Dynamic Memory Allocation

using new and delete

• new(std::nothrow) 可以避免 memory 不夠時 std::bad_alloc exception 被丟出。我們只需要檢查得到的 pointer 是否為 nullptr 即可。

#include <iostream>
class base {
    int* b;
    public:
    base();
    ~base();
};
 
base::base() {
    b = new(std::nothrow) int[10];
}
 
base::~base() {
    delete[] b;
}
 
int main()
{
    base* myBase = new(std::nothrow) base();
    delete myBase;
 
    int* a = new(std::nothrow) int(3);
    delete a;
 
    // terminate called after throwing an instance of 'std::bad_alloc'
    // what():  std::bad_alloc
    // Program terminated with signal: SIGSEGV
    // int* b = new int[100000000000];
    
    int* b = new(std::nothrow) int[100000000000];
    if(b == nullptr) {
        std::cout << "There is no enough memory!\n";
    }
    delete[] b;
 
    return 0;
}

比較

new & delete	malloc() & free()
It is an operator.	It is a library function.
It (de)allocates the memory dynamically.	It create/destroys the memory at runtime.
It should only be used for deallocating the memory allocated either using the new operator or for a NULL pointer.	It should only be used for deallocating the memory allocated either using malloc(), calloc(), realloc() or for a NULL pointer.
This operator calls the destructor before it destroys the allocated memory.	This function only frees the memory from the heap. It does not call the destructor.
It is comparatively slower because it invokes the destructor for the object being deleted before deallocating the memory.	It is faster than delete operator.
new returns appropriate pointer	malloc returns void * and pointer needs to typecast to appropriate type.

#include <iostream>
class base {
    public:
    base();
    ~base();
};
 
base::base() {
    std::cout << "Constructor is called!\n";
}
 
base::~base() {
    std::cout << "Destructor is called!\n";
}
 
int main()
{
    base* myBase = new(std::nothrow) base();
    delete myBase;
 
    base* myBase2 = (base*) malloc(sizeof(base));
    free(myBase2);
 
    return 0;
}

Output:

Constructor is called!
Destructor is called!

可看到使用 malloc 和 free 並不會 call class object 的 constructor 和 destructor。

Constructor and Destructor on the stack

#include <iostream>
class base {
    public:
    base();
    ~base();
};
 
base::base() {
    std::cout << "Constructor is called!\n";
}
 
base::~base() {
    std::cout << "Destructor is called!\n";
}
 
int main()
{
    base myBase;
 
    exit(0);
    // return 0;
}

當使用 exit(0) 時，只有 Constructor is called! 的 output，使用 return 0 時，Destructor is called! 也會被輸出。