c++ - new operator for memory allocation on heap -

i looking @ signature of new operator. is:

void* operator new (std::size_t size) throw (std::bad_alloc); 

but when use operator, never use cast. i.e

 int *arr = new int; 

so, how c++ convert pointer of type void* int* in case. because, malloc returns void* , need explicitly use cast.

there subtle difference in c++ between operator new , new operator. (read on again... ordering important!)

the function operator new c++ analog of c's malloc function. it's raw memory allocator responsibility solely produce block of memory on construct objects. doesn't invoke constructors, because that's not job. usually, not see operator new used directly in c++ code; looks bit weird. example:

void* memory = operator new(137); // allocate @ least 137 bytes 

the new operator keyword responsible allocating memory object , invoking constructor. what's encountered commonly in c++ code. when write

int* myint = new int; 

you using new operator allocate new integer. internally, new operator works this:

1. allocate memory hold requested object using operator new.
2. invoke object constructor, if any. if throws exception, free above memory operator delete, propagate exception.
3. return pointer newly-constructed object.

because new operator , operator new separate, it's possible use new keyword construct objects without allocating memory. example, famous placement new allows build object @ arbitrary memory address in user-provided memory. example:

t* memory = (t*) malloc(sizeof(t)); // allocate raw buffer new (memory) t(); // construct new t in buffer pointed @ 'memory.' 

overloading new operator defining custom operator new function lets use new in way; specify how allocation occurs, , c++ compiler wire new operator.

in case you're curious, delete keyword works in same way. there's deallocation function called operator delete responsible disposing of memory, , delete operator responsible invoking object destructors , freeing memory. however, operator new , operator delete can used outside of these contexts in place of c's malloc , free, example.