What Is the Preferred/Idiomatic Way to Insert into a Map?

As of C++11, you have two major additional options. First, you can use insert() with list initialization syntax:

function.insert({0, 42});

This is functionally equivalent to

function.insert(std::map<int, int>::value_type(0, 42));

but much more concise and readable. As other answers have noted, this has several advantages over the other forms:

• The operator[] approach requires the mapped type to be assignable, which isn't always the case.
• The operator[] approach can overwrite existing elements, and gives you no way to tell whether this has happened.
• The other forms of insert that you list involve an implicit type conversion, which may slow your code down.

The major drawback is that this form used to require the key and value to be copyable, so it wouldn't work with e.g. a map with unique_ptr values. That has been fixed in the standard, but the fix may not have reached your standard library implementation yet.

Second, you can use the emplace() method:

function.emplace(0, 42);

This is more concise than any of the forms of insert(), works fine with move-only types like unique_ptr, and theoretically may be slightly more efficient (although a decent compiler should optimize away the difference). The only major drawback is that it may surprise your readers a little, since emplace methods aren't usually used that way.

The first version:

function[0] = 42; // version 1

may or may not insert the value 42 into the map. If the key 0 exists, then it will assign 42 to that key, overwriting whatever value that key had. Otherwise it inserts the key/value pair.

The insert functions:

function.insert(std::map<int, int>::value_type(0, 42));  // version 2
function.insert(std::pair<int, int>(0, 42));             // version 3
function.insert(std::make_pair(0, 42));                  // version 4

on the other hand, don't do anything if the key 0 already exists in the map. If the key doesn't exist, it inserts the key/value pair.

The three insert functions are almost identical. std::map<int, int>::value_type is the typedef for std::pair<const int, int>, and std::make_pair() obviously produces a std::pair<> via template deduction magic. The end result, however, should be the same for versions 2, 3, and 4.

Which one would I use? I personally prefer version 1; it's concise and "natural". Of course, if its overwriting behavior is not desired, then I would prefer version 4, since it requires less typing than versions 2 and 3. I don't know if there is a single de facto way of inserting key/value pairs into a std::map.

Another way to insert values into a map via one of its constructors:

std::map<int, int> quadratic_func;

quadratic_func[0] = 0;
quadratic_func[1] = 1;
quadratic_func[2] = 4;
quadratic_func[3] = 9;

std::map<int, int> my_func(quadratic_func.begin(), quadratic_func.end());

Since C++17 std::map offers two new insertion methods: insert_or_assign() and try_emplace(), as also mentioned in the comment by sp2danny.

insert_or_assign()

Basically, insert_or_assign() is an "improved" version of operator[]. In contrast to operator[], insert_or_assign() doesn't require the map's value type to be default constructible. For example, the following code doesn't compile, because MyClass does not have a default constructor:

class MyClass {
public:
    MyClass(int i) : m_i(i) {};
    int m_i;
};

int main() {
    std::map<int, MyClass> myMap;

    // VS2017: "C2512: 'MyClass::MyClass' : no appropriate default constructor available"
    // Coliru: "error: no matching function for call to 'MyClass::MyClass()"
    myMap[0] = MyClass(1);

    return 0;
}

However, if you replace myMap[0] = MyClass(1); by the following line, then the code compiles and the insertion takes place as intended:

myMap.insert_or_assign(0, MyClass(1));

Moreover, similar to insert(), insert_or_assign() returns a pair<iterator, bool>. The Boolean value is true if an insertion occurred and false if an assignment was done. The iterator points to the element that was inserted or updated.