template re-initialization question

Collapse
This topic is closed.
X
X
Collapse
 
  • Page of 1
  • Time
  • Show
Clear All
new posts
Previous template Next
  • zl2k
    #1

    template re-initialization question

    hi, all

    Suppose I have a template class A
    template<class Tclass A{
    T a;
    A(T &a){this->a = a;} //***
    }

    Then I declare something as following in a header file

    A<intabc;
    A<int*p;

    In somewhere in the cpp file, I'll need to re-initialize them, but not
    sure I am doing it correctly.

    p = new A<int>(123); //looks fine?
    How can I re-initialize abc?

    If I'll need other type (double, short, etc) in the runtime, I have to
    declare A<double>, A<shortfor each of them in the header file? That
    looks like redundant. Thanks for comments.

    BTW, in ***, if I write in that way, will the instance of A keeps a
    "hard copy" of a such that A keeps what every transfered to a during
    initialization from outside even after the outside object is deleted
    afterwards? Or maybe I should write A(T a){this->a = a;}?
    zl2k

    Tags: None
    • Victor Bazarov
      #2
      Re: template re-initialization question

      zl2k wrote:
      Suppose I have a template class A
      template<class Tclass A{
      T a;
      A(T &a){this->a = a;} //***
      Why not initialise it? Always prefer initialisation over assignment!
      }
      ^^^
      Semicolon is missing here
      >
      Then I declare something as following in a header file
      >
      A<intabc;
      A<int*p;
      If those are not inside a class, they are also _definitions_, since they
      do not have "extern".
      >
      In somewhere in the cpp file, I'll need to re-initialize them, but not
      sure I am doing it correctly.
      >
      p = new A<int>(123); //looks fine?
      That's not "initialise ", that's _assign_, again. And it has to be in
      a function, it cannot be outside of any function.
      How can I re-initialize abc?
      What do you mean? You already defined (and initialised) them.
      >
      If I'll need other type (double, short, etc) in the runtime, I have to
      declare A<double>, A<shortfor each of them in the header file? That
      looks like redundant. Thanks for comments.
      Why do you think you need to declare them in the first place? Only
      declare/define/initialise objects *right before you need them*.
      >
      BTW, in ***, if I write in that way, will the instance of A keeps a
      "hard copy" of a such that A keeps what every transfered to a during
      initialization from outside even after the outside object is deleted
      afterwards? Or maybe I should write A(T a){this->a = a;}?
      No, a reference should be fine, and even better is to use a reference
      to const 'T'.

      V
      --
      Please remove capital 'A's when replying by e-mail
      I do not respond to top-posted replies, please don't ask


        Comment

        • zl2k
          #3
          Re: template re-initialization question


          Victor Bazarov wrote:
          zl2k wrote:
          Suppose I have a template class A
          template<class Tclass A{
          T a;
          A(T &a){this->a = a;} //***
          >
          Why not initialise it? Always prefer initialisation over assignment!
          >
          }
          ^^^
          Semicolon is missing here
          >

          Then I declare something as following in a header file

          A<intabc;
          A<int*p;
          >
          If those are not inside a class, they are also _definitions_, since they
          do not have "extern".
          >

          In somewhere in the cpp file, I'll need to re-initialize them, but not
          sure I am doing it correctly.

          p = new A<int>(123); //looks fine?
          >
          That's not "initialise ", that's _assign_, again. And it has to be in
          a function, it cannot be outside of any function.
          >
          How can I re-initialize abc?
          >
          What do you mean? You already defined (and initialised) them.
          I mean how to assign another value. A pointer can do by new, I am not
          sure how abc can assigned to another value.
          >

          If I'll need other type (double, short, etc) in the runtime, I have to
          declare A<double>, A<shortfor each of them in the header file? That
          looks like redundant. Thanks for comments.
          >
          Why do you think you need to declare them in the first place? Only
          declare/define/initialise objects *right before you need them*.
          The problem is, if I declare/define/initialise objects *right before
          you need them* in somewhere, then in some other functions I may not be
          able to use it. I declare it in the header so that the program can use
          it everywhere, with different types defined at run time. Is it
          possible?

          >

          BTW, in ***, if I write in that way, will the instance of A keeps a
          "hard copy" of a such that A keeps what every transfered to a during
          initialization from outside even after the outside object is deleted
          afterwards? Or maybe I should write A(T a){this->a = a;}?
          >
          No, a reference should be fine, and even better is to use a reference
          to const 'T'.
          >
          V
          --
          Please remove capital 'A's when replying by e-mail
          I do not respond to top-posted replies, please don't ask

            Comment

            • Victor Bazarov
              #4
              Re: template re-initialization question

              zl2k wrote:
              [..]
              The problem is, if I declare/define/initialise objects *right before
              you need them* in somewhere, then in some other functions I may not be
              able to use it. I declare it in the header so that the program can use
              it everywhere, with different types defined at run time. Is it
              possible?
              What C++ book are you reading? You seem to misunderstand some fundamental
              concepts of C++. A variable has its type. A variable cannot change its
              type during run-time. It can only change its value.

              V
              --
              Please remove capital 'A's when replying by e-mail
              I do not respond to top-posted replies, please don't ask


                Comment

                Previous template Next
                Working...