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发信人: jek (好好学习天天向上), 信区: Program
标  题: C++ Builder常见问题解答(3)
发信站: BBS 荔园晨风站 (Wed Mar 15 12:43:51 2000), 转信

C++ Builder常见问题解答(3)
第三部分  C/C++ Language Issues
1、Sizeof template parameter

Question: BC++ 5.02 and C++Builder do not allow sizeof() to take a
template parameter. How do I get around this?

Answer: The behavior is a result of a bug in BC++ 5.02 and C++Builder.
Borland is aware of the bug. In the interim, the following hack will
get around the problem:
template <class T> class foo {
        struct hack {T t; };
        enum _sz {SIZE = sizeof(t) };
}

2、Calling Pascal from C++Builder

Question: How do I call Pascal routines and variables from C++ modules?

Answer:    An easy way to find out is to let C++Builder do it for you.
Put the Object Pascal declarations you need to translate in the interface
section of an Object {ascal unit.  Now use the command line compiler
DCC32.EXE with the options j, p, h, n, and v.
Below is an example of called DCC32 to translate a file called MyUnit.pas.

DCC32 -jphnv  MyUnit.pas   After the compliler finished you will have
a header file call MyUnit.hpp, open it in an editor and will see the
C++ declarations.

3、Converting AnsiString to numeric variables

Question:    How do you convert AnsiStrings to and from numeric values
in C++Builder?

Answer:    Use the built-in functions StrToInst() and StrToFloat
convert to numeric values and IntToStr() and StrToFloat() for the
reverse process.
int intValue = StrToInt(Edit1->Text); //String to Integer double
double doubleValue = StrToFloat(Edit1->Text);//String to Double
Edit1->Text = IntToStr(IntValue);         // Integer to String
Edit1->Text = FloatToStr(doubleValue);    // Double to String

4、C++ and Pascal
Question: I am C++ programmer and not familiar with pascal.
Can you give me a few pointers that will help?

Answer: Here are some pointers:
- Function ( procedure ) calls don't require parenthesis.
- When you see '.' in pascal, you probably need the '->' in C/C++.
Form2.Show;     (* Opascal *)
Form2->Show();  geBox(0, E->Message.c_str(), "Trapped Exception", MB_OK);
        }
      else
        {
       // This is not the error you are looking for, Raise the exception.
       // ...
        }
      }
  void __fastcall TForm1::FormCreate(TObject *Sender)
  {       Application->OnException = MyException;    }

  12、Why can't I use a member function of an object to create a
  thread

  Question: I'm trying to spin a thread, and I want to use a member
  function as the thread procedure. But I can't get the compiler to
  accept it. Why not?

  Answer: C++ does not allow this behavior.
  Class methods are stored in memory in exactly (1) location.
  The way that a class method accesses other members of a class is via
  a pointer which is silently passed as the first argument to the
  method; windows knows nothing about this pointer, and is unable to
  pass it.   The compiler will not allow you to call a non-static
  member function without an object because it does not know what to
  store in the this pointer; if it did, you would still not be happy
  with the result, because windows would NOT pass a this pointer as
  the first argument.   To get around this, you can:
  (a) use a static method. (b) have a global c-style function which
  calls the method of a particular object by invoking it explicitly,
  eg.   int foo() {    object.method() }   and then pass that to the
  thread-invocation function.

  13、Errno, BC5.02, and C++Builder

  Question: I've created this static library in BC++ 5.0,
  and i'm trying to use it in C++Builder, but i'm getting linker
  errors regarding _errno. Why? Shouldn't this be the same in both
  products?

  Answer: At first glance, it seems like it should.
  But if you look at the header file, you will see that _errno is
  _different_ depending on whether or not _MT is defined. This is
  because the default errno is not thread-safe, and the one provided
  when _MT (which determines whether the compiler should use the
  multi-threaded or non-multi-threaded versions of the RTL) is defined
  is.   The defaults for this compiler define vary between BC++ 5.02
  and BC++Builder. Thus, you cannot link; the symbol is different.
  You need to rebuild your .lib in BC++ 5.02 _with _MT defined._

  14、Fatal General error in .#nn files or assert failures from ilink

  Question: Problems at link time Linker Fatal General error in
  .#nn files or assert failures from ILINK   Problem description:
  At link time Fatal General error in module ..\lib\vcld.#02.
  Sometimes it is vcld.#00 or vcld.#01.   Using the incremental linker
  the error is  Fatal: Assertion failed: recLen == 0 at "I info is turned on.
The problem will show up as general
  failures in .#nn with the standard linker or asserts with the
  incremental linker.   The workaround is to manage your precompiled
  headers with #pragma hdrstop being sure to exclude the header(s)
  with template typedefs from the pch.
  #include <vcl\vcl.h>
  #include "mypch.h"
  // headers that seldom change and no templates
  #pragma hdrstop
  #include "mytemplates.h"
  Note that the problem is really with the external-type OBJ, those .
  #nn files ( -He in the readme ).
  So you could also just turn that switch off. Open the .MAK file and
  add -He- to the CFLAG1 line.

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