What's the equivalent of new/delete of C++ in C?

What's the equivalent of new/delete of C++ in C?

Or it's the s开发者_开发百科ame in C/C++?

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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;

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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);

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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)

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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

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Use malloc / free functions.

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Late, but I really like this syntax, although I'm not sure if it fits ways of C

#include <stdlib.h>

#define new(type, length) malloc(sizeof(type)*(length))
#define delete(x) free(x)
int main()
{
    int *test = new(int, 30);
    delete(test);
}