c++11 - How to initialize a nested map in constructor initialization list in c++ -

class test{    public:       test();    ...    private:       map<int, string> map1;       map<int, map<int, string>> map2; //two dimensional map    ... };  test::test():map1() {} 

i trying initialise map using initialization list. simple(single) map, can initialize call map constructor(map1()) in initialization list. how can initialize nested map(map2) in constructor initialization list.

thanks in advance help.