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Casting

Posted on 1998-09-15
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Last Modified: 2010-04-02
Why does this work in C, but not C++? When compiled as a .c-file, it works, but not as a C++-file.

Atype A;
A = func();

It says can't cast (void *) to (void struct *)    ... or something like that.
If you can't solve it now, I can give some more hints.
Mike
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Question by:mdoland
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3 Comments
 
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Expert Comment

by:snoegler
ID: 1172719
The error you described says 'cannot cast (void*) to (void struct *)'
So i think Atype is a typedef to a struct pointer, for example
struct A {
.
};
typedef struct A *Atype;

C allows pointer casts with no limits, at least at the standard warning level. That means
you can cast a float* to a char* without a (char*) before the float.
Under C++, this is treated much more critical as under C ... I think because too many errors
resulted of such typecasts.

P.S: In my personal opinion, it's not good to read if you typedef pointers without
some hints ... for example, 'typedef struct A *Atype' -> no one will see that 'Atype' is a pointer
without seeing the typedef line. But 'typedef struct A *pAType' gives some hint that the
'pAType' type refers to a pointer :)
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Accepted Solution

by:
alexo earned 60 total points
ID: 1172720
In C, a "general" pointer (void*) can be IMPLICITLY converted to any type of pointer.  Thus:
    char* p = malloc(5); /* Legal C */

However, C++ requires an EXPLICIT conversion, thus:
    char* p = (char*) malloc(5); // Legal C++
    char* p = static_cast<char*>(malloc(5)); // Better C++

Given the usial declaration: void* malloc(size_t);

But then:
    char* p = new char[5]; // Best C++

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LVL 11

Expert Comment

by:alexo
ID: 1172721
And the autograder hits again!
Thank you *so much* for your consideration!
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