What's the equivalent of new/delete of C++ in C?
Solution 1
There's no new
/delete
expression in C.
The closest equivalent are the malloc
and free
functions, if you ignore the constructors/destructors and type safety.
#include <stdlib.h>
int* p = malloc(sizeof(*p)); // int* p = new int;
...
free(p); // delete p;
int* a = malloc(12*sizeof(*a)); // int* a = new int[12];
...
free(a); // delete[] a;
Solution 2
Note that constructors might throw exceptions in C++. The equivalent of player* p = new player();
would be something like this in C.
struct player *p = malloc(sizeof *p);
if (!p) handle_out_of_memory();
int err = construct_player(p);
if (err)
{
free(p);
handle_constructor_error();
}
The equivalent of delete p
is simpler, because destructors should never "throw".
destruct(p);
free(p);
Solution 3
Use of new
and delete
in C++ combines two responsibility - allocating/releasing dynamic memory, and initialising/releasing an object.
As all the other answers say, the most common way to allocate and release dynamic memory is calling malloc
and free
. You also can use OS-specific functions to get a large chunk of memory and allocate your objects in that, but that is rarer - only if you have fairly specific requirements that malloc does not satisfy.
In C, most APIs will provide a pair of functions which fulfil the other roles of new
and delete
.
For example, the file api uses a pair of open and close functions:
// C++
fstream* fp = new fstream("c:\\test.txt", "r");
delete fp;
// C
FILE *fp=fopen("c:\\test.txt", "r");
fclose(fp);
It may be that fopen
uses malloc
to allocate the storage for the FILE
struct, or it may statically allocate a table for the maximum number of file pointers on process start. The point is, the API doesn't require the client to use malloc
and free
.
Other APIs provide functions which just perform the initialisation and releasing part of the contract - equivalent to the constructor and destructor, which allows the client code to use either automatic , static or dynamic storage. One example is the pthreads API:
pthread_t thread;
pthread_create( &thread, NULL, thread_function, (void*) param);
This allows the client more flexibility, but increases the coupling between the library and the client - the client needs to know the size of the pthread_t
type, whereas if the library handles both allocation and initialisation the client does not need to know the size of the type, so the implementation can vary without changing the client at all. Neither introduces as much coupling between the client and the implementation as C++ does. (It's often better to think of C++ as a template metaprogramming language with vtables than an OO language)
Solution 4
Not directly an exact replica but compatible equivalents are malloc and free.
<data-type>* variable = (<data-type> *) malloc(memory-size);
free(variable);
No constructors/destructors - C anyway doesn't have them :)
To get the memory-size, you can use sizeof
operator.
If you want to work with multidimensional arrays, you will need to use it multiple times (like new):
int** ptr_to_ptr = (int **) malloc(12 * sizeof(int *)); //assuming an array with length 12.
ptr[0] = (int *) malloc(10 * sizeof(int)); //1st element is an array of 10 items
ptr[1] = (int *) malloc(5 * sizeof(int)); //2nd element an array of 5 elements etc
Solution 5
Use malloc / free functions.
httpinterpret
Updated on February 10, 2020Comments
-
httpinterpret over 4 years
What's the equivalent of new/delete of C++ in C?
Or it's the same in C/C++?
-
Chris Becke about 14 yearsIn C you don't need to cast from void* to other pointers. Its just int* p = malloc( sizeof(int) * cElements);
-
gvaish about 14 yearsChris: Are you sure? I haven't worked with C for quite some time now, but IIRC, I had to do it because the return type of malloc is void *.
-
Scott Wales about 14 yearsIn c
void*
automatically converts to other pointer types. Casting the return of malloc can cause errors if you haven't includedstdlib.h
, as the parameters will have been assumed to beint
. -
stakx - no longer contributing about 14 years@KennyTM: Does
sizeof(*p)
actually dereferencep
, or is it fully equivalent to writingsizeof(int)
? It seems that in the former case, this expression would potentially cause a segmentation fault (becausep
is not yet assigned at this point). In the latter case, I would probably still prefer writingsizeof(int)
because there's less potential for misunderstanding what this statement does. -
kennytm about 14 years@stakx: The
sizeof
operator is evaluated at compile time. There's no dereferencing.sizeof(*p)
is preferred tosizeof(int)
because if you change the type ofp
todouble
the compiler cannot warn you of size mismatch. -
fredoverflow about 14 years@stakx The
sizeof
operator is a mapping from type tosize_t
. The value of its operand is not interesting at all. For example, insizeof(1 + 2)
, there is absolutely no need to compute the result3
. Thesizeof
operator simply sees an expression of typeint + int
and infers that the result is also anint
. Then it mapsint
to 4 (or 2 or 8, depending on the platform). It's the same thing withsizeof(*p)
. The type system knows that, on the type level, dereferencing anint*
yields anint
.sizeof
is not interested in the value of*p
at all, only the type matters. -
fredoverflow about 14 years@stakx For exactly the same reason,
sizeof(1/0)
does NOT crash horribly with an arithmetic exception, but instead yieldssizeof(int)
, because1
is of typeint
,0
is of typeint
, and the division of twoint
s also yields anint
on the type level. The division is never executed, neither at compile time nor at runtime! -
stakx - no longer contributing about 14 yearsThanks to both of you for replying. I thought I'd ask because I haven't seen this usage of
sizeof
before. Good thing to know, though! -
jamesdlin about 14 yearsExplicitly casting from
void*
to other pointer types is necessary in C++ but not in C. -
gvaish about 14 years@Scott, @Jamesdlin: Thanks. It seems I've become rusty in C ;)
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Pacerier over 10 yearsCan you explain exactly what specific requirements malloc does not satisfy?
-
Pete Kirkham over 10 years@Pacerier
malloc
allocates memory.operator new
(usually) allocates memory and initialises the memory to contain values supplied by the constructor of the class of object specified. ( there is a variable ofoperator new
called 'placement new' which does not allocate memory, so you can use malloc for the first part, and new for the second if you so wish) -
Unheilig over 10 years@KennyTM Nice answer. Would like to ask: why is the following considered a more proper way when using sizeof: malloc(sizeof (*p)); note the single space between sizeof and (*p)? Thanks.
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kennytm over 10 years@Unheilig:
sizeof (*p)
andsizeof(*p)
are just equivalent. You could also omit the parenthesis for expressionssizeof *p
. -
Nic over 7 yearsWhat would
construct_player
look like?