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C++11中“= delete；“的使用

news 来源：原创 2024/5/19 3:33:49

C++11中，对于deleted函数，编译器会对其禁用，从而避免某些非法的函数调用或者类型转换，从而提高代码的安全性。

对于 C++ 的类，如果程序员没有为其定义特殊成员函数，那么在需要用到某个特殊成员函数的时候，编译器会隐式的自动生成一个默认的特殊成员函数，比如默认的构造函数、析构函数、拷贝构造函数以及拷贝赋值运算符。

为了能够让程序员显式的禁用某个函数，C++11标准引入了一个新特性：deleted函数。程序员只需在函数声明后加上”=delete;”，就可将该函数禁用。

deleted函数特性还可用于禁用类的某些转换构造函数，从而避免不期望的类型转换。

deleted函数特性还可以用来禁用某些用户自定义的类的new操作符，从而避免在自由存储区创建类的对象。

必须在函数第一次声明的时候将其声明为deleted函数，否则编译器会报错。即对于类的成员函数而言，deleted函数必须在类体里(inline)定义，而不能在类体外(out-of-line)定义。

虽然defaulted函数特性规定了只有类的特殊成员函数才能被声明为defaulted函数，但是deleted函数特性并没有此限制。非类的成员函数，即普通函数也可以被声明为deleted函数。

In C++11, defaulted and deleted functions give you explicit control over whether the special member functions are automatically generated.

The opposite of a defaulted function is a deleted function.

Deleted functions are useful for preventing object copying, among the rest. Recall that C++ automatically declares a copy constructor and an assignment operator for classes. To disable copying, declare these two special member functions “=delete”.

The delete specifier can also be used to make sure member functions with particular parameters aren’t called.

=default: it means that you want to use the compiler-generated version of that function, so you don't need to specify a body.

=delete: it means that you don't want the compiler to generate that function automatically.

下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：

#include "delete.hpp"
#include <iostream>
 
//
// reference: http://www.learncpp.com/cpp-tutorial/b-6-new-virtual-function-controls-override-final-default-and-delete/
class Foo
{
	Foo& operator=(const Foo&) = delete; // disallow use of assignment operator
	Foo(const Foo&) = delete; // disallow copy construction
};
 
class Foo_1
{
	Foo_1(long long); // Can create Foo() with a long long
	Foo_1(long) = delete; // But can't create it with anything smaller
};
 
class Foo_2
{
	Foo_2(long long); // Can create Foo() with a long long
	template<typename T> Foo_2(T) = delete; // But can't create it with anything else
};
 
///
// reference: http://www.bogotobogo.com/cplusplus/C11/C11_default_delete_specifier.php
class A_
{
public:
	A_(int a){};
	A_(double) = delete;         // conversion disabled
	A_& operator=(const A_&) = delete;  // assignment operator disabled
};
 
int test_delete1()
{
	A_ a_(10);     // OK
	// A_ b(3.14);   // Error: conversion from double to int disabled
	// a = b;       // Error: assignment operator disabled
 
	return 0;
}
 
 
// reference: https://msdn.microsoft.com/zh-cn/library/dn457344.aspx
struct widget
{
	// deleted operator new prevents widget from being dynamically allocated.
	void* operator new(std::size_t) = delete;
};