PRB: Unexpected C2440 Error in C++ Typecast (104243)

The information in this article applies to:

Microsoft C/C++ for MS-DOS 7.0
Microsoft Visual C++ for Windows, 16-bit edition 1.0
Microsoft Visual C++ for Windows, 16-bit edition 1.5
Microsoft Visual C++ for Windows, 16-bit edition 1.51
Microsoft Visual C++ for Windows, 16-bit edition 1.52
Microsoft Visual C++, 32-bit Editions 1.0
Microsoft Visual C++, 32-bit Editions 2.0
Microsoft Visual C++, 32-bit Editions 2.1
Microsoft Visual C++, 32-bit Editions 4.0

This article was previously published under Q104243

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

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);
}

Modification Type:	Minor	Last Reviewed:	7/5/2005
Keywords:	kbprb KB104243