Related
What is the difference between new/delete and malloc/free?
Related (duplicate?): In what cases do I use malloc vs new?
new / delete
Allocate / release memory
Memory allocated from 'Free Store'.
Returns a fully typed pointer.
new (standard version) never returns a NULL (will throw on failure).
Are called with Type-ID (compiler calculates the size).
Has a version explicitly to handle arrays.
Reallocating (to get more space) not handled intuitively (because of copy constructor).
Whether they call malloc / free is implementation defined.
Can add a new memory allocator to deal with low memory (std::set_new_handler).
operator new / operator delete can be overridden legally.
Constructor / destructor used to initialize / destroy the object.
malloc / free
Allocate / release memory
Memory allocated from 'Heap'.
Returns a void*.
Returns NULL on failure.
Must specify the size required in bytes.
Allocating array requires manual calculation of space.
Reallocating larger chunk of memory simple (no copy constructor to worry about).
They will NOT call new / delete.
No way to splice user code into the allocation sequence to help with low memory.
malloc / free can NOT be overridden legally.
Table comparison of the features:
Feature
new / delete
malloc / free
Memory allocated from
'Free Store'
'Heap'
Returns
Fully typed pointer
void*
On failure
Throws (never returns NULL)
Returns NULL
Required size
Calculated by compiler
Must be specified in bytes
Handling arrays
Has an explicit version
Requires manual calculations
Reallocating
Not handled intuitively
Simple (no copy constructor)
Call of reverse
Implementation defined
No
Low memory cases
Can add a new memory allocator
Not handled by user code
Overridable
Yes
No
Use of constructor / destructor
Yes
No
Technically, memory allocated by new comes from the 'Free Store' while memory allocated by malloc comes from the 'Heap'. Whether these two areas are the same is an implementation detail, which is another reason that malloc and new cannot be mixed.
The most relevant difference is that the new operator allocates memory then calls the constructor, and delete calls the destructor then deallocates the memory.
new calls the ctor of the object, delete call the dtor.
malloc & free just allocate and release raw memory.
new/delete is C++, malloc/free comes from good old C.
In C++, new calls an objects constructor and delete calls the destructor.
malloc and free, coming from the dark ages before OO, only allocate and free the memory, without executing any code of the object.
In C++ new/delete call the Constructor/Destructor accordingly.
malloc/free simply allocate memory from the heap. new/delete allocate memory as well.
The main difference between new and malloc is that new invokes the object's constructor and the corresponding call to delete invokes the object's destructor.
There are other differences:
new is type-safe, malloc returns objects of type void*
new throws an exception on error, malloc returns NULL and sets errno
new is an operator and can be overloaded, malloc is a function and cannot be overloaded
new[], which allocates arrays, is more intuitive and type-safe than malloc
malloc-derived allocations can be resized via realloc, new-derived allocations cannot be resized
malloc can allocate an N-byte chunk of memory, new must be asked to allocate an array of, say, char types
Looking at the differences, a summary is malloc is C-esque, new is C++-esque. Use the one that feels right for your code base.
Although it is legal for new and malloc to be implemented using different memory allocation algorithms, on most systems new is internally implemented using malloc, yielding no system-level difference.
The only similarities are that malloc/new both return a pointer which addresses some memory on the heap, and they both guarantee that once such a block of memory has been returned, it won't be returned again unless you free/delete it first. That is, they both "allocate" memory.
However, new/delete perform arbitrary other work in addition, via constructors, destructors and operator overloading. malloc/free only ever allocate and free memory.
In fact, new is sufficiently customisable that it doesn't necessarily return memory from the heap, or even allocate memory at all. However the default new does.
There are a few things which new does that malloc doesn’t:
new constructs the object by calling the constructor of that object
new doesn’t require typecasting of allocated memory.
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.
In a word, if you use C++, try to use new as much as possible.
also,
the global new and delete can be overridden, malloc/free cannot.
further more new and delete can be overridden per type.
new and delete are C++ primitives which declare a new instance of a class or delete it (thus invoking the destructor of the class for the instance).
malloc and free are C functions and they allocate and free memory blocks (in size).
Both use the heap to make the allocation. malloc and free are nonetheless more "low level" as they just reserve a chunk of memory space which will probably be associated with a pointer. No structures are created around that memory (unless you consider a C array to be a structure).
new is an operator, whereas malloc() is a fucntion.
new returns exact data type, while malloc() returns void * (pointer of type void).
malloc(), memory is not initialized and default value is garbage, whereas in case of new, memory is initialized with default value, like with 'zero (0)' in case on int.
delete and free() both can be used for 'NULL' pointers.
new and delete are operators in c++; which can be overloaded too.
malloc and free are function in c;
malloc returns null ptr when fails while new throws exception.
address returned by malloc need to by type casted again as it returns the (void*)malloc(size)
New return the typed pointer.
To use the malloc(), we need to include <stdlib.h> or
<alloc.h> in the program which is not required for new.
new and delete can be overloaded but malloc can not.
Using the placement new, we can pass the address where we want to
allocate memory but this is not possible in case of malloc.
This code for use of delete keyword or free function. But when create a
pointer object using 'malloc' or 'new' and deallocate object memory using
delete even that object pointer can be call function in the class. After
that use free instead of delete then also it works after free statement ,
but when use both then only pointer object can't call to function in class..
the code is as follows :
#include<iostream>
using namespace std;
class ABC{
public: ABC(){
cout<<"Hello"<<endl;
}
void disp(){
cout<<"Hi\n";
}
};
int main(){
ABC* b=(ABC*)malloc(sizeof(ABC));
int* q = new int[20];
ABC *a=new ABC();
b->disp();
cout<<b<<endl;
free(b);
delete b;
//a=NULL;
b->disp();
ABC();
cout<<b;
return 0;
}
output :
Hello
Hi
0x2abfef37cc20
1.new syntex is simpler than malloc()
2.new/delete is a operator where malloc()/free()
is a function.
3.new/delete execute faster than malloc()/free() because new assemly code directly pasted by the compiler.
4.we can change new/delete meaning in program with the help of operator overlading.
This question already has answers here:
new, delete ,malloc & free
(2 answers)
Closed 8 years ago.
I'm working on a C++ library, one of whose functions returns a (freshly allocated) pointer to an array of doubles. The API states that it is the responsibility of the caller to deallocate the memory.
However, that C++ library used to be implemented in C and the function in question allocates the memory with malloc(). It also assumes that the caller will deallocate that memory with free().
Can I safely replace the call to malloc() with a call to new? Will the existing client code (that uses free() break if I do so? All I could find so far was the official documentation of free(), which states that
If ptr does not point to a block of memory allocated with [malloc, calloc or realloc], it causes undefined behavior.
But I believe this was written before C++ came along with its own allocation operators.
You MUST match calls to malloc with free and new with delete. Mixing/matching them is not an option.
You are not allowed to mix and match malloc and free with new and delete the draft C++ standard refers back to the C99 standard for this and if we go to the draft C++ standard section 20.6.13 C library it says (emphasis mine going forward):
The contents are the same as the Standard C library header stdlib.h, with the following changes:
and:
The functions calloc(), malloc(), and realloc() do not attempt to allocate storage by calling ::operator
new() (18.6).
and:
The function free() does not attempt to deallocate storage by calling ::operator delete().
See also: ISO C Clause 7.11.2.
and includes other changes, none of which state that we can use free on contents allocated with new. So section 7.20.3.2 The free function from the draft C99 standard is still the proper reference and it says:
Otherwise, if the argument does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.
As you've heard now, you can't mix them.
Keep in mind that in C++ it's common to have lots of relatively small temporary objects dynamically allocated (for instance, it's easy to write code like my_string + ' ' + your_string + '\n'), while in C memory allocation's typically more deliberate, often with a larger average allocation size and longer lifetime (much more likely someone would directly malloc(strlen(my_string) + strlen(your_string) + 3) for the result without any temporary buffers). For that reason, some C++ libraries will optimise for large numbers of small transient objects. They might, for instance, use malloc() to get three 16k blocks, then use each for fixed-size requests of up to 16, 32 and 64 bytes respectively. If you call delete in such a situation, it doesn't free anything - it just returns the particular entry in the 16k buffer to a C++-library free list. If you called free() and the pointer happened to be to the first element in the 16k buffer, you'd accidentally deallocate all the elements; if it wasn't to the first you have undefined behaviour (but some implementations like Visual C++ apparently still free blocks given a pointer anywhere inside them).
So - really, really don't do it.
Even if it ostensibly works on your current system, it's a bomb waiting to go off. Different runtime behaviour (based on different inputs, thread race conditions etc.) could cause a later failure. Compilation with different optimisation flags, compiler version, OS etc. could all break it at any time.
The library should really provide a deallocation function that forwards to the correct function.
In addition to what the others already said (no compatibility guarantee), there is also the possibility that the library is linked to a different C library than your program, and so invoking free() on a pointer received from them would pass it to the wrong deallocation function even if the function names are correct.
You must use delete operator to deallocate memory when it is allocated by new operator.
malloc() allocates the memory and sends the address of the first block to the assigned pointer variable,in the case of new it allocates the memory and returns the address .it is a convention that when you use a malloc() function we should use delete function and when you are allocating memory with the help of new function the usage of free() function is comfortable.when malloc()it is a convention that we should use the corresponding realloc(),calloc(),delete() functions and similarly,when you use new() function the corresponding free()function is used.
What is the use of malloc and free when we have new and delete in C++. I guess function of both free and delete is same.
They're not the same. new calls the constructor, malloc just allocates the memory.
Also, it's undefined behavior mixing the two (i.e. using new with free and malloc with delete).
In C++, you're supposed to use new and delete, malloc and free are there for compatibility reasons with C.
In C++, it is rarely useful that one would use malloc & free instead of new& delete.
One Scenario I can think of is:
If you do not want to get your memory initialized by implicit constructor calls, and just need an assured memory allocation for placement new then it is perfectly fine to use malloc and free instead of new and delete.
On the other hand, it is important to know that mallocand new are not same!
Two important differences straight up are:
new guarantees callng of constructors of your class for initializing the class members while mallocdoes not, One would have to do an additional memset or related function calls post an malloc to initialize the allocated memory to do something meaningful.
A big advantage is that for new you do not need to check for NULL after every allocation, just enclosing exception handlers will do the job saving you redundant error checking unlike malloc.
First, when you speak of new and delete, I assume you mean the
expressions, and not the operator new and operator delete functions.
The new and delete expressions are not related to malloc and
free, and only manage memory incidentally; their main role is to
manage object lifetime: a new expression will call the operator new
function to obtain memory, and then call the constructor; a delete
expression will call the destructor before calling operator delete to
free the memory. For the most part, objects should be created, and
not simply allocated, which means using the expressions exclusively.
There are some rare cases where one wants to separate allocation and
initialization (creation); implementing things like std::vector is a
classical example, where you'll allocate for many objects in one go, but
only construct one at a time. In such cases, you'll use the operator
new function for allocation, and placement new for initialization; at
the other end, you'll explicitly call the constructor (something like
p->~T()) for destruction, and use the operator delete function to
free the memory.
Off hand, I can only think of two cases where you'd use malloc and
free in C++. The first is to implement your own replacements of the
::operator new and ::operator delete functions. (I often replace
the global ::operator new and ::operator delete with debugging
versions, which trace allocations, put guard zones around the allocated
memory, etc.) The other is when interacting with a legacy library
written in C: if the library says to pass a pointer to memory allocated
by malloc (because it will free it itself using free), or more
commonly, returns a pointer to memory allocated by malloc, which
you're expected to free, then you must use malloc and free. (The
better libraries will provide their own allocation and deallocation
functions, which do more or less what the new and delete operators
do, but there will always be things like strdup().)
What is the difference between new/delete and malloc/free?
Related (duplicate?): In what cases do I use malloc vs new?
new / delete
Allocate / release memory
Memory allocated from 'Free Store'.
Returns a fully typed pointer.
new (standard version) never returns a NULL (will throw on failure).
Are called with Type-ID (compiler calculates the size).
Has a version explicitly to handle arrays.
Reallocating (to get more space) not handled intuitively (because of copy constructor).
Whether they call malloc / free is implementation defined.
Can add a new memory allocator to deal with low memory (std::set_new_handler).
operator new / operator delete can be overridden legally.
Constructor / destructor used to initialize / destroy the object.
malloc / free
Allocate / release memory
Memory allocated from 'Heap'.
Returns a void*.
Returns NULL on failure.
Must specify the size required in bytes.
Allocating array requires manual calculation of space.
Reallocating larger chunk of memory simple (no copy constructor to worry about).
They will NOT call new / delete.
No way to splice user code into the allocation sequence to help with low memory.
malloc / free can NOT be overridden legally.
Table comparison of the features:
Feature
new / delete
malloc / free
Memory allocated from
'Free Store'
'Heap'
Returns
Fully typed pointer
void*
On failure
Throws (never returns NULL)
Returns NULL
Required size
Calculated by compiler
Must be specified in bytes
Handling arrays
Has an explicit version
Requires manual calculations
Reallocating
Not handled intuitively
Simple (no copy constructor)
Call of reverse
Implementation defined
No
Low memory cases
Can add a new memory allocator
Not handled by user code
Overridable
Yes
No
Use of constructor / destructor
Yes
No
Technically, memory allocated by new comes from the 'Free Store' while memory allocated by malloc comes from the 'Heap'. Whether these two areas are the same is an implementation detail, which is another reason that malloc and new cannot be mixed.
The most relevant difference is that the new operator allocates memory then calls the constructor, and delete calls the destructor then deallocates the memory.
new calls the ctor of the object, delete call the dtor.
malloc & free just allocate and release raw memory.
new/delete is C++, malloc/free comes from good old C.
In C++, new calls an objects constructor and delete calls the destructor.
malloc and free, coming from the dark ages before OO, only allocate and free the memory, without executing any code of the object.
In C++ new/delete call the Constructor/Destructor accordingly.
malloc/free simply allocate memory from the heap. new/delete allocate memory as well.
The main difference between new and malloc is that new invokes the object's constructor and the corresponding call to delete invokes the object's destructor.
There are other differences:
new is type-safe, malloc returns objects of type void*
new throws an exception on error, malloc returns NULL and sets errno
new is an operator and can be overloaded, malloc is a function and cannot be overloaded
new[], which allocates arrays, is more intuitive and type-safe than malloc
malloc-derived allocations can be resized via realloc, new-derived allocations cannot be resized
malloc can allocate an N-byte chunk of memory, new must be asked to allocate an array of, say, char types
Looking at the differences, a summary is malloc is C-esque, new is C++-esque. Use the one that feels right for your code base.
Although it is legal for new and malloc to be implemented using different memory allocation algorithms, on most systems new is internally implemented using malloc, yielding no system-level difference.
The only similarities are that malloc/new both return a pointer which addresses some memory on the heap, and they both guarantee that once such a block of memory has been returned, it won't be returned again unless you free/delete it first. That is, they both "allocate" memory.
However, new/delete perform arbitrary other work in addition, via constructors, destructors and operator overloading. malloc/free only ever allocate and free memory.
In fact, new is sufficiently customisable that it doesn't necessarily return memory from the heap, or even allocate memory at all. However the default new does.
There are a few things which new does that malloc doesn’t:
new constructs the object by calling the constructor of that object
new doesn’t require typecasting of allocated memory.
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.
In a word, if you use C++, try to use new as much as possible.
also,
the global new and delete can be overridden, malloc/free cannot.
further more new and delete can be overridden per type.
new and delete are C++ primitives which declare a new instance of a class or delete it (thus invoking the destructor of the class for the instance).
malloc and free are C functions and they allocate and free memory blocks (in size).
Both use the heap to make the allocation. malloc and free are nonetheless more "low level" as they just reserve a chunk of memory space which will probably be associated with a pointer. No structures are created around that memory (unless you consider a C array to be a structure).
new is an operator, whereas malloc() is a fucntion.
new returns exact data type, while malloc() returns void * (pointer of type void).
malloc(), memory is not initialized and default value is garbage, whereas in case of new, memory is initialized with default value, like with 'zero (0)' in case on int.
delete and free() both can be used for 'NULL' pointers.
new and delete are operators in c++; which can be overloaded too.
malloc and free are function in c;
malloc returns null ptr when fails while new throws exception.
address returned by malloc need to by type casted again as it returns the (void*)malloc(size)
New return the typed pointer.
To use the malloc(), we need to include <stdlib.h> or
<alloc.h> in the program which is not required for new.
new and delete can be overloaded but malloc can not.
Using the placement new, we can pass the address where we want to
allocate memory but this is not possible in case of malloc.
This code for use of delete keyword or free function. But when create a
pointer object using 'malloc' or 'new' and deallocate object memory using
delete even that object pointer can be call function in the class. After
that use free instead of delete then also it works after free statement ,
but when use both then only pointer object can't call to function in class..
the code is as follows :
#include<iostream>
using namespace std;
class ABC{
public: ABC(){
cout<<"Hello"<<endl;
}
void disp(){
cout<<"Hi\n";
}
};
int main(){
ABC* b=(ABC*)malloc(sizeof(ABC));
int* q = new int[20];
ABC *a=new ABC();
b->disp();
cout<<b<<endl;
free(b);
delete b;
//a=NULL;
b->disp();
ABC();
cout<<b;
return 0;
}
output :
Hello
Hi
0x2abfef37cc20
1.new syntex is simpler than malloc()
2.new/delete is a operator where malloc()/free()
is a function.
3.new/delete execute faster than malloc()/free() because new assemly code directly pasted by the compiler.
4.we can change new/delete meaning in program with the help of operator overlading.
What is the difference between new/delete and malloc/free?
Related (duplicate?): In what cases do I use malloc vs new?
new / delete
Allocate / release memory
Memory allocated from 'Free Store'.
Returns a fully typed pointer.
new (standard version) never returns a NULL (will throw on failure).
Are called with Type-ID (compiler calculates the size).
Has a version explicitly to handle arrays.
Reallocating (to get more space) not handled intuitively (because of copy constructor).
Whether they call malloc / free is implementation defined.
Can add a new memory allocator to deal with low memory (std::set_new_handler).
operator new / operator delete can be overridden legally.
Constructor / destructor used to initialize / destroy the object.
malloc / free
Allocate / release memory
Memory allocated from 'Heap'.
Returns a void*.
Returns NULL on failure.
Must specify the size required in bytes.
Allocating array requires manual calculation of space.
Reallocating larger chunk of memory simple (no copy constructor to worry about).
They will NOT call new / delete.
No way to splice user code into the allocation sequence to help with low memory.
malloc / free can NOT be overridden legally.
Table comparison of the features:
Feature
new / delete
malloc / free
Memory allocated from
'Free Store'
'Heap'
Returns
Fully typed pointer
void*
On failure
Throws (never returns NULL)
Returns NULL
Required size
Calculated by compiler
Must be specified in bytes
Handling arrays
Has an explicit version
Requires manual calculations
Reallocating
Not handled intuitively
Simple (no copy constructor)
Call of reverse
Implementation defined
No
Low memory cases
Can add a new memory allocator
Not handled by user code
Overridable
Yes
No
Use of constructor / destructor
Yes
No
Technically, memory allocated by new comes from the 'Free Store' while memory allocated by malloc comes from the 'Heap'. Whether these two areas are the same is an implementation detail, which is another reason that malloc and new cannot be mixed.
The most relevant difference is that the new operator allocates memory then calls the constructor, and delete calls the destructor then deallocates the memory.
new calls the ctor of the object, delete call the dtor.
malloc & free just allocate and release raw memory.
new/delete is C++, malloc/free comes from good old C.
In C++, new calls an objects constructor and delete calls the destructor.
malloc and free, coming from the dark ages before OO, only allocate and free the memory, without executing any code of the object.
In C++ new/delete call the Constructor/Destructor accordingly.
malloc/free simply allocate memory from the heap. new/delete allocate memory as well.
The main difference between new and malloc is that new invokes the object's constructor and the corresponding call to delete invokes the object's destructor.
There are other differences:
new is type-safe, malloc returns objects of type void*
new throws an exception on error, malloc returns NULL and sets errno
new is an operator and can be overloaded, malloc is a function and cannot be overloaded
new[], which allocates arrays, is more intuitive and type-safe than malloc
malloc-derived allocations can be resized via realloc, new-derived allocations cannot be resized
malloc can allocate an N-byte chunk of memory, new must be asked to allocate an array of, say, char types
Looking at the differences, a summary is malloc is C-esque, new is C++-esque. Use the one that feels right for your code base.
Although it is legal for new and malloc to be implemented using different memory allocation algorithms, on most systems new is internally implemented using malloc, yielding no system-level difference.
The only similarities are that malloc/new both return a pointer which addresses some memory on the heap, and they both guarantee that once such a block of memory has been returned, it won't be returned again unless you free/delete it first. That is, they both "allocate" memory.
However, new/delete perform arbitrary other work in addition, via constructors, destructors and operator overloading. malloc/free only ever allocate and free memory.
In fact, new is sufficiently customisable that it doesn't necessarily return memory from the heap, or even allocate memory at all. However the default new does.
There are a few things which new does that malloc doesn’t:
new constructs the object by calling the constructor of that object
new doesn’t require typecasting of allocated memory.
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.
In a word, if you use C++, try to use new as much as possible.
also,
the global new and delete can be overridden, malloc/free cannot.
further more new and delete can be overridden per type.
new and delete are C++ primitives which declare a new instance of a class or delete it (thus invoking the destructor of the class for the instance).
malloc and free are C functions and they allocate and free memory blocks (in size).
Both use the heap to make the allocation. malloc and free are nonetheless more "low level" as they just reserve a chunk of memory space which will probably be associated with a pointer. No structures are created around that memory (unless you consider a C array to be a structure).
new is an operator, whereas malloc() is a fucntion.
new returns exact data type, while malloc() returns void * (pointer of type void).
malloc(), memory is not initialized and default value is garbage, whereas in case of new, memory is initialized with default value, like with 'zero (0)' in case on int.
delete and free() both can be used for 'NULL' pointers.
new and delete are operators in c++; which can be overloaded too.
malloc and free are function in c;
malloc returns null ptr when fails while new throws exception.
address returned by malloc need to by type casted again as it returns the (void*)malloc(size)
New return the typed pointer.
To use the malloc(), we need to include <stdlib.h> or
<alloc.h> in the program which is not required for new.
new and delete can be overloaded but malloc can not.
Using the placement new, we can pass the address where we want to
allocate memory but this is not possible in case of malloc.
This code for use of delete keyword or free function. But when create a
pointer object using 'malloc' or 'new' and deallocate object memory using
delete even that object pointer can be call function in the class. After
that use free instead of delete then also it works after free statement ,
but when use both then only pointer object can't call to function in class..
the code is as follows :
#include<iostream>
using namespace std;
class ABC{
public: ABC(){
cout<<"Hello"<<endl;
}
void disp(){
cout<<"Hi\n";
}
};
int main(){
ABC* b=(ABC*)malloc(sizeof(ABC));
int* q = new int[20];
ABC *a=new ABC();
b->disp();
cout<<b<<endl;
free(b);
delete b;
//a=NULL;
b->disp();
ABC();
cout<<b;
return 0;
}
output :
Hello
Hi
0x2abfef37cc20
1.new syntex is simpler than malloc()
2.new/delete is a operator where malloc()/free()
is a function.
3.new/delete execute faster than malloc()/free() because new assemly code directly pasted by the compiler.
4.we can change new/delete meaning in program with the help of operator overlading.