What are some uses for =delete? [duplicate]

Question

This question already has an answer here:

• c++ syntax: default and delete modifiers 1 answer

Earlier today I asked a question that led to another one: When should I use =delete? I don't think there is a post dedicated solely to =delete on SO, so I looked it up in a book called "The C++ Programming Language". I will list my findings in my answer below.

Please comment or answer if there's more to say or if I'm mistaken.

Answer 1

It turns out that =delete is extremely useful! Here are a few examples:

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Basically we can prevent copying base classes because it might often lead to slicing:

struct Base {

    Base(){}

    Base& operator=(const Base&) = delete; // disallow copying
    Base(const Base&) = delete;

    Base& operator=(Base && ) = delete;      // disallow moving
    Base(Base && ) = delete;

};

struct Der : public Base {};

void func() {

    Der d;
    Base base = d; // this won't work because the copy constructor is deleted!
                   // this behavior is desired - otherwise slicing would occur

}

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It's also useful when a template function cannot run with a certain type:

template<class T>
void fn(T p) { /* ... */ }; // do something with T

void fn(int) = delete; // disallow use with int

void fun() {

    fn(4);      // aha! cannot use fn() with int!
    fn(-4.5);   // fine
    fn("hello");// fine
}

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=delete can also disallow undesired conversions:

struct Z {

    Z(double); // can initialize with a double
    Z(int) = delete; // but not with an integer

};

void f() {

    Z z1 { 1 }; // error! can't use int
    Z z2 { 1.0 }; // double is ok

}

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Some more advanced uses of =delete include prohibiting stack or free store allocation:

class FS_Only {
    ~FS_Only() = delete;  // disallow stack allocation
};

class Stack_Only {
    void* operator new(size_t) = delete;   // disallow heap allocation
};

... You get the idea. Hope this helps someone! =delete can help write readable, bugless and elegant code.

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Edit:

As it was correctly pointed out in the comments, it is now impossible to delete FS_Only objects, so this one isn't such a good use of =delete after all.