type casting

[Sukhbir]

what happened if we do't typecast return value of malloc
like that

int *a=malloc(10);

[Dave Sinkula]

Casting malloc

int *a = malloc(10 * sizeof *a);

[Chainsaw]

Most compilers will complain because malloc() returns void* and, in C++, you need to cast void* to the type you are mallocing.

Generally, you should use 'new' and 'delete' rather than malloc, for at least these three good reasons:
1) You don't need to cast
2) As Dave pointed out, malloc(10) allocates 10 BYTES, not 10 ints
3) Constructors are not called. Not a big deal with ints, but a huge deal with objects.

In this case,

int* a = new int[10];
delete [] a;

note the need for [] in the delete; if you used [] in new, use them in delete.

[subtronic]

Using C memory allocation routines in C++ programs is BAD FORM.

[Narue]

>int *a = malloc(10 * sizeof *a);
Most compilers will complain because malloc() returns void* and, in C++, you need to cast void* to the type you are mallocing.

Generally, you should use 'new' and 'delete' rather than malloc, for at least these three good reasons:
1) You don't need to cast
2) As Dave pointed out, malloc(10) allocates 10 BYTES, not 10 ints
3) Constructors are not called. Not a big deal with ints, but a huge deal with objects.

In this case,

int* a = new int[10];
delete [] a;

note the need for [] in the delete; if you used [] in new, use them in delete.
>Using C memory allocation routines in C++ programs is BAD FORM.
I see nothing in the original post that suggests either C++ or C. As the answer is directly related to the language, don't you three think it makes sense to ask before going off on your favorite tangent?

>what happened if we do't typecast return value of malloc
In C, nothing because pointers are implicitly converted to and from void without the need for a cast. In fact, it is best to avoid the cast in C because it hides errors.

In C++, you would get a compile-time error because C++ does not bless implicit pointer conversions to and from void. A cast is required, but it's better to use new in C++ for several reasons that I won't mention because that wasn't your question.