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  4. Passing a class to a sprintf() and similar

Passing a class to a sprintf() and similar

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  • C Crazy Joe Devola

    I am trying to pass a class I wrote to sprintf(). Not the class itself actually, but a pointer to a string the class holds. With the VC++ CString class it works fine: sprintf(t, "%s", cst); But with my class, it either crashes the program or puts garbage into the t buffer: sprintf(t, "%s", c); Unless I use casting, and then it works ok: sprintf(t, "%s", (char *)c); What is my class missing? Some kind of an operator? Here is my code:

    class C1
    {
    public:
    C1()
    {
    sprintf(m_Data,"C1 class");
    }

    operator char *() { return m_Data; }

    private:
    char m_Data[100];
    };

    int main(int argc, char* argv[])
    {
    char t[1000]="";
    C1 c;
    CString cst("Test c string");

    sprintf(t, "%s", cst);
    sprintf(t, "%s", c);
    return 0;
    }

    L Offline
    L Offline
    Lost User
    wrote on last edited by
    #8

    OK, I finally figured it out. CString is a bit more of a sophisticated class, and will not use a fixed length buffer for its data. It uses a dynamically allocated one, and as long as that is the first data item in the class definition, the sprintf call will work; although it is luck rather than anything else. You can check this out with the following code:

    class C1
    {
    public:
    C1()
    {
    // allocate space for the text and then copy to the dynamic buffer
    m_Data = (char*)malloc(100);
    sprintf(m_Data,"C1 class");
    }

    operator char \*() { return m\_Data; }

    private:
    char* m_Data; // make buffer a pointer

    };

    When you now use the object without the cast, it just copies the object's data onto the stack, and since this data is a pointer to the actual text it all works as you expect. But be aware that this may have unintended consequences, so always use the cast when you want this behaviour.

    CPalliniC J A C 4 Replies Last reply
    0
    • CPalliniC CPallini

      It is not rubbish, in my test (VS2012). It is pointer to the C1 object, having the same address of its first data member (m_Data) in this simple case, so everything goes right (of course if you insert, before m_Data, another member variable, for instance an int, the sprintf gets some garbage). In the linked MSDN page they indded suggest to explicitely cast the CString object when used as sprintf parameter.

      THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

      L Offline
      L Offline
      Lost User
      wrote on last edited by
      #9

      See my answer to OP below.

      1 Reply Last reply
      0
      • L Lost User

        OK, I finally figured it out. CString is a bit more of a sophisticated class, and will not use a fixed length buffer for its data. It uses a dynamically allocated one, and as long as that is the first data item in the class definition, the sprintf call will work; although it is luck rather than anything else. You can check this out with the following code:

        class C1
        {
        public:
        C1()
        {
        // allocate space for the text and then copy to the dynamic buffer
        m_Data = (char*)malloc(100);
        sprintf(m_Data,"C1 class");
        }

        operator char \*() { return m\_Data; }

        private:
        char* m_Data; // make buffer a pointer

        };

        When you now use the object without the cast, it just copies the object's data onto the stack, and since this data is a pointer to the actual text it all works as you expect. But be aware that this may have unintended consequences, so always use the cast when you want this behaviour.

        CPalliniC Offline
        CPalliniC Offline
        CPallini
        wrote on last edited by
        #10

        However, I don't see the difference on my system (that is it works with m_Data as provided in the OP).

        THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

        In testa che avete, signor di Ceprano?

        L 1 Reply Last reply
        0
        • CPalliniC CPallini

          However, I don't see the difference on my system (that is it works with m_Data as provided in the OP).

          THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

          L Offline
          L Offline
          Lost User
          wrote on last edited by
          #11

          Definitely different for me, it only worked with the modified code.

          CPalliniC 1 Reply Last reply
          0
          • L Lost User

            Definitely different for me, it only worked with the modified code.

            CPalliniC Offline
            CPalliniC Offline
            CPallini
            wrote on last edited by
            #12

            And apparently the 'strange case' is mine.

            THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

            In testa che avete, signor di Ceprano?

            L 1 Reply Last reply
            0
            • CPalliniC CPallini

              And apparently the 'strange case' is mine.

              THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

              L Offline
              L Offline
              Lost User
              wrote on last edited by
              #13

              Well it can't be just you, there must be something else that is not obvious that happens in your compiler. I only managed to work out what was happening on my system by getting the assembler listing from the compiler, so maybe your version of the compiler is different from the two I used.

              1 Reply Last reply
              0
              • L Lost User

                OK, I finally figured it out. CString is a bit more of a sophisticated class, and will not use a fixed length buffer for its data. It uses a dynamically allocated one, and as long as that is the first data item in the class definition, the sprintf call will work; although it is luck rather than anything else. You can check this out with the following code:

                class C1
                {
                public:
                C1()
                {
                // allocate space for the text and then copy to the dynamic buffer
                m_Data = (char*)malloc(100);
                sprintf(m_Data,"C1 class");
                }

                operator char \*() { return m\_Data; }

                private:
                char* m_Data; // make buffer a pointer

                };

                When you now use the object without the cast, it just copies the object's data onto the stack, and since this data is a pointer to the actual text it all works as you expect. But be aware that this may have unintended consequences, so always use the cast when you want this behaviour.

                J Offline
                J Offline
                jschell
                wrote on last edited by
                #14

                Richard MacCutchan wrote:

                although it is luck rather than anything else

                The class has been that way for a very long time because I remember being mystified by exactly the same thing a long, long time ago. I think when I looked at source for the class it was also set up in such a way that there was in fact basically one data member (there was some tricky code going on.) So even if you used it in the middle everything still works. Because of that I wonder if there has been a deliberate effort to insure that behavior (versus luck.)

                1 Reply Last reply
                0
                • L Lost User

                  OK, I finally figured it out. CString is a bit more of a sophisticated class, and will not use a fixed length buffer for its data. It uses a dynamically allocated one, and as long as that is the first data item in the class definition, the sprintf call will work; although it is luck rather than anything else. You can check this out with the following code:

                  class C1
                  {
                  public:
                  C1()
                  {
                  // allocate space for the text and then copy to the dynamic buffer
                  m_Data = (char*)malloc(100);
                  sprintf(m_Data,"C1 class");
                  }

                  operator char \*() { return m\_Data; }

                  private:
                  char* m_Data; // make buffer a pointer

                  };

                  When you now use the object without the cast, it just copies the object's data onto the stack, and since this data is a pointer to the actual text it all works as you expect. But be aware that this may have unintended consequences, so always use the cast when you want this behaviour.

                  A Offline
                  A Offline
                  Albert Holguin
                  wrote on last edited by
                  #15

                  I thought this was done on pupose on CString for this very reason... (not sure if you mean the luck part referring to CString)

                  J 1 Reply Last reply
                  0
                  • L Lost User

                    OK, I finally figured it out. CString is a bit more of a sophisticated class, and will not use a fixed length buffer for its data. It uses a dynamically allocated one, and as long as that is the first data item in the class definition, the sprintf call will work; although it is luck rather than anything else. You can check this out with the following code:

                    class C1
                    {
                    public:
                    C1()
                    {
                    // allocate space for the text and then copy to the dynamic buffer
                    m_Data = (char*)malloc(100);
                    sprintf(m_Data,"C1 class");
                    }

                    operator char \*() { return m\_Data; }

                    private:
                    char* m_Data; // make buffer a pointer

                    };

                    When you now use the object without the cast, it just copies the object's data onto the stack, and since this data is a pointer to the actual text it all works as you expect. But be aware that this may have unintended consequences, so always use the cast when you want this behaviour.

                    C Offline
                    C Offline
                    Crazy Joe Devola
                    wrote on last edited by
                    #16

                    Cool! thank you very much Richard. :thumbsup:

                    1 Reply Last reply
                    0
                    • CPalliniC CPallini

                      You are right, of course. And that is, at the moment, a mystery to me too. :-) However, have a look at this MSDN page: CString::operator LPCTSTR[^]. That's make me guess in your actual code m_Data is not the unique (and not the first) data member.

                      THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

                      B Offline
                      B Offline
                      Bram van Kampen
                      wrote on last edited by
                      #17

                      Well I have noticed, VC 2012 has different parsing and scanning rules than earlier versions. The question is: Did the rules change in the standard, or was it incorrectly implemented by Microsoft. :)

                      Bram van Kampen

                      1 Reply Last reply
                      0
                      • A Albert Holguin

                        I thought this was done on pupose on CString for this very reason... (not sure if you mean the luck part referring to CString)

                        J Offline
                        J Offline
                        jschell
                        wrote on last edited by
                        #18

                        Albert Holguin wrote:

                        I thought this was done on pupose on CString for this very reason... (not sure if you mean the luck part referring to CString)

                        Yes that is what I meant - I was supposing that it was possible that it was planned that way from the start.

                        1 Reply Last reply
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