Microsoft KB Archive/104243

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

Article ID: 104243

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft C/C++ Professional Development System 7.0
 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 1.5 Professional Edition
 * Microsoft Visual C++ 1.51
 * Microsoft Visual C++ 1.52 Professional Edition
 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft Visual C++ 4.0 Standard Edition

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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); }

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 KB104243

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