c++ - template of template without default constructor -

i'm trying write template class doesn't have default constructor.
a<int> works fine, a<a<int>> don't know how work.

  1   #include <iostream>   2   using namespace std;   3    4   template <typename t>   5   class {   6     t x;   7    8    public:   9     a(t y) { x = y; }  10   };  11   12   int main() {  13     a<int> a(0);  14     a<a<int> > b(a<int>(0));  15   16     return 0;  17   } 

error list clang

    test.cpp:9:5: error: constructor 'a<a<int> >' must explicitly initialize member 'x' not have default constructor         a(t y) { x = y; }         ^     test.cpp:14:16: note: in instantiation of member function 'a<a<int> >::a' requested here         a<a<int> > b(a<int>(0));                    ^     test.cpp:6:7: note: member declared here         t x;           ^     test.cpp:5:9: note: 'a<int>' declared here       class {             ^ 

you not constructing x in initializer list of constructor, hence a(t y) must default construct x before invoking operator= copy assign y it.

int provides default constructor, let value uninitialized, a<int> not.

your constructor should be

a(t y) : x(y) { }