if (!haveValue) 里面感叹号什么意思
#include <iostream>
using namespace std;
struct Student
{
int id;
float gpa;
};
template <class T>
class Store
{
public:
Store();
T &getElem();
void putElem(const T &x);
private:
T item;
bool haveValue;
};
template <class T>
Store<T>::Store():haveValue(false){}
template <class T>
T &Store<T>::getElem()
{
if (!haveValue)
{
cout<<"No item present!"<<endl;
exit(1);
}
return item;
}
template <class T>
void Store<T>::putElem(const T &x)
{
haveValue=true;
item=x;
}
int main()
{
Store<int>s1,s2;
s1.putElem(3);
s2.putElem(-7);
cout<<s1.getElem()<<" "<<s2.getElem()<<endl;
Student g={1000,23};
Store<Student>s3;
s3.putElem(g);
cout<<"The student id is "<<s3.getElem().id<<endl;
Store<double>d;
cout<<"Retrieving object d...";
cout<<d.getElem()<<endl;
return 0;
}
if(!haveValue) //非真的情况
//C++ Operators
// Operators specify an evaluation to be performed on one of the following:
// One operand (unary operator)
// Two operands (binary operator)
// Three operands (ternary operator)
// The C++ language includes all C operators and adds several new operators.
// Table 1.1 lists the operators available in Microsoft C++.
// Operators follow a strict precedence which defines the evaluation order of
//expressions containing these operators. Operators associate with either the
//expression on their left or the expression on their right; this is called
//“associativity.” Operators in the same group have equal precedence and are
//evaluated left to right in an expression unless explicitly forced by a pair of
//parentheses, ( ).
// Table 1.1 shows the precedence and associativity of C++ operators
// (from highest to lowest precedence).
//
//Table 1.1 C++ Operator Precedence and Associativity
// The highest precedence level is at the top of the table.
//+------------------+-----------------------------------------+---------------+
//
[解决办法]
Operator
[解决办法]
Name or Meaning
[解决办法]
Associativity
[解决办法]
//+------------------+-----------------------------------------+---------------+
//
[解决办法]
::
[解决办法]
Scope resolution
[解决办法]
None
[解决办法]
//
[解决办法]
::
[解决办法]
Global
[解决办法]
None
[解决办法]
//
[解决办法]
[ ]
[解决办法]
Array subscript
[解决办法]
Left to right
[解决办法]
//
[解决办法]
( )
[解决办法]
Function call
[解决办法]
Left to right
[解决办法]
//
[解决办法]
( )
[解决办法]
Conversion
[解决办法]
None
[解决办法]
//
[解决办法]
.
[解决办法]
Member selection (object)
[解决办法]
Left to right
[解决办法]
//
[解决办法]
->
[解决办法]
Member selection (pointer)
[解决办法]
Left to right
[解决办法]
//
[解决办法]
++
[解决办法]
Postfix increment
[解决办法]
None
[解决办法]
//
[解决办法]
--
[解决办法]
Postfix decrement
[解决办法]
None
[解决办法]
//
[解决办法]
new
[解决办法]
Allocate object
[解决办法]
None
[解决办法]
//
[解决办法]
delete
[解决办法]
Deallocate object
[解决办法]
None
[解决办法]
//
[解决办法]
delete[ ]
[解决办法]
Deallocate object
[解决办法]
None
[解决办法]
//
[解决办法]
++
[解决办法]
Prefix increment
[解决办法]
None
[解决办法]
//
[解决办法]
--
[解决办法]
Prefix decrement
[解决办法]
None
[解决办法]
//
[解决办法]
*
[解决办法]
Dereference
[解决办法]
None
[解决办法]
//
[解决办法]
&
[解决办法]
Address-of
[解决办法]
None
[解决办法]
//
[解决办法]
+
[解决办法]
Unary plus
[解决办法]
None
[解决办法]
//
[解决办法]
-
[解决办法]
Arithmetic negation (unary)
[解决办法]
None
[解决办法]
//
[解决办法]
!
[解决办法]
Logical NOT
[解决办法]
None
[解决办法]
//
[解决办法]
~
[解决办法]
Bitwise complement
[解决办法]
None
[解决办法]
//
[解决办法]
sizeof
[解决办法]
Size of object
[解决办法]
None
[解决办法]
//
[解决办法]
sizeof ( )
[解决办法]
Size of type
[解决办法]
None
[解决办法]
//
[解决办法]
typeid( )
[解决办法]
type name
[解决办法]
None
[解决办法]
//
[解决办法]
(type)
[解决办法]
Type cast (conversion)
[解决办法]
Right to left
[解决办法]
//
[解决办法]
const_cast
[解决办法]
Type cast (conversion)
[解决办法]
None
[解决办法]
//
[解决办法]
dynamic_cast
[解决办法]
Type cast (conversion)
[解决办法]
None
[解决办法]
//
[解决办法]
reinterpret_cast
[解决办法]
Type cast (conversion)
[解决办法]
None
[解决办法]
//
[解决办法]
static_cast
[解决办法]
Type cast (conversion)
[解决办法]
None
[解决办法]
//
[解决办法]
.*
[解决办法]
Apply pointer to class member (objects)
[解决办法]
Left to right
[解决办法]
//
[解决办法]
->*
[解决办法]
Dereference pointer to class member
[解决办法]
Left to right
[解决办法]
//
[解决办法]
*
[解决办法]
Multiplication
[解决办法]
Left to right
[解决办法]
//
[解决办法]
/
[解决办法]
Division
[解决办法]
Left to right
[解决办法]
//
[解决办法]
%
[解决办法]
Remainder (modulus)
------解决方案--------------------
Left to right
[解决办法]
//
[解决办法]
+
[解决办法]
Addition
[解决办法]
Left to right
[解决办法]
//
[解决办法]
-
[解决办法]
Subtraction
[解决办法]
Left to right
[解决办法]
//
[解决办法]
<<
[解决办法]
Left shift
[解决办法]
Left to right
[解决办法]
//
[解决办法]
>>
[解决办法]
Right shift
[解决办法]
Left to right
[解决办法]
//
[解决办法]
<
[解决办法]
Less than
[解决办法]
Left to right
[解决办法]
//
[解决办法]
>
[解决办法]
Greater than
[解决办法]
Left to right
[解决办法]
//
[解决办法]
<=
[解决办法]
Less than or equal to
[解决办法]
Left to right
[解决办法]
//
[解决办法]
>=
[解决办法]
Greater than or equal to
[解决办法]
Left to right
[解决办法]
//
[解决办法]
==
[解决办法]
Equality
[解决办法]
Left to right
[解决办法]
//
[解决办法]
!=
[解决办法]
Inequality
[解决办法]
Left to right
------解决方案--------------------
//
[解决办法]
&
[解决办法]
Bitwise AND
[解决办法]
Left to right
[解决办法]
//
[解决办法]
^
[解决办法]
Bitwise exclusive OR
[解决办法]
Left to right
[解决办法]
//
[解决办法]
[解决办法]
[解决办法]
Bitwise OR
[解决办法]
Left to right
[解决办法]
//
[解决办法]
&&
[解决办法]
Logical AND
[解决办法]
Left to right
[解决办法]
//
[解决办法]
[解决办法]
[解决办法]
Logical OR
[解决办法]
Left to right
[解决办法]
//
[解决办法]
e1?e2:e3
[解决办法]
Conditional
[解决办法]
Right to left
[解决办法]
//
[解决办法]
=
[解决办法]
Assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
*=
[解决办法]
Multiplication assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
/=
[解决办法]
Division assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
%=
[解决办法]
Modulus assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
+=
[解决办法]
Addition assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
-=
[解决办法]
Subtraction assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
<<=
[解决办法]
Left-shift assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
>>=
[解决办法]
Right-shift assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
&=
[解决办法]
Bitwise AND assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
[解决办法]
=
[解决办法]
Bitwise inclusive OR assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
^=
[解决办法]
Bitwise exclusive OR assignment
[解决办法]
Right to left
[解决办法]
//
[解决办法]
,
[解决办法]
Comma
[解决办法]
Left to right
[解决办法]
//+------------------+-----------------------------------------+---------------+
