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C Programming for Visual Basic Lovers

Jul 19, 2002

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Visual C++ for Visual Basic Developers

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Expressions

Expressions cover assigning or computing values and use a sequence of operators and operands. The operators and operands are combined in such a way as to obtain the desired results.

There are an unlimited number of ways to build expressions, but there are some basic rules that are fairly easy to follow. Most of these rules in C are similar to those in Visual Basic. C has some "shortcut" operators that make statements shorter, like the ++ operator that can increment a variable.

This section covers the operators, precedence, conversions, and type casts. This gives you the building blocks for expressions.

Operators and Precedence

In this section I discuss operators, their uses, and their precedence. This provides much of the information you need in order to successfully use these operators within expressions.

Unary Operators

The unary operators in C include *, &, sizeof, -, ~, !, ++, and --. You've seen and used the indirection (*) and address-of operators (&) earlier in this chapter in the section on pointers:

Indirection operator—This is used to assign a value to the variable that a pointer variable is pointing to.
Address-of operator—This operator is used to assign a variable's address to a pointer. These are demonstrated following:

int x=10;

int *px = &x; //address-of operator to assign pointer

*px = 20; //indirection to assign value

Sizeof operator—This operator provides the amount of storage required to store the item in the operand. This is expressed in bytes. Len in Visual Basic roughly equates to this operator.
Negation operator (-)—This operator does what you might expect: It returns the negative of the operand. Visual Basic has the same operator.
Bitwise not (~)—This operator produces the bitwise complement of the operand. The logical not (!) produces the logical opposite of the operand. Visual Basic uses the NOT keyword for the logical not.
Increment (++)and decrement (--)operators—These operators can be used in a couple ways. When used within an expression, it is significant if these operators are used as prefix operators or suffix operators. Basically they increment or decrement the variable they are affixed to. Used creatively they can save code. The code following shows how to use these operators.
```
x = 0;
```
```
item[++x] = 1; //item[1]
```
```
x = 0;
```
```
item[x++] = 1; //item[0]
```

Using the increment operator as a prefix changes the value before the assignment. Using it as a suffix operator increments the value after the assignment.

Binary Operators

The binary operators include *, /, %, +, -, <<, >>, <, >, <=, >=, ==, !, =, &, |, ^, &&, ||, and ,; only some of these have counterparts in Visual Basic. And unlike VB, C also allows a combination of the assignment operator and other binary operators.

The multiplicative operators—the multiplication (*), division (/), and remainder (%) —all have counterparts in Visual Basic. Only the remainder operator is different in VB, the Mod operator is used in VB instead of the %. The multiplication and division operators are the same in C and VB. These three operators work pretty much like you'd expect.

The additive operators plus(+) and minus (-) have direct counterparts in Visual Basic and work just as you'd expect.

The bitwise shift operators shift their operand left(<<) or right (>>) by the number of positions specified. Visual Basic can simulate this by multiplying by a number, but the shift still does things a bit differently than a multiplication does and works across the entire unsigned number. Because of signed numbers, bit shifts in Visual Basic are fairly complex issues.

Logical comparison operators are the same as Visual Basic in many cases, but different enough in a couple of cases to be aggravating. The <, >, <=, and >= operators are all the same as VB and work like you would expect them. The equivalent comparison is different (==) in C than in VB (=). Also the not operator in C (!) is different than VB (NOT). A common way of using the not operator (!) in C has a different counterpart in VB: not equal in C is !=, in VB it is <>.

Bitwise operators AND (&), OR (|), and exclusive OR (^) compare to the Visual Basic keywords AND, OR, and XOR. The problem with the Visual Basic operators is that context decides whether they are bit operators or logical operators and this can produce unexpected results in some cases. In addition you have the fact that there are no unsigned types in Visual Basic which hinder the use of bitwise operators.

Logical operators AND (&&) and OR (||) compare to the Visual Basic operators AND and OR. Again in VB, context determines which operation is performed, logical or bitwise.

The == operator is worth further mention. This is an equal comparison operator. In Visual Basic the equal operator does double duty, both as assignment and comparison. Not so in C. This has both good points and bad points and it can be especially troublesome to a VB programmer.

VB programmers are used to using the equal sign for comparison and so typically make the mistake of trying to use it in C. This is a simple thing, but sometimes habit takes over. Many times misuse of this operator will not produce an error, just an unwanted result.

C operators are summarized in Table 3.2.

Table 3.2 C Operators

Operator	Basic Use	Visual Basic
Unary
`*`	Indirection	None
`&`	Address of	`Varptr`
`sizeof`	Size of	`Len(x)`—sort of
`-`	Negation	`-`
`~`	Bitwise not	`NOT`
`!`	Logical not	`NOT`
`++`	Increment	`x = x + 1`
`--`	Decrement	`x = x - 1`
Binary
`*`	Multiplication	`*`
`/`	Division	`/`
`%`	Remainder	`MOD`
`+`	Addition	`+`
`-`	Subtraction	`-`
`<<`	Left bit shift	None
`>>`	Right bit shift	None
`<`	Less than	`<`
`>`	Greater then	`>`
`<=`	Less then or equal	`<=`
`>=`	Greater than or equal	`>=`
`==`	Logical equal	`=`
`!`	Not	`NOT`
`=`	Equal assignment	`=`
`&`	Bitwise AND	`AND`
`\|`	Bitwise inclusive OR	`OR`
`^`	Bitwise exclusive or	`XOR`
`&&`	Logical AND	`AND`
`\|\|`	Logical OR	`OR`
`,`	Sequential evaluation	None
`? :`	Conditional Evaluator	IIF
Assignment
`=`	Simple assignment	`=`
`*=`	Multiplication assignment	`x = x * y`
`/=`	Division assignment	`x = x / y`
`%=`	Remainder assignment	`x = x MOD` y
`+=`	Addition assignment	`x = x +` y
`<<=`	Left shift assignment	None
`>>=`	Right shift assignment	None
`&=`	Bit AND assignment	`x = x AND` y
`\|=`	Bit OR assignment	`x = x OR y`
`^=`	Bit Exclusive OR assignment	`x = x XOR y`

Precedence

Operators listed at the same level have the same precedence. As in Visual Basic, grouping statement segments within parentheses can alter precedence.

Table 3.3 summarizes operator precedence.

Table 3.3 C Operator Precedence

Symbols	Basic Use	Associativity
`[]().-> postfix ++ postfix --`	Expression	Left to Right
`Prefix ++ prefix—sizeof & * - ~ !`	Unary	Right to Left
`Type casts`	Unary	Right to Left
`* / %`	Multiplicative	Left to Right
`+ -`	Additive	Left to Right
`<< >>`	Bitwise shift	Left to Right
`< > <= >=`	Relational	Left to Right
`== !=`	Logical Equality	Left to Right
`&`	Bitwise AND	Left to Right
`^`	Bitwise Exclusive OR	Left to Right
`\|`	Bitwise Inclusive OR	Left to Right
`&&`	Logical-AND	Left to Right
`\|\|`	Logical-OR	Left to Right
`?:`	Conditional-expression	Right to Left
`= *= /= %= += -= <<= >>= &= ^= \|=`	Assignment	Right to Left
`,`	Sequential evaluation	Left to Right

There are subtle problems that can creep into expressions with multiple operators depending on your compiler and the current compilation flags. For example, logical expressions will often "shortcut" the latter part of the expression if the first part is enough to determine the outcome of the logical expression.

Tip

Precedence is difficult to deal with in complex expressions. My advice is to group your expressions so there will be little ambiguity.

Conversions and Type Casts

Conversions and type casts follow a set of rules. These rules involve a couple of definitions that you need to know to understand the conversions involving numeric data:

Sign extend—This rule dictates how smaller integral types are converted to larger ones. The number must be represented in a different manner because the bit that denotes the negativity is in a different spot.
Preserve low order word or byte—This rule describes how a larger integral type is converted to a smaller one. The high order word or byte will be ignored, effectively it is dropped.
Loss of precision—This rule governs what happens when a number is converted to float and the significant digits are larger than 7. It can also happen with a double after 14 digits.

From Signed Integral Types

When converting from signed integral types, the main point to remember in integral conversions is that there is a loss of the upper bytes when going from a larger integral to a smaller one, like a long to a short. If there is information contained in the upper bytes, it will therefore be lost. Converting from a smaller int to a larger one involves no real problems. Converting a signed integer to an unsigned one doesn't change the data as long as they are the same size. It may change the way the bits are interpreted if the signed number is negative, but the in memory representation will remain unchanged as long as they are the same size.

Converting an integral type to a decimal type (float or double) should leave the number unchanged, unless the number is longer than the decimal type's significant digits, in which case a loss of precision will occur. A summary of signed integral conversions is shown in Table 3.4. Also a floating-point representation has gaps in its coverage of all decimal numbers (which is infinite).

Note

Because the long will be converted to a mantissa and an exponent (for example, 1000 becomes 1E+3), you can actually end up with a different number than what you started with. This has nothing to do with loss of precision that results from too few significant digits.

Table 3.4 Signed Integral Conversions

From	To	Method
char	short	Sign-extend
char	long	Sign-extend
char	unsigned char	Preserve pattern; high-order bit loses function as sign bit
char	unsigned short	Sign-extend to short; convert short to unsigned short
char	unsigned long	Sign-extend to long; convert long to unsigned long
char	float	Sign-extend to long; convert long to float
char	double	Sign-extend to long; convert long to double
char	long double	Sign-extend to long; convert long to double
short	char	Preserve low-order byte
short	long	Sign-extend
short	unsigned char	Preserve low-order byte
short	unsigned short	Preserve bit pattern; high-order bit loses function as sign bit
short	unsigned long	Sign-extend to long; convert long to unsigned long
short	float	Sign-extend to long; convert long to float
short	double	Sign-extend to long; convert long to double
short	long double	Sign-extend to long; convert long to double
long	char	Preserve low-order byte
long	short	Preserve low-order word
long	unsigned char	Preserve low-order byte
long	unsigned short	Preserve low-order word
long	unsigned long	Preserve bit pattern; high-order bit loses function as sign bit
long	float	-Represent as float. If long cannot be represented exactly, some precision is lost.
long	double	-Represent as double. If long cannot be represented exactly as a double, some precision is lost.
long	long double	-Represent as double. If long cannot be represented exactly as a double, some precision is lost.

From Unsigned Integral Types

The concerns with unsigned types are the same as with signed integral types. Again, going from unsigned to signed may change the way the bits are interpreted, but in memory it will still be the same if the types are the same size. A summary of unsigned integral conversions is shown in Table 3.5.

Table 3.5 Unsigned Integral Conversions

From	To	Method
unsigned char	char	Preserve bit pattern; high-order bit becomes sign bit
unsigned char	short	Zero-extend
unsigned char	long	Zero-extend
unsigned char	unsigned short	Zero-extend
unsigned char	unsigned long	Zero-extend
unsigned char	float	Convert to long; convert long to float
unsigned char	double	Convert to long; convert long to double
unsigned char	long double	Convert to long; convert long to double
unsigned short	char	Preserve low-order byte
unsigned short	short	Preserve bit pattern; high-order bit becomes sign bit
unsigned short	long	Zero-extend
unsigned short	unsigned char	Preserve low-order byte
unsigned short	unsigned long	Zero-extend
unsigned short	float	Convert to long; convert long to float
unsigned short	double	Convert to long; convert long to double
unsigned short	long double	Convert to long; convert long to double
unsigned long	char	Preserve low-order byte
unsigned long	short	Preserve low-order word
unsigned long	long	Preserve bit pattern; high-order bit becomes sign bit
unsigned long	unsigned char	Preserve low-order byte
unsigned long	unsigned short	Preserve low-order word
unsigned long	float	Convert to long; convert long to float
unsigned long	double	Convert directly to double
unsigned long	long double	Convert to long; convert long to double

From Floating Point Types

Floating point conversions can result in loss of precision or can even be undefined if, for example, the float value is converted to a short and the float value is out of the short's range. A summary of floating point conversions is shown in Table 3.6.

Table 3.6 Floating Point Conversions

From	To	Method
Float	char	Convert to long; convert long to char
Float	short	Convert to long; convert long to short
Float	Llong	Truncate at decimal point. If result is too large to be represented as long, result is undefined.
Float	unsigned short	Convert to long; convert long to unsigned short
Float	unsigned long	Convert to long; convert long to unsigned long
Float	double	Change internal representation
Float	long double	Change internal representation
Double	char	Convert to float; convert float to char
Double	short	Convert to float; convert float to short
Double	long	-Truncate at decimal point. If result is too large to be represented as long, result is undefined.
Double	unsigned short	Convert to long; convert long to unsigned short
Double	unsigned long	Convert to long; convert long to unsigned long
double	float	Represent as a float. If double value cannot be represented exactly as float, loss of precision occurs. If value is too large to be represented as float, the result is undefined.
long double	char	Convert to float; convert float to char
long double	short	Convert to float; convert float to short
long double	long	Truncate at decimal point. If result is too large to be represented as long, result is undefined.
long double	unsigned short	Convert to long; convert long to unsigned short
long double	unsigned long	Convert to long; convert long to unsigned long
long double	float	Represent as a float. If double value cannot be represented exactly as float, loss of precision occurs. If value is too large to be represented as float, the result is undefined.
long double	double	The long double value is treated as double.

To and From Pointers

Pointers can be converted to integral types and back to pointers. Here you simply need to be aware of the size of the pointer and the size of the integral, which may be determined by the memory-addressing model of your operating system (for example, 32 bit for WIN32).

Generally speaking pointers can be converted without loss of data or any consequences. However there can be some alignment issues with some types of pointers, as well as other issues with the data.

Note

The biggest issue in converting a pointer has to do with accessing the data after the pointer is converted. Usually this is why you convert the pointer, so you can access the data. However, there is no protection in C if you want to convert a pointer to a double to a pointer to a long and then try to use the data like it contains long data. This gives erroneous results, but you can do it.

Other Types

Because an enum is an integer, conversions for it work the same as for an integer. For the Microsoft compiler, an integer is the same as a long.

Void types have no value and cannot be converted—only void pointers may be converted to other pointer types with no loss of information.

Type Casts

Type casts are the way you change one variable type to another. Obviously some type casts are allowable and others are not. The conversion rules you saw in the previous sections apply to type casts as well as implicit conversions. The code following shows a few examples of type casts.

//type casts
//int to short
int i=2;
short sval = (short) i;
//long to pointer – common in handling lParam of SendMessage
char *p;
p = (char *)lParam;

Table 3.7 summarizes the allowable type casts.

Table 3.7 Type Cast Conversions

Destination Types	Potential Sources
Integral types	Any integer type or floating-point type, or pointer to an object
Floating-point	Any arithmetic type
A pointer to an object, or (void *)	Any integer type, (void *), a pointer to an object, or a function pointer
Function pointer	Any integral type, a pointer to an object, or a function pointer
A structure, union, or array	None
Void type	Any type

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