Problem with Copy Constructor and overloaded assignment operator with templates

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  • saxman
    #1

    Problem with Copy Constructor and overloaded assignment operator with templates

    Hi everyone,

    I'm writing a class that inherits from std::vector in order to add
    some additional functionality. I'm having a compiler problem, however,
    when I have a function that returns this class. I've drilled down and
    been able to create a very simple example of the problem. Here's my
    code:

    <code>
    #include <vector>
    #include <iostream>

    using std::vector;
    using std::cout;
    using std::endl;

    template <typename T>
    class B : public std::vector<T>
    {
    public:
    B(){ cout << "In B ctor" << endl; }

    B(B<T&b) { cout << "In B copy ctor" << endl; }

    B<T>& operator=(const B<T&b) { cout << "In B assign." << endl;
    return *this; }

    ~B(){ cout << "In B destructor" << endl; }

    };

    B<inttest()
    {
    B<intreturnVal ;
    return returnVal;
    }

    int main()
    {
    B<intb;
    b = test();
    return 0;
    }
    </code>

    I get the following error when compiling:
    test.cpp: In function `int main()':
    test.cpp:31: error: no matching function for call to
    `B<int>::B(B<in t>)'
    test.cpp:14: note: candidates are: B<T>::B(B<T>&) [with T = int]

    Does anyone know of a solution?

    Some additional information: any of the following code in 'main'
    compiles fine:
    <code>
    B<intb;
    B<intb2 = b;
    return 0;
    </code>

    or

    <code>
    B<intb;
    B<intb2;
    b2 = b;
    return 0;
    </code>

    or

    <code>
    B<intb;
    B<intb2(b);
    return 0;
    </code>

    Any help would be greatly appreciated.

    Tags: None
    • =?iso-8859-1?q?Erik_Wikstr=F6m?=
      #2
      Re: Problem with Copy Constructor and overloaded assignment operator with templates

      On 12 Juni, 16:08, saxman <erll...@gmail. comwrote:
      Hi everyone,
      >
      I'm writing a class that inherits from std::vector in order to add
      some additional functionality. I'm having a compiler problem, however,
      when I have a function that returns this class. I've drilled down and
      been able to create a very simple example of the problem. Here's my
      code:
      >
      <code>
      #include <vector>
      #include <iostream>
      >
      using std::vector;
      using std::cout;
      using std::endl;
      >
      template <typename T>
      class B : public std::vector<T>
      {
      public:
      B(){ cout << "In B ctor" << endl; }
      >
      B(B<T&b) { cout << "In B copy ctor" << endl; }
      >
      B<T>& operator=(const B<T&b) { cout << "In B assign." << endl;
      return *this; }
      >
      ~B(){ cout << "In B destructor" << endl; }
      >
      };
      >
      B<inttest()
      {
      B<intreturnVal ;
      return returnVal;
      >
      }
      >
      int main()
      {
      B<intb;
      b = test();
      The problem is that the compiler converts this to a copy-constructor
      call like this 'B<int>(test()) ', however since your copy-constructor
      takes a reference as a parameter this can not compile (since a
      reference can not bind to the temporary returned by test()). To solve
      this simply change the copy-constructor to 'B(const B<T&b)'. In
      general the parameters to copy-constructors should be const.

      --
      Erik Wikström

        Comment

        • saxman
          #3
          Re: Problem with Copy Constructor and overloaded assignment operator with templates

          On Jun 12, 10:16 am, Erik Wikström <eri...@student .chalmers.sewro te:
          On 12 Juni, 16:08, saxman <erll...@gmail. comwrote:
          >
          >
          >
          Hi everyone,
          >
          I'm writing a class that inherits from std::vector in order to add
          some additional functionality. I'm having a compiler problem, however,
          when I have a function that returns this class. I've drilled down and
          been able to create a very simple example of the problem. Here's my
          code:
          >
          <code>
          #include <vector>
          #include <iostream>
          >
          using std::vector;
          using std::cout;
          using std::endl;
          >
          template <typename T>
          class B : public std::vector<T>
          {
          public:
          B(){ cout << "In B ctor" << endl; }
          >
          B(B<T&b) { cout << "In B copy ctor" << endl; }
          >
          B<T>& operator=(const B<T&b) { cout << "In B assign." << endl;
          return *this; }
          >
          ~B(){ cout << "In B destructor" << endl; }
          >
          };
          >
          B<inttest()
          {
          B<intreturnVal ;
          return returnVal;
          >
          }
          >
          int main()
          {
          B<intb;
          b = test();
          >
          The problem is that the compiler converts this to a copy-constructor
          call like this 'B<int>(test()) ', however since your copy-constructor
          takes a reference as a parameter this can not compile (since a
          reference can not bind to the temporary returned by test()). To solve
          this simply change the copy-constructor to 'B(const B<T&b)'. In
          general the parameters to copy-constructors should be const.
          >
          --
          Erik Wikström
          Thanks for the help and the explanation, that worked perfectly

            Comment

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