Assignment in C++ occurs despite exception on the right side

Question

I have some (C++14) code that looks like this:

map<int, set<string>> junk;
for (int id : GenerateIds()) {
    try {
        set<string> stuff = GetStuff();
        junk[id] = stuff;
    } catch (const StuffException& e) {
        ...
    }
}

This works. Sometimes GetStuff() throws an exception, which works fine, because if it does, I don't want a value in the junk map then.

But at first I'd written this in the loop, which doesn't work:

junk[id] = GetStuff();

More precisely, even when GetStuff() throws an exception, junk[id] is created (and assigned an empty set).

This isn't what I'd expect: I'd expect them to function the same way.

Is there a principle of C++ that I've misunderstood here?

Answer 1

Before C++17 there was no sequencing between the left- and right-hand side of assignment operators.

It's first in C++17 that explicit sequencing was introduced (right-hand side is evaluated first).

That means the evaluation order is unspecified , which means it's up to the implementation to perform the evaluation in the order in which it wants, and in this case it evaluates the left-hand side first.

See this evaluation order reference for more details (especially point 20).

Answer 2

std::map::operator[]

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.

junk[id] causes the above mentioned insertion and after that has already happened GetStuff() throws. Note that in C++14 the order in which these things happen is implementation defined so with a different compiler your junk[id] = GetStuff(); may not do the insertion if GetStuff() throws.

Answer 3

You're misunderstanding how operator[] works on std::map .

It returns a reference to the mapped item. Therefore, your code is first inserting a default item in that position and then invoking operator= to set a new value.

To make this work the way you expect, you'll need to use std::map::insert (*):

junk.insert(std::make_pair(id, GetStuff()));

Caveat : insert will only add the value if id is not already mapped.