I present to you this code riddle:
Using this compiler:
user@bruh:~/test$ g++ --version
g++ (Ubuntu 7.3.0-16ubuntu3) 7.3.0 Copyright (C) 2017 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
...and this compile string:
g++ main.cpp class.cpp -o main -g
...and these files:
class.hpp:
class base {
public:
base();
virtual void f() = 0;
};
class derived : public base {
public:
derived( unsigned int id );
void f() override;
unsigned int id;
};
class.cpp:
#include <iostream>
#include "class.hpp"
base::base() {
return;
}
derived::derived( unsigned int id )
:
id( id ),
base() {
return;
}
void derived::f() {
std::cout << "Ahoy, Captain! I am " << id << std::endl;
return;
}
main.cpp:
#include <iostream>
#include <functional>
#include <vector>
#include "class.hpp"
int main() {
unsigned int n_elements;
std::cout << "enter the number of elements: ";
std::cin >> n_elements;
std::cout << std::endl;
std::vector< class derived > v;
std::vector< std::reference_wrapper< class base > > base_vector_0;
std::vector< std::reference_wrapper< class base > > base_vector_1;
for( unsigned int i = 0; i < n_elements; i++ ) {
v.emplace_back( i );
base_vector_0.emplace_back( v[ i ] );
}
for( unsigned int i = 0; i < n_elements; i++ ) {
base_vector_1.emplace_back( v[ i ] );
}
std::cout << "sanity check:" << std::endl;
for( unsigned int i = 0; i < n_elements; i++ ) {
class base &base = v[ i ];
base.f();
}
std::cout << "case 1:" << std::endl;
for( unsigned int i = 0; i < n_elements; i++ ) {
base_vector_1[ i ].get().f();
}
std::cout << "case 0:" << std::endl;
for( unsigned int i = 0; i < n_elements; i++ ) {
base_vector_0[ i ].get().f();
}
return 0;
}
...I get the following output:
user@bruh:~/test$ ./main
enter the number of elements: 1
sanity check:
Ahoy, Captain! I am 0
case 1:
Ahoy, Captain! I am 0
case 0:
Ahoy, Captain! I am 0
harrynh3@bruh:~/test$ ./main
enter the number of elements: 2
sanity check:
Ahoy, Captain! I am 0
Ahoy, Captain! I am 1
case 1:
Ahoy, Captain! I am 0
Ahoy, Captain! I am 1
case 0:
Segmentation fault (core dumped)
My questions:
Why does this not segfault when the user supplied argument = 1?
Why does this segfault when the user supplied argument > 1?
My short explanation of what the code does:
Creates many objects derived from an abstract base class. Stores references to the objects in containers as std::reference_wrapper around abstract base class reference. Creates the containers of std::reference_wrapper slightly differently. Calls the derived override of pure virtual function via the std::reference_wrappers. Segfaults specifically in the case denoted in the source code above.
I implore the C++ experts... Please help me! This is fascinating and I have no idea why it is happening! I've probably done something dumb.