I'm still wondering why in C you can't simply set something to be another thing using plain variables. A variable itself is a pointer to data, is it not? So why make pointers point to the data in the variable when you can simply use the original variable? Is it to access specific bits (or bytes, I guess) of data within said variable?
I'm sure it's logical, however I have never fully grasped the concept and when reading code seeing *pointers
always throws me off.
What are Pointers? A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address.
We usually use pointers simply as a reference to another variable in our programs. Well, for such cases, you can argue that there is no need to expose memory addresses and a programming language will do fine with just references (as Java does) . But, pointers are not there only for referencing existing variables.
Passing pointers to functions in C. Passing an argument by reference or by address enable the passed argument to be changed in the calling function by the called function. C allows a function to return a pointer to the local variable, static variable, and dynamically allocated memory as well.
In the example above, &myAge is also known as a pointer. A pointer is a variable that stores the memory address of another variable as its value. A pointer variable points to a data type (like int) of the same type, and is created with the * operator.
One common place where pointers are helpful is when you are writing functions. Functions take their arguments 'by value', which means that they get a copy of what is passed in and if a function assigns a new value to one of its arguments that will not affect the caller. This means that you couldn't write a "doubling" function like this:
void doubling(int x)
{
x = x * 2;
}
This makes sense because otherwise what would the program do if you called doubling like this:
doubling(5);
Pointers provide a tool for solving this problem because they let you write functions that take the address of a variable, for example:
void doubling2(int *x)
{
(*x) = (*x) * 2;
}
The function above takes the address of an integer as its argument. The one line in the function body dereferences that address twice: on the left-hand side of the equal sign we are storing into that address and on the right-hand side we are getting the integer value from that address and then multiply it by 2. The end result is that the value found at that address is now doubled.
As an aside, when we want to call this new function we can't pass in a literal value (e.g. doubling2(5)
) as it won't compile because we are not properly giving the function an address. One way to give it an address would look like this:
int a = 5;
doubling2(&a);
The end result of this would be that our variable a
would contain 10.
A variable itself is a pointer to data
No, it is not. A variable represents an object, an lvalue. The concept of lvalue is fundamentally different from the concept of a pointer. You seem to be mixing the two.
In C it is not possible to "rebind" an lvalue to make it "point" to a different location in memory. The binding between lvalues and their memory locations is determined and fixed at compile time. It is not always 100% specific (e.g. absolute location of a local variable is not known at compile time), but it is sufficiently specific to make it non-user-adjustable at run time.
The whole idea of a pointer is that its value is generally determined at run time and can be made to point to different memory locations at run time.
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