Q104243: PRB: Unexpected C2440 Error in C++ Typecast

Article: Q104243
Product(s): Microsoft C Compiler
Version(s): 
Operating System(s): 
Keyword(s): kbprb
Last Modified: 22-JUL-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft C/C++ for MS-DOS, version 7.0 
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0 
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SYMPTOMS
========

The following error

  Error C2440 : 'Conversion': cannot convert from 'type1' to 'type2'

is generated when a statement of the form

     TYPE ( identifier ) ( expression );

is used to type cast a function pointer and make a call simultaneously.

CAUSE
=====

This is expected C++ compiler behavior. The sample code below explains the
problem in a more understandable way.

If the following three statements taken from the sample code are compiled

     typedef void (* funcptr) (int);

...

     void f(int){...};

...

     funcptr (f)(1); // Error C2440

the following error message is generated:

  

  error C2440: 'initializing' : cannot convert from 'const int' to
  'void (__cdecl *) (int)'

Although the last statement appears to be typecasting the function "f" to
funcptr, the compiler thinks it is declaring an object of name "f" of type
funcptr and initializing it with the integer constant "1" equivalent to having

     funcptr f = 1;

-or-

     void (*f) (int) = 1;

which makes the error message much more understandable.

RESOLUTION
==========

The way to the achieve the desired typecast is to add another set of
parenthesis, for example

     ((funcptr)f)(1);

which resolves the confusion for the compiler, which thus treats the typecast in
the way it should.

NOTE: Using a C++ style cast, such as (funcptr(f))(1), produces a syntax error.
You must use C-style casting.

Another method is to have a pointer explicitly bind a new name to the function,
for example

     void (*f1)(int) = f;

and use: "

     f1(1);     // OK

" (without the quotation marks)

MORE INFORMATION
================

The sample below demonstrates the error and workarounds.

Sample Code
-----------

  typedef void (*funcptr) (int);

  void f(int);

  void main()
  {
      ((void (*) (int)) f)(1);
      funcptr (f) (1);             // Gives error C2440
      ((funcptr)f)(1);
      void (*f1) (int) = f;
      f1(1);
  }

Additional query words: 1.00 1.50 2.00 2.10 4.00 7.00 8.00 8.00c 9.00 9.10

======================================================================
Keywords          : kbprb 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Issue type        : kbprb

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