
    BlockChain myChain;
    cout << "current time is " << getTime();
    cout << myChain.chain[0].timestamp; //Is this correct?
}

I included a line in main() to access the string timestamp in my object mychain. I suspect this may be the problem, but i'm not sure how else I can access timestamp when its called over both Blockchain and Block classes.

Currently, BlockChain::chain is an array with unknown size. But when you access chain[0] in BlockChain's constructor, you're assuming that chain points to valid memory, which it doesn't because you never initialize it. That's why you're getting a crash due to a bad memory access. I would suggest the use of std::vector<Block> instead of Block[], which you can resize as needed:

class BlockChain {
public:
    std::vector<Block> chain;
    BlockChain() {
        // initialize and append a new block to chain
        chain.emplace_back(createGenesisBlock());
    }