问题
Suppose I have an array of objects of type Foo in C++:
Foo array[10];
In Java, I can set an object in this array to null simply by:
array[0] = null //the first one
How can I do this in C++?
回答1:
Use pointers instead:
Foo *array[10];
// Dynamically allocate the memory for the element in `array[0]`
array[0] = new Foo();
array[1] = new Foo();
...
// Make sure you free the memory before setting
// the array element to point to null
delete array[1];
delete array[0];
// Set the pointer in `array[0]` to point to nullptr
array[1] = nullptr;
array[0] = nullptr;
// Note the above frees the memory allocated for the first element then
// sets its pointer to nullptr. You'll have to do this for the rest of the array
// if you want to set the entire array to nullptr.
Note that you need to consider memory management in C++ because unlike Java, it does not have a Garbage Collector that will automatically clean up memory for you when you set a reference to nullptr. Also, nullptr is the modern and proper C++ way to do it, as rather than always is a pointer type rather than just zero.
回答2:
So, forget Java here, it's not helping you. Ask yourself; what does it mean for an object to be null? Can an object be null? The answer is no, an object can not be null, but a reference (pointer in C++ terms) to one can be.
In java you have reference types, which are similar to pointers under the hood. However, you cannot set objects to null even in java, only references.
In C++ you have fully fledged objects and pointers and references to objects. A pointer to an object (or primitive type) can be null, but an object itself cannot.
So, when you create an array of Foo objects you have, well, an array of Foo objects. You don't have pointers, you have objects. If your array were an array of pointers to objects then yes, you could initialize them to null (nullptr in C++0x), i.e., they don't refer to a valid object.
回答3:
Other way that you have is using one dynamic array of pointers of Foo like this:
Foo** array = new Foo*[10];// Pointer of array of pointers.
Then you can initialize all this pointers to objects Foo
for(int i=0;i<10;i++)
array[i] = new Foo();// Give memory to pointers of object Foo in array
assign null to one item of array:
array[0] = 0;
I think that in C++, NULL is 0. This means it can even be assigned to an int or anywhere a 0 is acceptable. Conventionally, it's only used with pointers.
For delete this memory:
for(int i=0;i<10;i++)
delete array[i];
and finally
delete[] array;
Code Example:
#include <iostream.h>
#include <stdio.h>
class Foo
{
private:
int a;
public:
Foo()
{
a = 0;
}
void Set_V(int p){a = p;}
int Get_V(){return a;}
};
int main()
{
Foo **array = new Foo*[10];
for(int i=0;i<10;i++)
{
array[i] = new Foo();
}
//array[0] = 0;
for(int i=0;i<10;i++)
array[i]->Set_V(i);
for(int i=0;i<10;i++)
cout<<array[i]->Get_V()<<endl;
for(int i=0;i<10;i++)
delete array[i];
delete[] array;
return 0;
}
回答4:
so is there away to "remove" an element in the array without creating a new one? i.e making a hole in the array of some sort?
In C++, if you have an array:
Foo array[10];
Then all 10 elements will be default-constructed. In general, there's no concept of array elements being in use vs. "removed". One way to track this idea is to make having actual values in the array elements optional, as in:
boost::optional<Foo> array[10];
You can read about the boost library's optional<> library at http://www.boost.org/doc/libs/release/libs/optional/
For the sake of documenting alternatives, I'll cover a hideous one. You could call the destructor on one of the elements: array[3].~Foo(); This is very dangerous as when the array itself goes out of scope, the destructor for each element will be called, and a precondition for that is to have a properly constructed object, so you have to be sure to have constructed a Foo in that element again beforehand (using "placement" new). There is nothing in the array itself that will help you keep track of which elements have had their destructors called by you - you'd need to track that yourself (having some record of this inside the objects would probably work in practice, but accessing the object after destruction is undefined behaviour). You'd need to be very careful that in all code paths - including those due to exceptions - you tracked the momentarily unused array elements. You really don't want to do this.
If your concern is to remove an element from the array to save memory, then use smart pointers:
shared_ptr<Foo> array[10];
You can then control specific array elements independently, with only still-populated elements being destructed when the element goes out of scope. Suitable smart pointers are available in boost or C++11, or you could use std::auto_ptr<> but should carefully read about its semantics beforehand to see if it's suitable and how to safely use it.
Separately, if there's a Foo::operator=(Some_Type*) then you could set array[3] = NULL;, and it would do whatever that assignment operator did to array[3]. Still, if Foo is not a pointer, it is probably a bad idea for it to allow someone to assign NULL to it.
来源：https://stackoverflow.com/questions/13114881/set-array-of-object-to-null-in-c