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iPtr =&iVar; // Let iPtr point to the variable iVar.
In a pointer declaration, the asterisk (*) is part of an individual declarator. We can thus define and initialize the variables iVar and iPtr in one declaration, as follows:
int iVar = 77, *iPtr =&iVar; // Define an int variable and a
// pointer to it.
The second of these two declarations initializes the pointer iPtr with the address of the variable iVar, so that iPtr points to iVar. Figure 4.1. illustrates one possible arrangement of the variables iVar and iPtr in memory. The addresses shown are purely fictitious examples. As Figure 4.1. shows, the value stored in the pointer iPtr is the address of the object iVar.
It is often useful to output addresses for verification and debugging purposes. The printf() functions provide a format specifier for pointers: %p. The following statement prints the address and contents of the variable iPtr:
printf("Value of iPtr (i.e. the address of iVar): %p\n"
"Address of iPtr: %p\n", iPtr,&iPtr);
The size of a pointer in memory given by the expression sizeof(iPtr)
The unary operator&gives the address of an object, so the statement
p =&c;
assigns the address of c to the variable p, and p is said to “point to” c. The&operator only applies to objects in memory: variables and array elements. It cannot be applied to expressions, constants, or register variables.
* is the operator that retrieves the value stored at the address held in the pointer. The indirection operator * yields the location in memory whose address is stored in a pointer. If ptr is a pointer, then *ptr designates the object that ptr points to. Using the indirection operator is sometimes called dereferencing a pointer. The type of the pointer determines the type of object that is assumed to be at that location in memory. For example, when you access a given location using an int pointer, you read or write an object of type int.
The indirection operator * is a unary operator; that is, it has only one operand.ptr points to the variable x. Hence the expression *ptr is equivalent to the variable x itself.
double x, y, *ptr; // Two double variables and a pointer to double.
ptr =&x; // Let ptr point to x.
*ptr = 7.8; // Assign the value 7.8 to the variable x.*ptr *= 2.5; // Multiply x by 2.5.
y = *ptr + 0.5; // Assign y the result of the addition x + 0.5. Do not confuse the asterisk (*) in a pointer declaration with the indirection operator. The syntax of the declaration can be seen as an illustration of how to use the pointer.
double *ptr;
As declared here, ptr has the type double * (read: "pointer to double"). Hence the expression *ptr would have the type double.
Of course, the indirection operator * must be used with only a pointer that contains a valid address. This usage requires careful programming! Without the assignment ptr =&x in the listing above, all of the statements containing *ptr would be senseless dereferencing an undefined pointer value and might well cause the program to crash.
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