Segmentation faults and erroneous output from std::sort - c++

Question

There is something very wrong with the following code, but I can't figure out what it is.
#include <map>
#include <iostream>
#include <random>
#include <algorithm>
class A
{
public:
A(int v) : val(v) { }
A() = delete;
const size_t val;
};
typedef std::shared_ptr<A> A_ptr;
class B
{
public:
B(unsigned int L)
{
std::mt19937 rand_engine(9341255);
std::uniform_int_distribution<int> dist(1,1000);
for (int i = 0; i < L; ++i)
{
m.insert(std::pair<unsigned int, A_ptr>(i, std::make_shared<A>(dist(rand_engine))));
}
}
B() = delete;
std::map<unsigned int, A_ptr> m;
};
int main()
{
unsigned int b_size = 47;
B b(b_size);
std::vector<A_ptr> v = {};
for (std::map<unsigned int, A_ptr>::iterator it = b.m.begin(); it != b.m.end(); ++it)
{
v.push_back(it->second);
}
std::cout << "v = ";
for (auto & i : v)
std::cout << i->val << " ";
std::cout << "\n";
std::sort(v.begin(), v.end(), [](A_ptr a1, A_ptr a2){ return (a1->val >= a2->val); });
std::cout << "v sorted = ";
for (auto & i : v)
std::cout << i->val << " ";
std::cout << "\n";
return 0;
}
This code compiles and executes, and yields the following output:
g++ -std=c++17 map_sort.cpp -o map_sort && ./map_sort
v = 634 739 51 906 227 185 738 302 310 888 886 644 191 719 68 212 124 732 879 724 671 167 367 313 813 1000 905 24 245 320 580 605 641 760 23 382 348 718 373 937 733 335 306 679 840 880 138
v sorted = 1000 937 906 905 880 888 886 245 879 840 813 760 733 719 718 644 641 739 738 732 724 679 634 671 605 580 382 373 367 320 348 335 313 310 306 302 227 68 191 185 167 212 138 124 51 24 23
Note that the output on the v sorted line is NOT correctly sorted.
Changing unsigned int b_size = 47 to unsigned int b_size = 48 yields the following output:
g++ -std=c++17 map_sort.cpp -o map_sort && ./map_sort
v = 634 739 51 906 227 185 738 302 310 888 886 644 191 719 68 212 124 732 879 724 671 167 367 313 813 1000 905 24 245 320 580 605 641 760 23 382 348 718 373 937 733 335 306 679 840 880 138 924
Segmentation fault: 11
Similar behaviors occur for other values of b_size as well. It wouldn't surprise me if they're coming from the same underlying problem. I'm clearly using std::sort incorrectly, but I have no idea how. Can anybody shed some light on this?