How to use Google Protobuf Map in C++?

Question

I am attempting to use the new protobuf Map functionality in C++.

The following was done on Ubuntu 14.04 with gcc 4.9.2 C++14 and protobuf 3.0.0-alpha-4:

Message definition:

message TestMap {
     map<string,uint64> map1 = 1;
}

Then, I tried to compile the following example program:

auto test = debug::TestMap::default_instance();
auto map = test.mutable_map1();
string key = "key";
uint64 val = 20;
map[key] = val;

Acccessing the map using the [] syntax works totally fine for std::unordered_map. But the protobuf implementation always yields the following compiler error:

error: no match for ‘operator[]’ (operand types are ‘google::protobuf::Map<std::basic_string<char>, long unsigned int>*’ and ‘std::string {aka std::basic_string<char>}’)

I do not understand why this operator is not known, because the header file google::protobuf::Map is clearly found and this should be an elementary operation.

Do you have any idea what goes wrong here? I would welcome any example of using the new protobuf maps, as I haven't found any online after researching for hours.

Solution

As Pixelchemist pointed out, the problem is that map is a pointer so the [] operator does not work.

Thus, the pointer needs to be dereferenced first. *map[key] also does not work, as the compiler first parses [] and then the *. The following does work:

(*map)[key] = val;

Although this is a quite basic problem, this represents a good learning opportunity for C++ and Protocol Buffers.

Comments

[Pixelchemist]

Your map variable seems to be a pointer. Did you try *map[key]?

[Daniel Becker]

@Pixelchemist: Do you want to post the answer or should I do it?

[Kenton Varda]

FWIW, you could do: auto& map = *test.mutable_map1();, and then map[key] would work.

[Daniel Becker]

Thanks Kenton! That is much more reable and easier to maintain.

[tigertang]

map->operator[](key) = val;

[Christopher Pisz]

is mutable_map the only method to use to access the map?

[Rick]

@ChristopherPisz No. test.map1() gives you a const reference, if you only need acess instead of modification. Doc is here developers.google.com/protocol-buffers/docs/reference/…