Get an object from a map by reference

Question

I am trying to get an object inside of a hash_map by reference. It seems simple enough, but I cannot figure out why it doesn't compile. The error is no match for operator=. In my mind, I am setting the value of fooptr to the address of the found Foo object in the map.

void FooManager::GetFoo(Foo *fooptr, std::string name){
    std::hash_map<std::string, Foo>::iterator it = this->foos.find(name);
    if(it != this->foos.end()){
       *fooptr = &it->second;
    }
}

Foo *foo = 0;
GetFoo(foo, "test");

If `fooptr` is of type `Foo *`, ask yourself what type does `*fooptr` have. — HolyBlackCat, Mar 11 '19 at 16:10
Seems like a [std::hash_map](https://stackoverflow.com/questions/5908581/is-hash-map-part-of-the-stl) does exist. — TrebledJ, Mar 11 '19 at 16:19
@TrebuchetMS - The Q&A you linked **explicitly** says the opposite. — StoryTeller - Unslander Monica, Mar 11 '19 at 17:01
@StoryTeller Perhaps I misread or misunderstood, but referring to [Lightness's second-to-last paragraph](https://stackoverflow.com/a/5908859/10239789): "_both MSVC++ and GCC... place it [hash_map] in the std namespace_". I myself wasn't aware that there was a hash_map container at all. I was merely saying that there _is_ a `std::hash_map` (albeit not necessarily on all implementations). :| — TrebledJ, Mar 12 '19 at 03:22

Thomas Sablik · Answer 1 · 2019-03-11T18:10:04.087

3

After reading the comments in john's answer I think I understand your problem. Call by pointer is also call by value. You copy the pointer address. You can call by reference to pointer:

void FooManager::GetFoo(Foo *&fooptr, std::string name){
    std::hash_map<std::string, Foo>::iterator it = this->foos.find(name);
    if(it != this->foos.end()){
       fooptr = &it->second;
    }
}

Foo *foo = 0;
GetFoo(foo, "test");

Now fooptr is a reference to a pointer. When you change the address (value) of fooptr, you also change the address (value) of foo.

edited Mar 11 '19 at 18:10

answered Mar 11 '19 at 16:27

Thomas Sablik

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That works, too. I think I'll just return the address itself. – peer_2_peer_2 Mar 11 '19 at 16:29
Correct me if I'm wrong... shouldn't it be `fooptr = &it->second;` (without the star) instead? – TrebledJ Mar 11 '19 at 16:36

john · Accepted Answer · 2019-03-11T16:23:00.637

1

You're confused about pointers.

*fooptr = it->second;

Foo foo;
GetFoo(&foo, "test");

Declare an object, pass a pointer to that object, dereference the pointer inside the function.

EDIT

Based on feed back below, the answer should be

*fooptr = &it->second;

Foo* foo = 0;
GetFoo(&foo, "test");

Declare a pointer, pass a pointer to that pointer to the function (a double pointer), dereference the double pointer (to get a pointer) and assign to that a pointer to the map element.

This code is horrendous.

-2 is nothing, i wouldn't worry.

edited Mar 11 '19 at 16:23

answered Mar 11 '19 at 16:14

john

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I feel like I've tried this, it was my original route. It worked but it didn't seem to be obtaining the actual object - more like it was obtaining a copy of it. I'll try again.. – peer_2_peer_2 Mar 11 '19 at 16:15
1

@peer_2_peer_2 Of course it's obtaining a copy. Sounds like you need a double pointer. – john Mar 11 '19 at 16:16
I guess I just don't get it? I have an object living inside a map. That object has an address. So why does declaring a pointer, and then passing it to a function, and then setting the value of that pointer to the address of the object, not work? – peer_2_peer_2 Mar 11 '19 at 16:18
1

to use `Foo * FooManager::GetFoo(std::string name)` is must more practical to get a pointer to, of course return nullptr if not found – bruno Mar 11 '19 at 16:19
2

Because the pointer in your function is only a copy of the pointer outside your function. If you want to change a pointer, you need to pass a pointer to the pointer, same as if you want to change an integer you need to pass a pointer to the integer. (references work as well). – john Mar 11 '19 at 16:20
Or do what bruno suggests which is equivalent but a little easier to understand. – john Mar 11 '19 at 16:20

Get an object from a map by reference

2 Answers2