In what cases do I use malloc and/or new?

Question

I see in C++ there are multiple ways to allocate and free data and I understand that when you call malloc you should call free and when you use the new operator you should pair with delete and it is a mistake to mix the two (e.g. Calling free() on something that was created with the new operator), but I'm not clear on when I should use malloc/ free and when I should use new/ delete in my real world programs.

If you're a C++ expert, please let me know any rules of thumb or conventions you follow in this regard.

Cordalia

I'd just like to add a reminder that you cannot mix the two styles - that is, you cannot use new to create an object and then call free() on it, nor attempt to delete a block allocated by malloc(). Probably obvious to say it, but nonetheless... – Ninebark

Good answers, all I have to add (that I haven't seen) is that new/delete calls the constructor/destructor for you, malloc/free does not. Just a difference worth mentioning. – Marder

With modern C++, I'm still trying to find a reason to use either. – Vuong

Or use neither and go with std:shared_ptr<T>. Then you don't have to delete at all. – Haphazard

std::unique_ptr<T> should be the default choice instead of std::shared_ptr<T>. – Decay

Answer 1

Unless you are forced to use C, you should never use malloc. Always use new.

If you need a big chunk of data just do something like:

char *pBuffer = new char[1024];

Be careful though this is not correct:

//This is incorrect - may delete only one element, may corrupt the heap, or worse...
delete pBuffer;

Instead you should do this when deleting an array of data:

//This deletes all items in the array
delete[] pBuffer;

The new keyword is the C++ way of doing it, and it will ensure that your type will have its constructor called. The new keyword is also more type-safe whereas malloc is not type-safe at all.

The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data. The new keyword does not have an analogous way like realloc. The realloc function might be able to extend the size of a chunk of memory for you more efficiently.

It is worth mentioning that you cannot mix new/free and malloc/delete.

Note: Some answers in this question are invalid.

int* p_scalar = new int(5);  // Does not create 5 elements, but initializes to 5
int* p_array  = new int[5];  // Creates 5 elements

Rodger

With regard to calling delete foo when you should call delete []foo, some compilers will fix this automagically for you and not leak and other will only delete the first entry and leak. I had a few of these in some code and valgrind will find them for you. – Humber

I verified this many years ago on VC++ with an array of objects and putting a breakpoint in my destructor. Only the first one got deleted. But ya, good point on some compilers catching this for you. – Rodger

I don't think there's any memory leak if you don't use the [] syntax. But, only the destructor for the first array item will be called. – Durazzo

If your compiler doesn't auto convert your delete to delete[] then there is definitely a memory leak. – Rodger

If you do not use the correct delete the result is undefined. It's incorrect. The fact that it might get part of the thing right or work sometimes is just blind luck. – Cabstand

I don't think it will cause heap corruption, it will just see it as an pointer to a single element. – Rodger

See items 16.12, 16.14, and 38.7 in the C++ FAQ: parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.12 parashift.com/c++-faq-lite/compiler-dependencies.html#faq-38.7 – Cabstand

@KPexEA: Even if some compilers might fix your mistakes, it's still wrong to make them in the first place :) Always use delete[] where appropriate. – Valladares

"Unless you are forced to use C, you should never use malloc. Always use new." Why? What is the win here? For objects we need construction, but for memory blocks, you clearly document two ways to make coding mistakes (the more easily caught () vs [] in new and the less easily caught mismatched array vs scaler new and delete). What is the motivation for using new/delete for blocks of raw memory? – Jealousy

type safe means? Can anyone please elaborate? – Errantry

"type safety is the extent to which a programming language discourages or prevents type errors" - wikipedia first line – Banderillero

Do not ever, ever use new[] and delete[], always use a std::vector. – Hibernicism

@DeadMG: If one is creating an array for use by an asynchronous API function, wouldn't new[] be much safer than std::vector? If one uses new[], the only way the pointer would become invalid would be via explicit delete, whereas the memory allocated for an std::vector could invalidated when the vector is resized or leaves scope. (Note that when using new[] one would have to allow for the possibility that one may not be able to call delete if the async method is still pending; if it may be necessary to abandon an async operation, one may have to arrange for delete via callback). – Mistook

What about in the multithreading context with OpenMP for example? Are they equally thread safe? – Bedight

What about realloc()?, there is no renew – Incorporated

Actually never use the new but prefer it before malloc if you really have to use one of them. They both require a manual memory management done by you. Under (almost) all circumstances you have std::vector and smart pointers which free the memory when they are destructed. – Inchoation

realloc is a poor reason to prefer malloc considering almost everyone calls realloc incorrectly. – Driscoll

another poor reason is calloc – Beria

@immibis This is not sufficient? "it will ensure that your type will have their constructor called [...] is also more type safe whereas malloc is not typesafe at all." – Membership

Why malloc is not type safe? – Circum

@PaulCoccoli and the alternative realloc for new initialized quantities is ? – Salvatore

@Salvatore It depends on what those new initialized quantities are. As stated multiple times above, there is no new-like version of realloc. – Driscoll

Would this: int* i = new int{ 5 }; be a better way of initializing it than using round brackets? – Heathheathberry

@scottxiao you can do int *array = (int *)malloc(15);//oops, last int would be corrupted but you won't make this mistake with new. – Terzas

Answer 2

The short answer is: don't use malloc for C++ without a really good reason for doing so. malloc has a number of deficiencies when used with C++, which new was defined to overcome.

Deficiencies fixed by new for C++ code

1. malloc is not typesafe in any meaningful way. In C++ you are required to cast the return from void*. This potentially introduces a lot of problems:

   #include <stdlib.h>
   
   struct foo {
     double d[5];
   }; 
   
   int main() {
     foo *f1 = malloc(1); // error, no cast
     foo *f2 = static_cast<foo*>(malloc(sizeof(foo)));
     foo *f3 = static_cast<foo*>(malloc(1)); // No error, bad
   }
   

2. It's worse than that though. If the type in question is POD (plain old data) then you can semi-sensibly use malloc to allocate memory for it, as f2 does in the first example.

   It's not so obvious though if a type is POD. The fact that it's possible for a given type to change from POD to non-POD with no resulting compiler error and potentially very hard to debug problems is a significant factor. For example if someone (possibly another programmer, during maintenance, much later on were to make a change that caused foo to no longer be POD then no obvious error would appear at compile time as you'd hope, e.g.:

   struct foo {
     double d[5];
     virtual ~foo() { }
   };
   

   would make the malloc of f2 also become bad, without any obvious diagnostics. The example here is trivial, but it's possible to accidentally introduce non-PODness much further away (e.g. in a base class, by adding a non-POD member). If you have C++11/boost you can use is_pod to check that this assumption is correct and produce an error if it's not:

   #include <type_traits>
   #include <stdlib.h>
   
   foo *safe_foo_malloc() {
     static_assert(std::is_pod<foo>::value, "foo must be POD");
     return static_cast<foo*>(malloc(sizeof(foo)));
   }
   

   Although boost is unable to determine if a type is POD without C++11 or some other compiler extensions.

3. malloc returns NULL if allocation fails. new will throw std::bad_alloc. The behaviour of later using a NULL pointer is undefined. An exception has clean semantics when it is thrown and it is thrown from the source of the error. Wrapping malloc with an appropriate test at every call seems tedious and error prone. (You only have to forget once to undo all that good work). An exception can be allowed to propagate to a level where a caller is able to sensibly process it, where as NULL is much harder to pass back meaningfully. We could extend our safe_foo_malloc function to throw an exception or exit the program or call some handler:

   #include <type_traits>
   #include <stdlib.h>
   
   void my_malloc_failed_handler();
   
   foo *safe_foo_malloc() {
     static_assert(std::is_pod<foo>::value, "foo must be POD");
     foo *mem = static_cast<foo*>(malloc(sizeof(foo)));
     if (!mem) {
        my_malloc_failed_handler();
        // or throw ...
     }
     return mem;
   }
   

4. Fundamentally malloc is a C feature and new is a C++ feature. As a result malloc does not play nicely with constructors, it only looks at allocating a chunk of bytes. We could extend our safe_foo_malloc further to use placement new:

   #include <stdlib.h>
   #include <new>
   
   void my_malloc_failed_handler();
   
   foo *safe_foo_malloc() {
     void *mem = malloc(sizeof(foo));
     if (!mem) {
        my_malloc_failed_handler();
        // or throw ...
     }
     return new (mem)foo();
   }
   

5. Our safe_foo_malloc function isn't very generic - ideally we'd want something that can handle any type, not just foo. We can achieve this with templates and variadic templates for non-default constructors:

   #include <functional>
   #include <new>
   #include <stdlib.h>
   
   void my_malloc_failed_handler();
   
   template <typename T>
   struct alloc {
     template <typename ...Args>
     static T *safe_malloc(Args&&... args) {
       void *mem = malloc(sizeof(T));
       if (!mem) {
          my_malloc_failed_handler();
          // or throw ...
       }
       return new (mem)T(std::forward(args)...);
     }
   };
   

   Now though in fixing all the issues we identified so far we've practically reinvented the default new operator. If you're going to use malloc and placement new then you might as well just use new to begin with!

Triple

It's too bad C++ made struct and class mean basically the same thing; I wonder if there would have been any problems with having struct be reserved for PODs and possibly having all class types be presumed to be non-PODs. Any types defined by code which predated the invention of C++ would necessarily be PODs, so I wouldn't think backward compatibility would be an issue there. Are there advantages to having non-PODs types declared as struct rather than class? – Mistook

@Mistook A bit late but as it turns out, making struct and class do almost the same thing was a wonderful design decision that now enables a neat feature called "metaclasses" (from Herb). – Caseworm

@Rakete1111: At first glance, that proposal looks like it preprocesses a version of the language that uses dollar-prefixed keywords like $class. I'm not sure what that has to do with class and struct being synonyms, however. – Mistook

@Mistook The type system would have been bifurcated more. By having class and struct mean effectively the same thing, you can do arbitrary transformations on them ($class) without worrying of making a class a struct and vice-versa. – Caseworm

@Rakete1111: If certain kinds of operations and transformations are safe with some types but not others, having the type directly identify that, and having a compiler reject unsafe operations and transformations, would seem better than having a change to a metaclass which was used in ways that are only suitable for a PODS, get silently changed to a non-PODS. – Mistook

@Mistook I think I wasn't clear, sorry. What I meant was something akin to template<class T> struct X { T t; }; only being valid for when T is a pod. The same can apply to metaclasses, when do you a transformation on the class that might change it's member in a way that makes the class a pod; either accidentally or not. – Caseworm

@Rakete1111: The requirements for something to be a PODS are a bit stricter than the requirements for presence within a struct should be; the biggest requirement should be standard layout. The notion of a struct in C was tied to the notion that each T occupies a range of sizeof (T) consecutive bytes, and a struct is the concatenation of such byte sequences with optional padding, while the notion of "size" is meaningless for class types. – Mistook

Answer 3

From the C++ FQA Lite:

[16.4] Why should I use new instead of trustworthy old malloc()?

FAQ: new/delete call the constructor/destructor; new is type safe, malloc is not; new can be overridden by a class.

FQA: The virtues of new mentioned by the FAQ are not virtues, because constructors, destructors, and operator overloading are garbage (see what happens when you have no garbage collection?), and the type safety issue is really tiny here (normally you have to cast the void* returned by malloc to the right pointer type to assign it to a typed pointer variable, which may be annoying, but far from "unsafe").

Oh, and using trustworthy old malloc makes it possible to use the equally trustworthy & old realloc. Too bad we don't have a shiny new operator renew or something.

Still, new is not bad enough to justify a deviation from the common style used throughout a language, even when the language is C++. In particular, classes with non-trivial constructors will misbehave in fatal ways if you simply malloc the objects. So why not use new throughout the code? People rarely overload operator new, so it probably won't get in your way too much. And if they do overload new, you can always ask them to stop.

Sorry, I just couldn't resist. :)

Unscathed

I cannot take this comment serious as it clearly projects the author's biased against C++. C++ is a language used to create performance oriented software, and a garbage collector could only be detrimental to its objective. I disagree with your entire answer! – Microstructure

@Microstructure You missed the joke. – Vaporize

Answer 4

Always use new in C++. If you need a block of untyped memory, you can use operator new directly:

void *p = operator new(size);
   ...
operator delete(p);

Durazzo

interesting, i always just allocated an array of unsigned char when i need a raw data buffer like this. – Caryl

@Greg - either way works the same. I just like the fact than calling operator new emphasizes that I'm doing a low-level untyped allocation. – Durazzo

The opposite of operator new is operator delete. It is not a well defined action to call delete on an expression with type void*. – Ogbomosho

Answer 5

new vs malloc()

1) new is an operator, while malloc() is a function.

2) new calls constructors, while malloc() does not.

3) new returns exact data type, while malloc() returns void *.

4) new never returns a NULL (will throw on failure) while malloc() returns NULL

5) Reallocation of memory not handled by new while malloc() can

Scheer

Hi , For point 4) , new can be instructed to return NULL on failure. char* ptr = new (std::nothrow) char [323232]; – Behavior

6) new creates from constructor arguments, while malloc uses size. – Arnhem

there is also a new function – Camden

If you were so inclined in C as to reallocate, I would hope that you'd use realloc rather than malloc, and start off with your pointer variable initialised to NULL. If you want a resizable chunk of memory in C++, on the other hand, I would be suggesting std::vector as opposed to realloc... That or a file. – Ream

Answer 6

To answer your question, you should know the difference between malloc and new. The difference is simple:

malloc allocates memory, while new allocates memory AND calls the constructor of the object you're allocating memory for.

So, unless you're restricted to C, you should never use malloc, especially when dealing with C++ objects. That would be a recipe for breaking your program.

Also the difference between free and delete is quite the same. The difference is that delete will call the destructor of your object in addition to freeing memory.

Ironmonger

Answer 7

Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs. Use new and delete (and the [] variants) for everything that you control.

Siderolite

Also notice that well written C library will hide malloc and free internally, this is how the C programmer should work. – Casimiracasimire

@dmckee do you have an example of C++ using c-centric libraries by malloc and free? – Desireah

@Dacav: If a C function will accept a pointer to an object that it will need to keep using after the function returns, and the caller will have no way of knowing when the object is still needed, it would be perfectly reasonable for the function to specify that the pointer must have been created with malloc. Likewise if a function like strdup needs to create an object and return it to a caller, it is perfectly reasonable to specify that the caller must call free on the object when it is no longer needed. How could such functions avoid exposing their use of malloc/free to the caller? – Mistook

@supercat, there's something inherently wrong in having a C function accept a pointer to objects, since C is not aware of objects at all. In general I believe the best approach is having semantic wrappers around allocation/deallocation also in C. It can be still acceptable, but less flexible, if a C library is asking the caller to pre-allocate and/or deallocate memory. If a C function is doing this and claiming ownership on the allocated memory, you are implicitly required to allocate it with malloc. – Casimiracasimire

@Mistook One example of an every-day package which I'm sure everyone has used is libgmp. If you've ever used any open-source encryption or software based on such encryption (which is very likely) then you've probably used an arbitrary precision arithmetic library which needs to grow and shrink its own internal data. This is done using an initialisation function... and then you gotta wonder, how do you use the C code which is libgmp, in C++, without recompiling it in C++? Now with that (the linker) in mind, think about it... why would any sensible person ever put malloc in C++? – Ream

@Sebivor: The C Standard overloads the term "object" for a number of different but related concepts which differ from how it's used in other contexts. – Mistook

I think that Dacav is suggesting that c libraries implementing opaque types ought to offer a facility for initializing the opaque storage so that the details of who those objects are managed internally are completely hidden and the caller doesn't even have to know what memory management scheme is used internally. But however you feel about that prescription, there are many places where the caller is expected to do the allocation (starting with unix FILEs). – Siderolite

Sebivor: I think, though, perhaps you meant your comment for @Decav? – Mistook

[I meant @Dacav] – Mistook

@dmckee: It was Dacav that said that C is not aware of objects at all. Neither the C core language nor the Standard Library has a broad-reaching concept of functions that create persistent storage or entities as part of their operation, other than those whose purpose is to create such entities (e.g. malloc, fopen, etc.); all other functions that operate on such entities expect them to have been created by their caller, either by using functions whose sole purpose was such creation, or by such means as the caller sees fit. – Mistook

@Sebivor: The C Standard overloads the term to describe variables, struct members, union members, array elements, malloc-allocated regions, and pointer targets, which leads to problems in places like e.g. 6.5p7 which limits the cases where an "object" can have its stored value accessed. – Mistook

@Mistook I assume the C++ standard inherits the C standards definition, since it doesn't otherwise seem to have one, and for every C++ compiler I've ever used there's been a compatible C ABI, which includes things like structure layout and internal representation of types. Additionally, the C++ standard actually talks about basic types like arrays of characters as though they are objects. – Ream

Wait, no, C++ does have a definition for "object". It says it plain and clear... "An object is a region of storage." – Ream

@Casimiracasimire From the C++ standard, the section entited The C++ object model, we can see a definition for object: "An object is a region of storage." There's a similar definition in the C standard; char c; in both C and C++, this variable denotes an object. The difference is that some (but not all) objects in C++ are also polymorphic (because C++ is OO, after all). Don't make the mistake of believing only object oriented code can use objects. – Ream

@Sebivor, these messages are quite old, is this comment thread still alive? At any rate, I'm using object-oriented design for C programs as well, so I totally agree with you – Casimiracasimire

@Casimiracasimire Really? If you design a function which requires a specific form of allocation (and forces the rest of the world to use that allocation when called upon), that's technically a violation of dependency inversion principle, one of the five core principles of object oriented design. Not everything you can do in C++ is object oriented, and there isn't a whole lot you can do with pure object oriented constructs. You can't compose logic (sequence, selection and iteration) without (typically) procedural or functional constructs. OO is just a more specific form of one of those with extensions. – Ream

Answer 8

There is one big difference between malloc and new. malloc allocates memory. This is fine for C, because in C, a lump of memory is an object.

In C++, if you're not dealing with POD types (which are similar to C types) you must call a constructor on a memory location to actually have an object there. Non-POD types are very common in C++, as many C++ features make an object automatically non-POD.

new allocates memory and creates an object on that memory location. For non-POD types this means calling a constructor.

If you do something like this:

non_pod_type* p = (non_pod_type*) malloc(sizeof *p);

The pointer you obtain cannot be dereferenced because it does not point to an object. You'd need to call a constructor on it before you can use it (and this is done using placement new).

If, on the other hand, you do:

non_pod_type* p = new non_pod_type();

You get a pointer that is always valid, because new created an object.

Even for POD types, there's a significant difference between the two:

pod_type* p = (pod_type*) malloc(sizeof *p);
std::cout << p->foo;

This piece of code would print an unspecified value, because the POD objects created by malloc are not initialised.

With new, you could specify a constructor to call, and thus get a well defined value.

pod_type* p = new pod_type();
std::cout << p->foo; // prints 0

If you really want it, you can use use new to obtain uninitialised POD objects. See this other answer for more information on that.

Another difference is the behaviour upon failure. When it fails to allocate memory, malloc returns a null pointer, while new throws an exception.

The former requires you to test every pointer returned before using it, while the later will always produce valid pointers.

For these reasons, in C++ code you should use new, and not malloc. But even then, you should not use new "in the open", because it acquires resources you need to release later on. When you use new you should pass its result immediately into a resource managing class:

std::unique_ptr<T> p = std::unique_ptr<T>(new T()); // this won't leak

Knop

Answer 9

Dynamic allocation is only required when the life-time of the object should be different than the scope it gets created in (This holds as well for making the scope smaller as larger) and you have a specific reason where storing it by value doesn't work.

For example:

 std::vector<int> *createVector(); // Bad
 std::vector<int> createVector();  // Good

 auto v = new std::vector<int>(); // Bad
 auto result = calculate(/*optional output = */ v);
 auto v = std::vector<int>(); // Good
 auto result = calculate(/*optional output = */ &v);

From C++11 on, we have std::unique_ptr for dealing with allocated memory, which contains the ownership of the allocated memory. std::shared_ptr was created for when you have to share ownership. (you'll need this less than you would expect in a good program)

Creating an instance becomes really easy:

auto instance = std::make_unique<Class>(/*args*/); // C++14
auto instance = std::unique_ptr<Class>(new Class(/*args*/)); // C++11
auto instance = std::make_unique<Class[]>(42); // C++14
auto instance = std::unique_ptr<Class[]>(new Class[](42)); // C++11

C++17 also adds std::optional which can prevent you from requiring memory allocations

auto optInstance = std::optional<Class>{};
if (condition)
    optInstance = Class{};

As soon as 'instance' goes out of scope, the memory gets cleaned up. Transferring ownership is also easy:

 auto vector = std::vector<std::unique_ptr<Interface>>{};
 auto instance = std::make_unique<Class>();
 vector.push_back(std::move(instance)); // std::move -> transfer (most of the time)

So when do you still need new? Almost never from C++11 on. Most of the you use std::make_unique until you get to a point where you hit an API that transfers ownership via raw pointers.

 auto instance = std::make_unique<Class>();
 legacyFunction(instance.release()); // Ownership being transferred

 auto instance = std::unique_ptr<Class>{legacyFunction()}; // Ownership being captured in unique_ptr

In C++98/03, you have to do manual memory management. If you are in this case, try upgrading to a more recent version of the standard. If you are stuck:

 auto instance = new Class(); // Allocate memory
 delete instance;             // Deallocate
 auto instances = new Class[42](); // Allocate memory
 delete[] instances;               // Deallocate

Make sure that you track the ownership correctly to not have any memory leaks! Move semantics don't work yet either.

So, when do we need malloc in C++? The only valid reason would be to allocate memory and initialize it later via placement new.

 auto instanceBlob = std::malloc(sizeof(Class)); // Allocate memory
 auto instance = new(instanceBlob)Class{}; // Initialize via constructor
 instance.~Class(); // Destroy via destructor
 std::free(instanceBlob); // Deallocate the memory

Even though, the above is valid, this can be done via a new-operator as well. std::vector is a good example for this.

Finally, we still have the elephant in the room: C. If you have to work with a C-library where memory gets allocated in the C++ code and freed in the C code (or the other way around), you are forced to use malloc/free.

If you are in this case, forget about virtual functions, member functions, classes ... Only structs with PODs in it are allowed.

Some exceptions to the rules:

• You are writing a standard library with advanced data structures where malloc is appropriate
• You have to allocate big amounts of memory (In memory copy of a 10GB file?)
• You have tooling preventing you to use certain constructs
• You need to store an incomplete type

Sumption

Definite +1! New/delete and malloc/free are for our grandparents. Those are not the suggested means of performing dynamic allocation in modern C++. Objects and containers on the stack, unique_ptr, and shared_ptr are the "proper" ways to handle objects. I have example projects with 100,000's of lines of code with NO malloc or new calls. – Dekaliter

Answer 10

There are a few things which new does that malloc doesn’t:

1. new constructs the object by calling the constructor of that object
2. new doesn’t require typecasting of allocated memory.
3. It doesn’t require an amount of memory to be allocated, rather it requires a number of objects to be constructed.

So, if you use malloc, then you need to do above things explicitly, which is not always practical. Additionally, new can be overloaded but malloc can’t be.

Acrilan

Answer 11

If you work with data that doesn't need construction/destruction and requires reallocations (e.g., a large array of ints), then I believe malloc/free is a good choice as it gives you realloc, which is way faster than new-memcpy-delete (it is on my Linux box, but I guess this may be platform dependent). If you work with C++ objects that are not POD and require construction/destruction, then you must use the new and delete operators.

Anyway, I don't see why you shouldn't use both (provided that you free your malloced memory and delete objects allocated with new) if can take advantage of the speed boost (sometimes a significant one, if you're reallocing large arrays of POD) that realloc can give you.

Unless you need it though, you should stick to new/delete in C++.

Palpate

Answer 12

If you are using C++, try to use new/delete instead of malloc/calloc as they are operators. For malloc/calloc, you need to include another header. Don't mix two different languages in the same code. Their work is similar in every manner, both allocates memory dynamically from heap segment in hash table.

Dike

Answer 13

new will initialise the default values of the struct and correctly links the references in it to itself.

E.g.

struct test_s {
    int some_strange_name = 1;
    int &easy = some_strange_name;
}

So new struct test_s will return an initialised structure with a working reference, while the malloc'ed version has no default values and the intern references aren't initialised.

Simmons

Answer 14

If you have C code you want to port over to C++, you might leave any malloc() calls in it. For any new C++ code, I'd recommend using new instead.

Malvinamalvino

Answer 15

From a lower perspective, new will initialize all the memory before giving the memory whereas malloc will keep the original content of the memory.

Imperfective

new does not in general initialize memory, although there are ways to make that happen: see #2204676 for one discussion about it. – Diller

Answer 16

In the following scenario, we can't use new since it calls constructor.

class  B  {
private:
    B *ptr;
    int x;
public:
    B(int n)  {
        cout<<"B: ctr"<<endl;
        //ptr = new B;  //keep calling ctr, result is segmentation fault
        ptr = (B *)malloc(sizeof(B));
        x = n;
        ptr->x = n + 10;
    }
    ~B()  {
        //delete ptr;
        free(ptr);
        cout<<"B: dtr"<<endl;
    }
};

Contraction

Answer 17

Rare case to consider using malloc/free instead of new/delete is when you're allocating and then reallocating (simple pod types, not objects) using realloc as there is no similar function to realloc in C++ (although this can be done using a more C++ approach).

Sorority

Answer 18

I had played before with few C/C++ applications for computer graphics. After so many time, some things are vanished and I missed them a lot.

The point is, that malloc and new, or free and delete, can work both, especially for certain basic types, which are the most common.

For instance, a char array, can be allocated both with malloc, or new. A main difference is, with new you can instantiate a fixed array size.

char* pWord = new char[5]; // allocation of char array of fixed size 

You cannot use a variable for the size of the array in this case. By the contrary, the malloc function could allow a variable size.

int size = 5; 
char* pWord = (char*)malloc(size); 

In this case, it might be required a conversion cast operator. For the returned type from malloc it's a pointer to void, not char. And sometimes the compiler could not know, how to convert this type.

After allocation the memory block, you can set the variable values. the memset function can be indeed slower for some bigger arrays. But all the bites must be set first to 0, before assigning a value. Because the values of an array could have an arbitrary content.

Suppose, the array is assigned with another array of smaller size. Part of the array element could still have arbitrary content. And a call to a memset function would be recomended in this case.

memset((void*)pWord, 0, sizeof(pWord) / sizeof(char)); 

The allocation functions are available for all C packages. So, these are general functions, that must work for more C types. And the C++ libraries are extensions of the older C libraries. Therefore the malloc function returns a generic void* pointer. The sructures do not have defined a new, or a delete operator. In this case, a custom variable can be allocated with malloc.

The new and delete keywords are actually some defined C operators. Maybe a custom union, or class, can have defined these operators. If new and delete are not defined in a class, these may not work. But if a class is derived from another, which has these operators, the new and delete keywords can have the basic class behavior.

About freeing an array, free can be only used in pair with malloc. Cannot allocate a variable with malloc, and then free with delete.

The simple delete operator references just first item of an array. Because the pWord array can be also written as:

pWord = &pWord[0]; // or *pWord = pWord[0]; 

When an array must be deleted, use the delete[] operator instead:

delete[] pWord; 

Casts are not bad, they just don't work for all the variable types. A conversion cast is also an operator function, that must be defined. If this operator is not defined for a certain type, it may not work. But not all the errors are because of this conversion cast operator.

Also a cast to a void pointer must be used when using a free call. This is because the argument of the free function is a void pointer.

free((void*)pWord); 

Some errors can arise, because the size of the array is too small. But this is another story, it is not because of using the cast.

With kind regards, Adrian Brinas

Stick

Answer 19

The new and delete operators can operate on classes and structures, whereas malloc and free only work with blocks of memory that need to be cast.

Using new/delete will help to improve your code as you will not need to cast allocated memory to the required data structure.

Ariew

Answer 20

malloc() is used to dynamically assign memory in C while the same work is done by new() in c++. So you cannot mix coding conventions of 2 languages. It would be good if you asked for difference between calloc and malloc()

Sergo

You can (but almost always shouldn't) use malloc in C++. – Middle

You also missed the prime point that you should aim to avoid dynamic memory allocation, unless doing so through smart pointers. You are just setting your self up for pain other wise – Adige