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Showing content from https://timsong-cpp.github.io/cppwp/n4140/dcl.fct.def.delete below:

[dcl.fct.def.delete]

A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed. [ Note: This includes calling the function implicitly or explicitly and forming a pointer or pointer-to-member to the function. It applies even for references in expressions that are not potentially-evaluated. If a function is overloaded, it is referenced only if the function is selected by overload resolution.  — end note ]

[ Example: One can enforce non-default initialization and non-integral initialization with

struct onlydouble {
  onlydouble() = delete;                onlydouble(std::intmax_t) = delete;
  onlydouble(double);
};

 — end example ]

[ Example: One can prevent use of a class in certain new expressions by using deleted definitions of a user-declared operator new for that class.

struct sometype {
  void* operator new(std::size_t) = delete;
  void* operator new[](std::size_t) = delete;
};
sometype* p = new sometype;     sometype* q = new sometype[3];  

 — end example ]

[ Example: One can make a class uncopyable, i.e. move-only, by using deleted definitions of the copy constructor and copy assignment operator, and then providing defaulted definitions of the move constructor and move assignment operator.

struct moveonly {
  moveonly() = default;
  moveonly(const moveonly&) = delete;
  moveonly(moveonly&&) = default;
  moveonly& operator=(const moveonly&) = delete;
  moveonly& operator=(moveonly&&) = default;
  ~moveonly() = default;
};
moveonly* p;
moveonly q(*p); 

 — end example ]

A deleted function is implicitly inline. [ Note: The one-definition rule ([basic.def.odr]) applies to deleted definitions.  — end note ] A deleted definition of a function shall be the first declaration of the function or, for an explicit specialization of a function template, the first declaration of that specialization. [ Example:

struct sometype {
  sometype();
};
sometype::sometype() = delete;      

 — end example ]

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