PRB: Unexpected C2440 Error in C++ Typecast

MS-DOS | WINDOWS             | WINDOWS NT

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with:

      - Microsoft C/C++ for MS-DOS, versions 7.0
      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, and
        1.52
      - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1 and
        4.0
  

SYMPTOMS

The following error

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

   TYPE ( identifier ) ( expression );

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
     ...

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

   funcptr f = 1;

   or

   void (*f) (int) = 1;

RESOLUTION

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

   ((funcptr)f)(1);

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;

   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);