Lifetime extension of temporary container's element

Question

Does prolonging the lifetime of a temporary object through const references also apply for elements of temporary containers, such as in the case of reference r in the program below?
Is that guaranteed by the C++ standard?
I've read that this sort of lifetime extension does apply for subobjects of complete objects, but do elements of standard containers qualify as subobjects of their respective container?

#include <unordered_map>
#include <string>
#include <iostream>

using my_map = std::unordered_map<std::string, int>;

my_map func() {
    return my_map{{"foo", 42}};
}

int main() {
    const auto& r = func()["foo"];
    std::cout << "r = " << r << std::endl; // Is r valid here?
    return 0;
}

Answer 1

No, r is not valid. r becomes an invalid reference to a temporary as soon as the statement is r = func()["foo"]; is over.

Here's a proof of concept that extends upon your example code:

#include <unordered_map>
#include <string>
#include <iostream>


class Foo {
public:
    int value;

    Foo() : value(0) {
        value = 0;
    }
    Foo(int n) : value(n) {
    }
    ~Foo() {
        std::cout << "~Foo(" << value << ")\n";
    }
};

using my_map = std::unordered_map<std::string, Foo>;

my_map func() {
    return my_map{ {"foo", 42} };
}

int main() {
    const auto& r = func()["foo"];
    std::cout << "r.value = " << r.value << std::endl; // Is r valid here?
}

When I run the following program, I get these results:

~Foo(42)
~Foo(42)
r.value = -572662307    # GARBAGE OUTPUT

-572662307 is 0xdddddddd in hex, which is what Visual Studio's debug assigns to deleted memory that hasn't been reclaimed. So it's a strong indication that r is definitely invalid.

This would be valid in your code and would print the expected value of 42:

std::cout << "func['foo'] = " << func()["foo"] << std::endl;