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14.7.4 Addition operator

王朝java/jsp·作者佚名  2006-03-10
窄屏简体版  字體: |||超大  

For an operation of the form x + y, binary operator overload resolution (?4.

2.4) is applied to select a

specific operator implementation. The operands are converted to the

parameter types of the selected

operator, and the type of the result is the return type of the operator.

The predefined addition operators are listed below. For numeric and

enumeration types, the predefined

addition operators compute the sum of the two operands. When one or both

operands are of type string,

the predefined addition operators concatenate the string representation of

the operands.

?Integer addition:

int operator +(int x, int y);

uint operator +(uint x, uint y);

long operator +(long x, long y);

ulong operator +(ulong x, ulong y);

In a checked context, if the sum is outside the range of the result type, a

System.OverflowException is thrown. In an unchecked context, overflows are

not reported and

any significant high-order bits outside the range of the result type are

discarded.

?Floating-point addition:

float operator +(float x, float y);

double operator +(double x, double y);

The sum is computed according to the rules of IEC 60559 arithmetic. The

following table lists the

results of all possible combinations of nonzero finite values, zeros,

infinities, and NaN?s. In the table, x

and y are nonzero finite values, and z is the result of x + y. If x and y

have the same magnitude but

opposite signs, z is positive zero. If x + y is too large to represent in

the destination type, z is an

infinity with the same sign as x + y. If x + y is too small to represent in

the destination type, z is a

zero with the same sign as x + y.

y +0 -0 +8 -8 NaN

x z x x +8 -8 NaN

+0 y +0 +0 +8 -8 NaN

-0 y +0 -0 +8 -8 NaN

+8 +8 +8 +8 +8 NaN NaN

-8 -8 -8 -8 NaN -8 NaN

NaN NaN NaN NaN NaN NaN NaN

?Decimal addition:

decimal operator +(decimal x, decimal y);

If the resulting value is too large to represent in the decimal format, a

System.OverflowException

is thrown. The scale of the result, before any rounding, is the larger of

the scales of the two operands.

Decimal addition is equivalent to using the addition operator of type

System.Decimal.

?Enumeration addition. Every enumeration type implicitly provides the

following predefined operators,

where E is the enum type, and U is the underlying type of E:

E operator +(E x, U y);

E operator +(U x, E y);

The operators are evaluated exactly as (E)((U)x + (U)y).

?String concatenation:

string operator +(string x, string y);

string operator +(string x, object y);

string operator +(object x, string y);

The binary + operator performs string concatenation when one or both

operands are of type string. If

an operand of string concatenation is null, an empty string is substituted.

Otherwise, any non-string

argument is converted to its string representation by invoking the virtual

ToString method inherited

from type object. If ToString returns null, an empty string is substituted.

[Example:

using System;

class Test

{

static void Main() {

string s = null;

Console.WriteLine("s = >" + s + "<"); // displays s = ><

int i = 1;

Console.WriteLine("i = " + i); // displays i = 1

float f = 1.2300E+15F;

Console.WriteLine("f = " + f); // displays f = 1.23E+15

decimal d = 2.900m;

Console.WriteLine("d = " + d); // displays d = 2.900

}

}

end example]

The result of the string concatenation operator is a string that consists

of the characters of the left

operand followed by the characters of the right operand. The string

concatenation operator never returns

a null value. A System.OutOfMemoryException may be thrown if there is not

enough memory

available to allocate the resulting string.

?Delegate combination. Every delegate type implicitly provides the

following predefined operator, where

D is the delegate type:

D operator +(D x, D y);

The binary + operator performs delegate combination when both operands are

of some delegate type D.

(If the operands have different delegate types, a compile-time error

occurs.) If the first operand is null,

Chapter 14 Expressions

161

the result of the operation is the value of the second operand (even if

that is also null). Otherwise, if the

second operand is null, then the result of the operation is the value of

the first operand. Otherwise, the

result of the operation is a new delegate instance that, when invoked,

invokes the first operand and then

invokes the second operand. [Note: For examples of delegate combination,

see ?4.7.5 and ?2.3. Since

System.Delegate is not a delegate type, operator + is not defined for it.

end note]

 
 
 
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