Pointers in C vs C++

Question

Possible Duplicate:
Why does C++ require a cast for malloc() but C doesn’t?

This particular piece of code runs fine in C, but gives compilation error when compiled as a C++ program.

#include<stdio.h>
#include<stdlib.h>
int main(){
    int (*b)[10];
    b = calloc(20, sizeof(int));
    return 0;
}

The error in C++ compilation is:

test.cpp: In function ‘int main()’:
test.cpp:9:28: error: invalid conversion from ‘void*’ to ‘int (*)[10]’ [-fpermissive]

Any idea what might be the reason?

Answer 1

While in C you may cast from/to void pointer to other pointer types implicitly, it is not allowed in c++, and you need to cast it explicitly:

b = (int (*)[10])calloc(20, sizeof(int));

Answer 2

C++ is more strict type checking language than C. So, you need to typecast it manually but in C it's typecasted automatically.

Here calloc returns void* and b is of type int(*)[] hence typecasting is mandatory.

In C++ other type castes are also available you need to keep in mind

<static_cast>
<const_cast>
<reinterpret_cast>
<dynamic_cast>

For more see this When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used?

Answer 3

C is more permissive when it comes to casting, C++ requires you to be explicit when casting. You should be casting it in the C version as well.