如何使用指针在 C++ 中创建输入法

Question

I want to create an input method that creates two ints with Pointers. It would be nice if you could help me or give me any tips. :)

my method:

void inputTest(int* x, int* y) {
    cin >> x;
    cin >> y;
}

my main:

int *x = 0;
int *y = 0;
cout << "Input: " << endl;


//set input from user x,y with input method 
inputTest(x,y);

Answer 1

First of all, you want to read int s, not int* s, so you need to dereference the pointers:

void inputTest(int* x, int* y) {
    cin >> *x;
    cin >> *y;
}

Then you need to pass valid pointers to the function - yours are null pointers and point nowhere at all.
The best way to do this is to first create two int s and then acquire their locations with the "address-of" operator, & .

int x = 0;
int y = 0;
cout << "Input: " << endl;
inputTest(&x, &y);

Answer 2

I'm new to pointers so I want to try it that way.. :)

Ok, then first lesson: Do not use pointers when you don't have to. Pointers can cause the most nasty bugs that you dont get without them.

Next: Pointers are just pointers. The can point to something. A int* can point to an int . Your pointers do not point to anything meaningful.

To store integer values you need int s somewhere. Having pointers pointing somewhere is not sufficient. Once you have a int , eg int x; then &x will give you a int* namely the address of x ( & is called the address-of operator, but dont get confused, & can have a different meaning, see below). If you have the pointer, int* p = &x; then you can dereference the pointer to get back x: *p = 5; will set the value of x to 5 . Using that you could write

void inputTest(int* x, int* y) {
    std::cin >> *x;    
    std::cin >> *y;
}
int main() {
   int x,y;
   inputTest(&x,&y);
   std::cout << x << " " << y;
}

BUT (would like to make it even more bold, because it really is a big "but"). There is an alternative and this is what you should use here. Pointers as parameters are useful when "not pointing anywhere" is an allowed parameter. For a fucntion that wants to read input from user and store that somewhere an invalid pointer is of little use. Better is to disallow such invalid input and use references:

void inputTest(int& x, int& y) {
    std::cin >> x;    
    std::cin >> y;
}
int main() {
   int x,y;
   inputTest(x,y);
   std::cout << x << " " << y;
}

I feel a bit bad for writing this answer, because when you are completely new to pointers, reading an answer here will not be enough to get a proper understanding. Get a book and read it.

Answer 3

You need to dereference the pointer in order to assign a value to the pointed location in memory.

void inputTest(int* xptr, int* yptr) {
    cin >> *xptr;
    cin >> *yptr;
}

int* x = 0 creates a pointer to location 0 in memory. Instead, we want to allocate memory, and then point to that memory. We can initialize x as int x = 0 and then get a pointer to it by using &x.

int x = 0;
int y = 0;
cout << "Input: " << endl;
inputTest(&x,&y);

You can also use references...

void inputTest(int& x, int& y) {
    cin >> x;
    cin >> y;
}

int x = 0;
int y = 0;
cout << "Input: " << endl;
inputTest(x,y);

Answer 4

You may choose one of two ways:

1. Declare your variables as regular int s and pass their addresses to the function inputTest() which is the recommended way.

2. Declare your variables as pointers to int s and allocate valid memory to them.

Example for approach 1:

int x = 0;
int y = 0;
cout << "Input: " << endl;


//set input from user x,y with input method 
inputTest(&x, &y);

Example for approach 2:

int *x = new int;
int *y = new int;
cout << "Input: " << endl;


//set input from user x,y with input method 
inputTest(x,y);

and don't forget to release/delete the allocated memory when you're done.

Also you should use:

cin >> *x;
cin >> *y;

inside your function.

Answer 5

Solution with pointers

With int *x = 0 you have only declared a pointer which points to nowhere. And pointer shall store only 'pointing' information, not a data itself.

    void inputTest(int* x, int* y) {
        cin >> *x; // dereference a pointer to get to the 'real' variable
        cin >> *y; // this is also a disadventage of pointers, pointer can point to nowhere, 
                   // and your programwould crach in that situation
    }

    int main()
    {
       int x = 0; // declare a normal variable, instead of a pointer
       int y = 0; 

       cout << "Please insert an input: " << endl;

       inputTest(&x, &y); // pass addresses that point to your variables

       cout << "Your input: " << x << " " << y << endl;
    }

Solution with references

You can also achive this same functionality with references. Maybe you will like them.

    void inputTest(int& x, int& y) // mind & instead of *
    {
        cin >> x; // no need of dereferencing,
        cin >> y;
    }

    int main()
    {
       int x = 0; // a normal variable
       int y = 0; 

       cout << "Please insert an input: " << endl;

       inputTest(x, y); // simple pass your variables

       cout << "Your input: " << x << " " << y << endl;
    }