Constructing a vector of structs (with some custom constructors) from exactly two string literals crashes. Why?

Question

Can you guess the output of this trivial program?

#include <vector>
#include <string>
#include <exception>
#include <iostream>

int main()
{
    try { 
        struct X {
            explicit X(int) {}
            X(std::string) {} // Just to confuse you more...
        };
        std::vector<X>{"a", "b"};
    } catch (std::exception& x) {
        std::cerr << x.what();
    }
}

Well, I couldn't, which cost me a day of "research", before landing here, with finally having it distilled from some complex real-life code (with type aliases everywhere, anon. unions with non-POD members & hand-orchestrated ctors/dtors etc., just for the vibe).

And... I still can't see what's going on! Can someone please give a gentle hint? (Hopefully just a blind spot. I no longer do C++ professionally.)

Note: clean compile* with both (latest) MSVC /W4 and GCC -Wall; same output in both (semantically).

_{* Even without the "confuse-the-reader" line. Which I think I'm gonna have nightmares from.}

---

(Please bear with me for trying not to spoiler it by spelling everything out even more — after all, this truly is self-explanatory as-is, right? Except, the exact opposite for me...)

Answer 1

std::vector<X>{"a", "b"};

This creates a vector from two iterators of type const char* using the constructor that takes two iterators:

template< class InputIt >
constexpr vector( InputIt first, InputIt last,
                  const Allocator& alloc = Allocator() );

Constructs the container with the contents of the range [first, last). This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator, to avoid ambiguity with the overload (3). (below)

constexpr vector( size_type count,
                  const T& value,
                  const Allocator& alloc = Allocator() );

It's just bad luck that the decay of the two const char[]s becomes perfect iterators that fulfills the LegacyInputIterator requirement.

The iterators do not point to an array/contiguous area and the program therefore has undefined behavior.

What happens under the hood is most likely that it'll try to get from the first const char* to the second and run out of bounds as soon as its passing the null terminator after the 'a'.

A similar construction that would actually work:

const char* arr = "working";

struct X {
    explicit X(int i) {
        std::cout << static_cast<char>(i); 
    }
};

const char* first = arr;     // begin iterator
const char* last = arr + 7;  // end iterator

std::vector<X>{first, last}; // prints "working"