PRB: Unexpected C2440 Error in C++ Typecast

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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

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);
}
Lastnosti

ID članka: 104243 – Zadnji pregled: 5. jul. 2005 – Revizija: 1

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