[BUG] Assignment from value to value not working for non-trivial types

[dsiroky]

Hi! This issues seems to appear only if the value is of a non-trivial type.

This code

    ankerl::unordered_dense::map<int, std::string> m{};
    m[1] = "abc";
    m[2] = m[1];
    std::cout << '"' << m[1] << "\"\n";
    std::cout << '"' << m[2] << "\"\n";

prints

"abc"
""

Expected output is

"abc"
"abc"

If I "pre-create" the value in the map then it works:

    ankerl::unordered_dense::map<int, std::string> m{};
    m[1] = "abc";
    m[2]; // pre-create
    m[2] = m[1];
    std::cout << '"' << m[1] << "\"\n";
    std::cout << '"' << m[2] << "\"\n";

System:

• OS: Linux
• Compiler: g++ 13.2, clang++ 16.0.6
• unordered_dense git commit d911053

[martinus]

Hi @dsiroky, thanks for the bug report! This seems to work for the try_emplace methods, but not for operator[]. I'll have a look

[glebm]

A repro test case for test/assignment_combinations.cpp:

TEST_CASE_MAP("assignment_combinations_11", int, std::string) {
    map_t m;
    m[1] = "abc";
    m[2] = m[1];
    REQUIRE(m[1] == "abc");
    REQUIRE(m[2] == "abc");
}

[glebm]

Let me rewrite the failing example above to an equivalent order of operations:

std::string &a = m[1]; // evaluate right-hand side
std::string &b = m[2]; // evaluate left-hand side
b = a; // assign

The problem is that the reference to a is invalidated in when the left-hand side is evaluated (because the underlying storage is resized), so it is invalid at the time of the assignment.

I don't think this is actually a bug in unordered_dense.

[martinus]

@glebm you are of course right, there's nothing I can do about this since the map doesn't guarantee reference stability.