Pointers in .NET

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Pointers
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THE FOUNDATION OF C++: THE C SUBSET
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int x, *p; x = 10; p = x; printf("%d", *p); return 0; }
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The call to printf( ) does not print the value of x, which is 10, on the screen It prints some unknown value because the assignment
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p = x;
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is wrong That statement assigns the value 10 to the pointer p However, p is supposed to contain an address, not a value To correct the program, write
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p = &x;
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Another error that sometimes occurs is caused by incorrect assumptions about the placement of variables in memory You can never know where your data will be placed in memory, or if it will be placed there the same way again, or whether each compiler will treat it in the same way For these reasons, making any comparisons between pointers that do not point to a common object may yield unexpected results For example,
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char s[80], y[80]; char *p1, *p2; p1 = s; p2 = y; if(p1 < p2)
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is generally an invalid concept (In very unusual situations, you might use something like this to determine the relative position of the variables But this would be rare) A related error results when you assume that two adjacent arrays may be indexed as one by simply incrementing a pointer across the array boundaries For example,
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int first[10], second[10]; int *p, t;
C++: The Complete Reference
p = first; for(t=0; t<20; ++t)
*p++ = t;
This is not a good way to initialize the arrays first and second with the numbers 0 through 19 Even though it may work on some compilers under certain circumstances, it assumes that both arrays will be placed back to back in memory with first first This may not always be the case The next program illustrates a very dangerous type of bug See if you can find it
/* This program has a bug */ #include <stringh> #include <stdioh> int main(void) { char *p1; char s[80]; p1 = s; do { gets(s);
/* read a string */
/* print the decimal equivalent of each character */ while(*p1) printf(" %d", *p1++); } while(strcmp(s, "done")); return 0; }
This program uses p1 to print the ASCII values associated with the characters contained in s The problem is that p1 is assigned the address of s only once The first time through the loop, p1 points to the first character in s However, the second time through, it continues where it left off because it is not reset to the start of s This next character may be part of the second string, another variable, or a piece of the program! The proper way to write this program is
/* This program is now correct */ #include <stringh>
5:
Pointers
THE FOUNDATION OF C++: THE C SUBSET
#include <stdioh> int main(void) { char *p1; char s[80]; do { p1 = s; gets(s);
/* read a string */
/* print the decimal equivalent of each character */ while(*p1) printf(" %d", *p1++); } while(strcmp(s, "done")); return 0; }
Here, each time the loop iterates, p1 is set to the start of the string In general, you should remember to reinitialize a pointer if it is to be reused The fact that handling pointers incorrectly can cause tricky bugs is no reason to avoid using them Just be careful, and make sure that you know where each pointer is pointing before you use it
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6
Functions
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C++: The Complete Reference
unctions are the building blocks of C and C++ and the place where all program activity occurs This chapter examines their C-like features, including passing arguments, returning values, prototypes, and recursion Part Two discusses the C++-specific features of functions, such as function overloading and reference parameters
The General Form of a Function
The general form of a function is ret-type function-name(parameter list) { body of the function } The ret-type specifies the type of data that the function returns A function may return any type of data except an array The parameter list is a comma-separated list of variable names and their associated types that receive the values of the arguments when the function is called A function may be without parameters, in which case the parameter list is empty However, even if there are no parameters, the parentheses are still required In variable declarations, you can declare many variables to be of a common type by using a comma-separated list of variable names In contrast, all function parameters must be declared individually, each including both the type and name That is, the parameter declaration list for a function takes this general form: f(type varname1, type varname2, , type varnameN) For example, here are correct and incorrect function parameter declarations:
f(int i, int k, int j) /* correct */ f(int i, k, float j) /* incorrect */
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