作者：I技男 | 来源：互联网 | 2021-12-12 16:53
枚举 枚举是用户定义的类型，其中包含一组称为枚举器的命名的整型常数。
// unscoped enum:
enum [identifier] [: type]
{enum-list};
// scoped enum:
enum [class|struct]
[identifier] [: type]
{enum-list};
// Forward declaration of enumerations (C++11):
enum A : int; // non-scoped enum must have type specified
enum class B; // scoped enum defaults to int
enum class C : short;
参数
namespace CardGame_Scoped
{
enum class Suit { Diamonds, Hearts, Clubs, Spades };
void PlayCard(Suit suit)
{
if (suit == Suit::Clubs) // Enumerator must be qualified by enum type
{ /*...*/}
}
}
namespace CardGame_NonScoped
{
enum Suit { Diamonds, Hearts, Clubs, Spades };
void PlayCard(Suit suit)
{
if (suit == Clubs) // Enumerator is visible without qualification
{ /*...*/
}
}
}
将为枚举中的每个名称分配一个整数值，该值与其在枚举中的顺序相对应。默认情况下，为第一个值分配 0，为下一个值分配 1，以此类推，但你可以显式设置枚举器的值，如下所示：
enum Suit { DiamOnds= 1, Hearts, Clubs, Spades };
为枚举器 Diamonds 分配值 1。后续枚举器接收的值会在前一个枚举器的值的基础上加一（如果没有显式赋值）。在前面的示例中，Hearts 将具有值 2，Clubs 将具有值 3，依此类推。
enum Suit { DiamOnds= 5, Hearts, Clubs = 4, Spades };
Diamonds、Hearts、Clubs 和 Spades 的值分别是 5、6、4 和 5。请注意，5 使用了多次；尽管这并不符合预期，但是允许的。对于区分范围的枚举来说，这些规则是相同的。
int account_num = 135692;
Suit hand;
hand = account_num; // error C2440: '=' : cannot convert from 'int' to 'Suit'
将 int 转换为区分范围或未区分范围的枚举器时，需要强制转换。但是，你可以将区分范围的枚举器提升为整数值，而不进行强制转换。
int account_num = Hearts; //OK if Hearts is in a unscoped enum
按照这种方式使用隐式转换可能导致意外副作用。若要帮助消除与区分范围的枚举相关的编程错误，区分范围的枚举值必须是强类型值。区分范围的枚举器必须由枚举类型名称（标识符）限定，并且无法进行隐式转换，如以下示例所示：
namespace ScopedEnumConversions
{
enum class Suit { Diamonds, Hearts, Clubs, Spades };
void AttemptConversions()
{
Suit hand;
hand = Clubs; // error C2065: 'Clubs' : undeclared identifier
hand = Suit::Clubs; //Correct.
int account_num = 135692;
hand = account_num; // error C2440: '=' : cannot convert from 'int' to 'Suit'
hand = static_cast(account_num); // OK, but probably a bug!!!
account_num = Suit::Hearts; // error C2440: '=' : cannot convert from 'Suit' to 'int'
account_num = static_cast(Suit::Hearts); // OK
}
注意，hand = account_num; 行仍会导致对未区分范围的枚举发生的错误，如前面所示。它可以与显式强制转换一起使用。但是，借助区分范围的枚举，不再允许在没有显式强制转换的情况下在下一条语句 account_num = Suit::Hearts; 中尝试转换。
联合
union 是用户定义的类型，其中所有成员都共享同一个内存位置。 这意味着在任何给定时间，联合都不能包含来自其成员列表的多个对象。 这还意味着无论联合具有多少成员，它始终仅使用足以存储最大成员的内存。
union [name] { member-list };
参数
// declaring_a_union.cpp
union RecordType // Declare a simple union type
{
char ch;
int i;
long l;
float f;
double d;
int *int_ptr;
};
int main()
{
RecordType t;
t.i = 5; // t holds an int
t.f = 7.25 // t now holds a float
}
使用联合
#include "stdafx.h"
#include
using namespace std;
enum class WeatherDataType
{
Temperature, Wind
};
struct TempData
{
int StationId;
time_t time;
double current;
double max;
double min;
};
struct WindData
{
int StationId;
time_t time;
int speed;
short direction;
};
struct Input
{
WeatherDataType type;
union
{
TempData temp;
WindData wind;
};
};
// Functions that are specific to data types
void Process_Temp(TempData t) {}
void Process_Wind(WindData w) {}
// Container for all the data records
queue
在前面的示例中，请注意 Input 结构中的联合没有名称。 这是匿名联合，可以访问其成员，如同它们是结构的直接成员一样。 有关匿名联合的详细信息，请参阅下面一节。
// for MyVariant
#include
#include
#include
// for sample objects and output
#include
#include
#include
using namespace std;
struct A
{
A() = default;
A(int i, const string& str) : num(i), name(str) {}
int num;
string name;
//...
};
struct B
{
B() = default;
B(int i, const string& str) : num(i), name(str) {}
int num;
string name;
vector vec;
// ...
};
enum class Kind { None, A, B, Integer };
#pragma warning (push)
#pragma warning(disable:4624)
class MyVariant
{
public:
MyVariant()
: kind_(Kind::None)
{
}
MyVariant(Kind kind)
: kind_(kind)
{
switch (kind_)
{
case Kind::None:
break;
case Kind::A:
new (&a_) A();
break;
case Kind::B:
new (&b_) B();
break;
case Kind::Integer:
i_ = 0;
break;
default:
_ASSERT(false);
break;
}
}
~MyVariant()
{
switch (kind_)
{
case Kind::None:
break;
case Kind::A:
a_.~A();
break;
case Kind::B:
b_.~B();
break;
case Kind::Integer:
break;
default:
_ASSERT(false);
break;
}
kind_ = Kind::None;
}
MyVariant(const MyVariant& other)
: kind_(other.kind_)
{
switch (kind_)
{
case Kind::None:
break;
case Kind::A:
new (&a_) A(other.a_);
break;
case Kind::B:
new (&b_) B(other.b_);
break;
case Kind::Integer:
i_ = other.i_;
break;
default:
_ASSERT(false);
break;
}
}
MyVariant(MyVariant&& other)
: kind_(other.kind_)
{
switch (kind_)
{
case Kind::None:
break;
case Kind::A:
new (&a_) A(move(other.a_));
break;
case Kind::B:
new (&b_) B(move(other.b_));
break;
case Kind::Integer:
i_ = other.i_;
break;
default:
_ASSERT(false);
break;
}
other.kind_ = Kind::None;
}
MyVariant& operator=(const MyVariant& other)
{
if (&other != this)
{
switch (other.kind_)
{
case Kind::None:
this->~MyVariant();
break;
case Kind::A:
*this = other.a_;
break;
case Kind::B:
*this = other.b_;
break;
case Kind::Integer:
*this = other.i_;
break;
default:
_ASSERT(false);
break;
}
}
return *this;
}
MyVariant& operator=(MyVariant&& other)
{
_ASSERT(this != &other);
switch (other.kind_)
{
case Kind::None:
this->~MyVariant();
break;
case Kind::A:
*this = move(other.a_);
break;
case Kind::B:
*this = move(other.b_);
break;
case Kind::Integer:
*this = other.i_;
break;
default:
_ASSERT(false);
break;
}
other.kind_ = Kind::None;
return *this;
}
MyVariant(const A& a)
: kind_(Kind::A), a_(a)
{
}
MyVariant(A&& a)
: kind_(Kind::A), a_(move(a))
{
}
MyVariant& operator=(const A& a)
{
if (kind_ != Kind::A)
{
this->~MyVariant();
new (this) MyVariant(a);
}
else
{
a_ = a;
}
return *this;
}
MyVariant& operator=(A&& a)
{
if (kind_ != Kind::A)
{
this->~MyVariant();
new (this) MyVariant(move(a));
}
else
{
a_ = move(a);
}
return *this;
}
MyVariant(const B& b)
: kind_(Kind::B), b_(b)
{
}
MyVariant(B&& b)
: kind_(Kind::B), b_(move(b))
{
}
MyVariant& operator=(const B& b)
{
if (kind_ != Kind::B)
{
this->~MyVariant();
new (this) MyVariant(b);
}
else
{
b_ = b;
}
return *this;
}
MyVariant& operator=(B&& b)
{
if (kind_ != Kind::B)
{
this->~MyVariant();
new (this) MyVariant(move(b));
}
else
{
b_ = move(b);
}
return *this;
}
MyVariant(int i)
: kind_(Kind::Integer), i_(i)
{
}
MyVariant& operator=(int i)
{
if (kind_ != Kind::Integer)
{
this->~MyVariant();
new (this) MyVariant(i);
}
else
{
i_ = i;
}
return *this;
}
Kind GetKind() const
{
return kind_;
}
A& GetA()
{
_ASSERT(kind_ == Kind::A);
return a_;
}
const A& GetA() const
{
_ASSERT(kind_ == Kind::A);
return a_;
}
B& GetB()
{
_ASSERT(kind_ == Kind::B);
return b_;
}
const B& GetB() const
{
_ASSERT(kind_ == Kind::B);
return b_;
}
int& GetInteger()
{
_ASSERT(kind_ == Kind::Integer);
return i_;
}
const int& GetInteger() const
{
_ASSERT(kind_ == Kind::Integer);
return i_;
}
private:
Kind kind_;
union
{
A a_;
B b_;
int i_;
};
};
#pragma warning (pop)
int main()
{
A a(1, "Hello from A");
B b(2, "Hello from B");
MyVariant mv_1 = a;
cout <<"mv_1 = a: " <> c;
}
#include
#include
using namespace std;
enum class WeatherDataType
{
Temperature, Wind
};
struct TempData
{
TempData() : StationId(""), time(0), current(0), maxTemp(0), minTemp(0) {}
TempData(string id, time_t t, double cur, double max, double min)
: StationId(id), time(t), current(cur), maxTemp(max), minTemp(0) {}
string StationId;
time_t time = 0;
double current;
double maxTemp;
double minTemp;
};
struct WindData
{
int StationId;
time_t time;
int speed;
short direction;
};
struct Input
{
Input() {}
Input(const Input&) {}
~Input()
{
if (type == WeatherDataType::Temperature)
{
temp.StationId.~string();
}
}
WeatherDataType type;
void SetTemp(const TempData& td)
{
type = WeatherDataType::Temperature;
// must use placement new because of string member!
new(&temp) TempData(td);
}
TempData GetTemp()
{
if (type == WeatherDataType::Temperature)
return temp;
else
throw logic_error("Can't return TempData when Input holds a WindData");
}
void SetWind(WindData wd)
{
// Explicitly delete struct member that has a
// non-trivial constructor
if (type == WeatherDataType::Temperature)
{
temp.StationId.~string();
}
wind = wd; //placement new not required.
}
WindData GetWind()
{
if (type == WeatherDataType::Wind)
{
return wind;
}
else
throw logic_error("Can't return WindData when Input holds a TempData");
}
private:
union
{
TempData temp;
WindData wind;
};
};
联合不能存储引用。 联合不支持继承，因此联合本身不能用作基类、继承自另一个类或具有虚函数。
#include
using namespace std;
union NumericType
{
short iValue;
long lValue;
double dValue;
};
int main()
{
union NumericType Values = { 10 }; // iValue = 10
cout <
NumericType 联合排列在内存中（概念性的），如下图所示。
匿名联合
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