C++ call a member function of an anonymous object instance? - c++

Question

I have the following piece of code:
map<int, set<int>> my_map;
int my_int1, my_int2;
// some code
set<int> temp;
temp.insert(my_int2);
my_map.insert(make_pair(my_int1, temp));
Is there some way to avoid using the temp (at least just for the code to look nicer, not necessarily for performance) and use an anonymous object? Something like:
my_map.insert(make_pair(my_int1, (set<int>{}).insert(my_int2)));
Except, the above line doesn't compile.

Answer 1

Not tested but try something like this:
my_map.insert({my_int1, {my_int2}});
Ok, let's sumarize. There is an important thing to know about insert: the insert doesn't insert if key already exists.
//create and initialise inline a map with int keys and string values
map<int, string> x {
{10, "hello"},
{123, "bye"},
{234, "world"}
};
x.insert({567, "alien"}); //inserts an element initialised inline
x.insert({567, "bar"}); //does nothing, according to specs
x[124] = "buzz"; //inserts an element
x[567] = "bar"; //modifies the element with key 567
//create and initialise inline a map with int keys and int values
map<int, string> y {
{10, 1},
{123, 2},
{234, 3}
};
y.insert({567, 4}); //insert an element initialised inline
z[124] = 5; //inserts
z[124] = 6; //modifies the above element
z[125]++; //inserts and modifies, now z[125] is 1
z[125]++; //modifies the above element to 2
It is very flexible. If you create a map of containers
//create and initialise inline a map with int keys and set values
map<int, set<int>> x {
{10, {1, 2, 3}},
{123, {2, 3}},
{234, {1, 2, 3}}}
};
//calling insert on map:
y.insert({567, {4, 5, 6}}); //element 567 has the value {4, 5, 6}
//now this calls insert in the set:
y[567].insert(7); //element 567 exists, so it is modified to {4, 5, 6, 7}
y[568].insert(7); //element 569 doesn't exist, so it is created first
//assign
y[235] = {4, 5, 6}; //element 235 has the value {4, 5, 6}
y[235] = {7, 8, 9}; //element 235 has the value {7, 8, 9}
y[235].insert(10); //element 235 has the value {7, 8, 9, 10}

Answer 2

You need none of your intermediate steps!
int main()
{
std::map<int, std::set<int>> my_map;
my_map.insert({1,{}});
}

Answer 3

You don't need the temporary std::set at all. Simply use std::map::operator[] to create the std::set in the std::map if it doesn't already exist, and then you can insert() into that std::set directly, eg:
map<int, set<int>> my_map;
int my_int1, my_int2;
// some code
my_map[my_int1].insert(my_int2);