Creating array of objects on the stack and heap

问题:

Consider the following code:

class myarray {     int i;      public:             myarray(int a) : i(a){ }  } 

How can you create an array of objects of myarray on the stack and how can you create an array of objects on the heap?

回答1:

You can create an array of objects on the stack via:

myarray stackArray[100]; // 100 objects 

And on the heap (or "freestore"):

myarray* heapArray = new myarray[100]; delete [] heapArray; // when you're done 

But it's best not manage memory yourself. Instead, use a std::vector:

#include <vector> std::vector<myarray> bestArray(100); 

A vector is a dynamic array, which (by default) allocates elements from the heap.

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Because your class has no default constructor, to create it on the stack you need to let the compiler know what to pass into the constructor:

myarray stackArray[3] = { 1, 2, 3 }; 

Or with a vector:

// C++11: std::vector<myarray> bestArray{ 1, 2, 3 };  // C++03: std::vector<myarray> bestArray; bestArray.push_back(myarray(1)); bestArray.push_back(myarray(2)); bestArray.push_back(myarray(3)); 

Of course, you could always give it a default constructor:

class myarray {     int i;     public:     myarray(int a = 0) :     i(a)     {} }; 

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回答2:

Since C++11 std::array<T,size> is available for arrays allocated on the stack. It wraps T[size] providing the interface of std::vector, but the most of the methods are constexpr. The downside here is that you never know when you overflow the stack.

std::array<myarray, 3> stack_array; // Size must be declared explicitly.VLAs 

For arrays allocated with heap memory use std::vector<T>. Unless you specify a custom allocator the standard implementation will use heap memory to allocate the array members.

std::vector<myarray> heap_array (3); // Size is optional. 

Note that in both cases a default constructor is required to initialize the array, so you must define

myarray::myarray() { ... } 

There are also options to use C's VLAs or C++'s new, but you should refrain from using them as much as possible, because their usage makes the code prone to segmentation faults and memory leaks.

回答3:

If you create an array of objects of class myarray ( either on stack or on heap) you would have to define a default constructor.

There is no way to pass arguments to the constructor when creating an array of objects.

回答4:

I know how to create object with out of the default constructor, but only on stack:

Suppose you want to create 10 objects for MyArray class with a = 1..10:

MyArray objArray[] = { MyArray[1], MyArray[2]......MyArray[10]} 

No need to call the destructor, because they are created in the stack.

回答5:

#include <stdio.h> class A { public:   A(int a){         printf("\nConstructor Called : %d\n",a);         aM = a;       }     ~A(){      printf("\ndestructor Called : %d\n",aM); } private:   int aM; };  int main() {                                                                                                      A **a = new A*[10];   for (int i = 0;i<10;i++)     a[i] = new A(i+1);     for (int i = 0;i<10;i++)       delete a[i];// = new A(i+1);                                                                                        delete []a; } 

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