使用动态分配的内存（指针）

Question

I was playing around with pointers and dynamic memory as I'm trying to learn C++ and I keep getting this error when I compile.

error C2678: binary '>>' : no operator found which takes a left-hand operand of type 'std::istream' (or there is no acceptable conversion)

My code is as follows:

int * ageP;    
ageP = new (nothrow) int;

if (ageP == 0)
{
    cout << "Error: memory could not be allocated";
}
else
{
    cout<<"What is your age?"<<endl;
    cin>> ageP;                       <--this is the error line
    youDoneIt(ageP);                                            
    delete ageP;
}

Any ideas? Thanks in advance for the help.

Answer 1

You have pointer ageP that points to memory, allocated by this call: ageP = new int; You can access this memory by dereferencing your pointer (ie by using the dereference operator : *ageP ):

  MEMORY
|        |
|--------|
|  ageP  | - - - 
|--------|      |
|  ...   |      |
|--------|      |
| *ageP  | < - -
|--------|
|        |

And then it's same like you would be working with variable of type int , so before when you worked with variable of type int like this:

int age;
cin >> age;

now it will become:

int *ageP = new int;
cin >> *ageP;

Answer 2

The problem is that you need a reference to an int, not an int*. For instance

int ageP;
cin >> ageP;

Thus, the delete is also unnecessary as you won't be utilizing a pointer.

Hope it helps.

Answer 3

John is essentially correct that your problem is supplying a pointer where a reference is expected.

However, since you're trying to learn about dynamic allocation, using an automatic variable is not a good solution. Instead, you can create a reference from a pointer using the * dereference operator.

int* ageP = new (nothrow) int;
std::cout << "What is your age?" << std::endl;
std::cin >> *ageP;                                           
delete ageP;