虚函数和std :: function? [英] Virtual functions and std::function?

问题描述

在C ++ 17中考虑以下代码:

Consider the following code in C++17:

#include <iostream>
#include <functional>

struct base
{
    base() {std::cout << "base::base" << std::endl;}
    virtual ~base() {std::cout << "base::~base" << std::endl;}
    virtual void operator()() {std::cout << "base::operator()" << std::endl;}
};

struct derived1: base
{
    derived1() {std::cout << "derived1::derived1" << std::endl;}
    virtual ~derived1() {std::cout << "derived1::~derived1" << std::endl;}
    virtual void operator()() {std::cout << "derived1::operator()" << std::endl;}
};

struct derived2: base
{
    derived2() {std::cout << "derived2::derived2" << std::endl;}
    virtual ~derived2() {std::cout << "derived2::~derived2" << std::endl;}
    virtual void operator()() {std::cout << "derived2::operator()" << std::endl;}
};

int main(int argc, char* argv[])
{
    base* ptr1 = new derived1();
    base* ptr2 = new derived2();
    std::function f1(*ptr1);
    std::function f2(*ptr2);
    std::invoke(*ptr1);     // calls derived1::operator()
    std::invoke(*ptr2);     // calls derived2::operator()
    std::invoke(f1);        // calls base::operator()
    std::invoke(f2);        // calls base::operator()
    delete ptr1;
    delete ptr2;
    return 0;
}

std::function似乎对虚函数没有做正确的事.有什么方法可以使std::invoke(*ptrN)和std::invoke(fN)行为相同?还是有什么办法可以创建一个新的处理虚拟函数的函数包装器?

std::function does not seem to do the right thing with virtual functions. Would there be any way to make std::invoke(*ptrN) and std::invoke(fN) behave the same way? Or would there be any way to create a new function wrapper that would deal with virtual functions?

推荐答案

您可以使用std::reference_wrapper或便捷的std::ref.在这种情况下，std::function将使用SOO(小对象优化)，因此不会复制/移动对象(避免使用

You can use a std::reference_wrapper, or the convenient std::ref. std::function will use SOO (small object optimization) in this case, so the object won't be copied/moved (avoiding the slicing problem). However, you won't get the deduction guide so you need to specify the template arguments.

std::function<void()> f1(std::ref(*ptr1));
std::function<void()> f2(std::ref(*ptr2));

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