non-assignable

Example

这个例子是取自 cppreference C++ named requirements: Swappable，它的non assignable的写法是:

    IntVector& operator=(IntVector) = delete; // not assignable

显然它的这种写法是包含了:

1、copy assign

class_name & class_name :: operator= ( const class_name & )=delete;    

2、move assign

class_name & class_name :: operator= ( class_name && )=delete;

使用copy assign

#include <iostream>
#include <vector>

class IntVector
{
    std::vector<int> v;

public:
    void swap(IntVector &other)
    {
        v.swap(other.v);
    }
    IntVector& operator=(IntVector) = delete; // not assignable
};

int main()
{
    IntVector v1, v2;
    v2 = v1;
}
// g++ --std=c++11 test.cpp -Wall -pedantic

编译报错如下:

test.cpp: 在函数‘int main()’中:
test.cpp:20:5: 错误：使用了被删除的函数‘IntVector& IntVector::operator=(IntVector)’
  v2 = std::move(v1);
     ^
test.cpp:14:13: 错误：在此声明
  IntVector& operator=(IntVector) = delete; // not assignable

使用move assign

#include <iostream>
#include <vector>
#include <utility>

class IntVector
{
        std::vector<int> v;

public:
        void swap(IntVector &other)
        {
                v.swap(other.v);
        }
        IntVector& operator=(IntVector) = delete; // not assignable
};

int main()
{
        IntVector v1, v2;
        v2 = std::move(v1);
}
// g++ --std=c++11 test.cpp -Wall -pedantic

编译报错如下:

test.cpp: 在函数‘int main()’中:
test.cpp:20:5: 错误：使用了被删除的函数‘IntVector& IntVector::operator=(IntVector)’
  v2 = std::move(v1);
     ^
test.cpp:14:13: 错误：在此声明
  IntVector& operator=(IntVector) = delete; // not assignable

Non-assignable mixin

Before C++11

class nonassignable
{
protected:
    nonassignable()
    {
    }
    ~nonassignable()
    {
    }
private:  // emphasize the following members are private
    nonassignable& operator=(nonassignable);
};

Since C++11

class nonassignable
{
protected:
    ~nonassignable() = default;
public:
    nonassignable& operator=(nonassignable) = delete;
};