「演算子ブール値()constは」の意味は何ですか
-
25-09-2019 - |
質問
operator bool() const
{
return col != 0;
}
col
はintです。
どのようにoperator bool() const
仕事?
解決
フォームのメンバ関数
operator TypeName()
変換演算子です。彼らはタイプTypeName
のものであったかのように、クラスタイプのオブジェクトを使用することを可能にし、それらがある場合、それらは、変換関数を使用してTypeName
に変換されます。
は、この特定のケースでは、operator bool()
は、bool
であるかのように、クラスタイプのオブジェクトを使用することを可能にします。あなたはobj
という名前のクラス型のオブジェクトを持っている場合たとえば、あなたは
if (obj)
これは、operator bool()
を呼び出して結果を返す、とif
の条件として結果を使用します。
operator bool()
非常に悪いアイデアであり、あなたはそれを使用しない、本当にんことに留意すべきです。それが悪い理由に、問題を解決するための詳細な説明については、「安全ブールイディオム」を参照してください。
(C ++ 0xを、C ++標準の今後の改正は、明示的な変換演算子のサポートを追加します。これらはあなたが安全なブールイディオムを実装するのフープを介してジャンプすることなく、正常に動作する安全なexplicit operator bool()
を書くことができます。 )
他のヒント
operator bool() const
{
return col != 0;
}
定義するクラスがブール値に変換可能である方法、const
この方法を示すために使用された後()
はのmutate(このクラスのメンバを変更)しない。
次のように、通常、このような演算子を使用することになります:
airplaysdk sdkInstance;
if (sdkInstance) {
std::cout << "Instance is active" << std::endl;
} else {
std::cout << "Instance is in-active error!" << std::endl;
}
私それを明確にするために多くのコードを与えるしたいと思います。
struct A
{
operator bool() const { return true; }
};
struct B
{
explicit operator bool() const { return true; }
};
int main()
{
A a1;
if (a1) cout << "true" << endl; // OK: A::operator bool()
bool na1 = a1; // OK: copy-initialization selects A::operator bool()
bool na2 = static_cast<bool>(a1); // OK: static_cast performs direct-initialization
B b1;
if (b1) cout << "true" << endl; // OK: B::operator bool()
// bool nb1 = b1; // error: copy-initialization does not consider B::operator bool()
bool nb2 = static_cast<bool>(b1); // OK: static_cast performs direct-initialization
}
これはimplicit
またはtrue
のいずれかにあなたのクラスを変換するために、ユーザー定義のfalse
変換機能です。
//usage
bool value = yourclassinstance; //yourclassinstance is converted into bool!
これはbool
への暗黙的な変換です。即ち暗黙的な変換が許可されているところはどこでも、あなたのクラスは、そのメソッドを呼び出すことによってbool
に変換することができます。
は、それがbool
に、この場合には、型変換のためです。たとえばます:
class A {
bool isItSafe;
public:
operator bool() const
{
return isItSafe;
}
...
};
さて、それはboolean型だかのように、私はこのクラスのオブジェクトを使用することができます:
A a;
...
if (a) {
....
}
私自身unique_ptrをを書くとき、私はこのケースを見つけました。与えられた std::unique_ptr
のoperator==
するます:
template<class T1, class D1, class T2, class D2>
bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
template <class T, class D>
bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
template <class T, class D>
bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;
と<のhref = "から、このテストケースhttps://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/testsuite/20_util/unique_ptr/comparison/42925.cc "REL =" nofollowをnoreferrer "> libstdcxxするます:
std::unique_ptr<int> ptr;
if (ptr == 0)
{ }
if (0 == ptr)
{ }
if (ptr != 0)
{ }
if (0 != ptr)
{ }
注ptr
がここに正常に動作しますので、そのexplicit operator bool() const noexcept;
は、operator overload resolution
を持っているので、例えば、ptr == 0
選択します。
template <class T, class D>
bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;`.
それがここにはexplicit
キーワードを持っていない場合は、は、ptr
でptr == 0
はbool
を内蔵し、bool
がint
ですので、その後bool operator==(int, int)
は、0
に変換され、int
に変換されます。どのような私たちを待っていると、あいまいなオーバーロードの解決エラーです。
ここで、最小完全、かつ検証例の
#include <cstddef>
struct A
{
constexpr A(std::nullptr_t) {}
operator bool()
{
return true;
}
};
constexpr bool operator ==(A, A) noexcept
{
return true;
}
constexpr bool operator ==(A, std::nullptr_t) noexcept
{
return true;
}
constexpr bool operator ==(std::nullptr_t, A) noexcept
{
return true;
}
int main()
{
A a1(nullptr);
A a2(0);
a1 == 0;
}
gccをするます:
prog.cc: In function 'int main()':
prog.cc:30:8: error: ambiguous overload for 'operator==' (operand types are 'A' and 'int')
30 | a1 == 0;
| ~~ ^~ ~
| | |
| A int
prog.cc:30:8: note: candidate: 'operator==(int, int)' <built-in>
30 | a1 == 0;
| ~~~^~~~
prog.cc:11:16: note: candidate: 'constexpr bool operator==(A, A)'
11 | constexpr bool operator ==(A, A) noexcept
| ^~~~~~~~
prog.cc:16:16: note: candidate: 'constexpr bool operator==(A, std::nullptr_t)'
16 | constexpr bool operator ==(A, std::nullptr_t) noexcept
| ^~~~~~~~
打ち鳴らすするます:
prog.cc:30:8: error: use of overloaded operator '==' is ambiguous (with operand types 'A' and 'int')
a1 == 0;
~~ ^ ~
prog.cc:16:16: note: candidate function
constexpr bool operator ==(A, std::nullptr_t) noexcept
^
prog.cc:11:16: note: candidate function
constexpr bool operator ==(A, A) noexcept
^
prog.cc:30:8: note: built-in candidate operator==(int, int)
a1 == 0;
^
prog.cc:30:8: note: built-in candidate operator==(float, int)
prog.cc:30:8: note: built-in candidate operator==(double, int)
prog.cc:30:8: note: built-in candidate operator==(long double, int)
prog.cc:30:8: note: built-in candidate operator==(__float128, int)
prog.cc:30:8: note: built-in candidate operator==(int, float)
prog.cc:30:8: note: built-in candidate operator==(int, double)
prog.cc:30:8: note: built-in candidate operator==(int, long double)
prog.cc:30:8: note: built-in candidate operator==(int, __float128)
prog.cc:30:8: note: built-in candidate operator==(int, long)
prog.cc:30:8: note: built-in candidate operator==(int, long long)
prog.cc:30:8: note: built-in candidate operator==(int, __int128)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long, int)
prog.cc:30:8: note: built-in candidate operator==(long long, int)
prog.cc:30:8: note: built-in candidate operator==(__int128, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, int)
prog.cc:30:8: note: built-in candidate operator==(float, float)
prog.cc:30:8: note: built-in candidate operator==(float, double)
prog.cc:30:8: note: built-in candidate operator==(float, long double)
prog.cc:30:8: note: built-in candidate operator==(float, __float128)
prog.cc:30:8: note: built-in candidate operator==(float, long)
prog.cc:30:8: note: built-in candidate operator==(float, long long)
prog.cc:30:8: note: built-in candidate operator==(float, __int128)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(double, float)
prog.cc:30:8: note: built-in candidate operator==(double, double)
prog.cc:30:8: note: built-in candidate operator==(double, long double)
prog.cc:30:8: note: built-in candidate operator==(double, __float128)
prog.cc:30:8: note: built-in candidate operator==(double, long)
prog.cc:30:8: note: built-in candidate operator==(double, long long)
prog.cc:30:8: note: built-in candidate operator==(double, __int128)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long double, float)
prog.cc:30:8: note: built-in candidate operator==(long double, double)
prog.cc:30:8: note: built-in candidate operator==(long double, long double)
prog.cc:30:8: note: built-in candidate operator==(long double, __float128)
prog.cc:30:8: note: built-in candidate operator==(long double, long)
prog.cc:30:8: note: built-in candidate operator==(long double, long long)
prog.cc:30:8: note: built-in candidate operator==(long double, __int128)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(__float128, float)
prog.cc:30:8: note: built-in candidate operator==(__float128, double)
prog.cc:30:8: note: built-in candidate operator==(__float128, long double)
prog.cc:30:8: note: built-in candidate operator==(__float128, __float128)
prog.cc:30:8: note: built-in candidate operator==(__float128, long)
prog.cc:30:8: note: built-in candidate operator==(__float128, long long)
prog.cc:30:8: note: built-in candidate operator==(__float128, __int128)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long, float)
prog.cc:30:8: note: built-in candidate operator==(long, double)
prog.cc:30:8: note: built-in candidate operator==(long, long double)
prog.cc:30:8: note: built-in candidate operator==(long, __float128)
prog.cc:30:8: note: built-in candidate operator==(long, long)
prog.cc:30:8: note: built-in candidate operator==(long, long long)
prog.cc:30:8: note: built-in candidate operator==(long, __int128)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long long, float)
prog.cc:30:8: note: built-in candidate operator==(long long, double)
prog.cc:30:8: note: built-in candidate operator==(long long, long double)
prog.cc:30:8: note: built-in candidate operator==(long long, __float128)
prog.cc:30:8: note: built-in candidate operator==(long long, long)
prog.cc:30:8: note: built-in candidate operator==(long long, long long)
prog.cc:30:8: note: built-in candidate operator==(long long, __int128)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(__int128, float)
prog.cc:30:8: note: built-in candidate operator==(__int128, double)
prog.cc:30:8: note: built-in candidate operator==(__int128, long double)
prog.cc:30:8: note: built-in candidate operator==(__int128, __float128)
prog.cc:30:8: note: built-in candidate operator==(__int128, long)
prog.cc:30:8: note: built-in candidate operator==(__int128, long long)
prog.cc:30:8: note: built-in candidate operator==(__int128, __int128)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned __int128)
1 error generated.
のstdコンテナは、カスタムオブジェクト内のキー値の等価比較を行うためにのために他の一般的な使用である。
class Foo
{
public: int val;
};
class Comparer { public:
bool operator () (Foo& a, Foo&b) const {
return a.val == b.val;
};
class Blah
{
std::set< Foo, Comparer > _mySet;
};