CODEWITHSUNDEEP
c++templates
titus
titus

Reputation: 5784

template of template without default constructor

I'm trying to write a template class that doesn't have default constructor.
For A<int> works fine, but for A<A<int>> I don't know how to get it to work.

  1   #include <iostream>
  2   using namespace std;
  3 
  4   template <typename T>
  5   class A {
  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 from clang

    test.cpp:9:5: error: constructor for 'A<A<int> >' must explicitly initialize the member 'x' which does not have a 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 is declared here
        T x;
          ^
    test.cpp:5:9: note: 'A<int>' declared here
      class A {
            ^

Upvotes: 0

Views: 436

Answers (1)

Jack
Jack

Reputation: 133609

You are not properly constructing x in the initializer list of the constructor, hence A(T y) must default construct x before invoking operator= to copy assign y to it.

int provides a default constructor, which just let the value uninitialized, but A<int> does not.

Your constructor should be

A(T y) : x(y) { }

Upvotes: 3

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