CODEWITHSUNDEEP
c++dictionary
cpp beginner
cpp beginner

Reputation: 570

Adding to a map with mutable values

Suppose I have a map<string, vector<int>> called my_map and I want to associate 42 to the key "foo". Coming from Java I would expect to have to do something like this

if (my_map.find("foo") != my_map.end()) {
    my_map["foo"].push_back(42);
} else {
    vector<int> my_vector;
    my_map["foo"] = my_vector;
    my_vector.push_back(42);
}

To my surprise I found that I can just do

my_map["foo"].push_back(42);

How does this work? What would my_map["foo"] do if the type of the map's values did not have a constructor with no parameters? Suppose you wanted to initialise all values in a manner other than using an empty constructor? In Java, I could do

map.computeIfAbsent("foo", **some supplier of Vectors**).add(42);

Is there an equivalent in C++?

Upvotes: 1

Views: 2091

Answers (2)

Quentin
Quentin

Reputation: 63114

my_map["foo"] default-constructs a new value if it doesn't already exist. Thus, it would cause a compilation error if the value type was not default-constructible (i.e. didn't have a constructor with no parameter). You would then have to do the following :

my_map.emplace("foo", param).first->second.push_back(42);

Where emplace would construct your value from param if it didn't already exist.

More parameters can be supplied with the piecewise constructor from std::pair :

my_map.emplace(
    std::piecewise_construct,
    std::forward_as_tuple("foo"),
    std::forward_as_tuple(param1, param2, param3)
).first->second.push_back(42);

Upvotes: 6

Unda
Unda

Reputation: 1899

You can read here for operator[] :

If k does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value. Notice that this always increases the container size by one, even if no mapped value is assigned to the element (the element is constructed using its default constructor).

If the mapped_type doesn't have a default constructor, you'd get a compilation error.

Upvotes: 2

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