C++: =default & =delete

参考：

• What does “default” mean after a class’ function declaration?
• Explicitly Defaulted and Deleted Functions
• Explicitly defaulted and deleted special member functions

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=default 和 =delete 一般用在函数声明处，=default 也可以在 class 外部定义 member function 时使用：

• =default means that you want to use the compiler-generated version of that function, so you don’t need to specify a body.
• =delete specifies that you don’t want the compiler to generate that function automatically.

所谓 “compiler-generated version” 包括默认构造器（Synthesized Default Constructor）、默认 copy-constructor、默认 operator= 等（其实还有 C++11 才有的 move constructor 和 move operator=，遇到的时候再讨论）：

struct SomeType {
    SomeType() = default; // The default constructor is explicitly stated.
    SomeType(OtherType value);
};

struct NonCopyable {
    NonCopyable() = default;
    NonCopyable(const NonCopyable&) = delete;
    NonCopyable & operator=(const NonCopyable&) = delete;
};

struct NoInt {
    void f(double i);
    void f(int) = delete;
};

struct OnlyDouble {
    void f(double d);
    template<class T> void f(T) = delete; // Generalized form to disallow calling f() with any type other than double
};