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Review: dangling references

std::string_view and references are cheap views of data owned by someone else. The classic review catch is a view that outlives what it points at. It compiles cleanly, often seems to work in testing (the freed memory still holds the old bytes for a while), and then prints garbage in production.

Red flags to look for:

• returning a string_view or a reference to a local or a temporary
• storing a string_view member initialized from a temporary std::string
• keeping a reference into a container that later grows

Who owns the characters?

For every view, find the std::string (or literal) that owns its characters, and check that the owner lives at least as long as the view.

#include <iostream>
#include <string>
#include <string_view>
#include <utility>

std::string_view first_word(std::string_view s) {      // fine: points into the caller's text
    return s.substr(0, s.find(' '));
}

struct Label {
    std::string text;                                   // owns its characters
    explicit Label(std::string t) : text(std::move(t)) {}
};

Label make_label(int n) {
    return Label("item " + std::to_string(n));          // the temporary is moved into the member
}

int main() {
    std::string line = "hello big world";
    std::cout << first_word(line) << "\n";              // line outlives the view: safe

    Label l = make_label(7);
    std::cout << l.text << "\n";
}

hello
item 7

• first_word is safe because it returns a view into the caller's string, which is still alive when the view is used.
• Label stores a std::string, not a view. The temporary "item " + ... would die at the end of the statement, so a string_view member would dangle; an owning member takes the characters with it.

The rule of thumb: views for parameters, owning strings for members and for return values built inside the function.

Your turn: this pull request adds a greeting helper and a Tag type. Both have lifetime bugs. Fix them by making each owner hold its own std::string. (The checks inspect the types too, since dangling-view bugs can seem to work by luck.)

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