类成员函数的std :: bind [英] std::bind of class member function

问题描述

我有以下代码：

#include <iostream>
#include <functional>

struct Foo
{
        int get(int n) { return 5+n; }
};

int main()
{
        Foo foo;
        auto L = std::bind(&Foo::get, &foo, 3);

        std::cout << L() << std::endl;

        return 0;
}

似乎是这样：

auto L = std::bind(&Foo::get, &foo, 3);

等价于：

auto L = std::bind(&Foo::get, foo, 3);

为什么？

推荐答案

std :: bind（）接受按值的参数。这意味着在第一种情况下，您将按值传递 pointer ，从而生成了指针的副本。在第二种情况下，将按值传递 foo 类型的对象，从而得到 Foo 。

std::bind() accepts its arguments by value. This means that in the first case you are passing a pointer by value, resulting in the copy of a pointer. In the second case, you are passing an object of type foo by value, resulting in a copy of an object of type Foo.

结果，在第二种情况下，对表达式 L（）的求值导致成员函数 get（）可以在原始对象 foo 的副本上调用

As a consequence, in the second case the evaluation of the expression L() causes the member function get() to be invoked on a copy of the original object foo, which may or may not be what you want.

此示例说明了差异（忘记违反三规则/五规则，这仅出于说明目的）：

This example illustrates the difference (forget the violation of the Rule of Three/Rule of Five, this is just for illustration purposes):

#include <iostream>
#include <functional>

struct Foo
{
    int _x;

    Foo(int x) : _x(x) { }

    Foo(Foo const& f) : _x(f._x)
    {
        std::cout << "Foo(Foo const&)" << std::endl;
    }

    int get(int n) { return _x + n; }
};

int main()
{
   Foo foo1(42);

   std::cout << "=== FIRST CALL ===" << std::endl;
   auto L1 = std::bind(&Foo::get, foo1, 3);
   foo1._x = 1729;
   std::cout << L1() << std::endl; // Prints 45

   Foo foo2(42);

   std::cout << "=== SECOND CALL ===" << std::endl;
   auto L2 = std::bind(&Foo::get, &foo2, 3);
   foo2._x = 1729;
   std::cout << L2() << std::endl; // Prints 1732
}

实时示例 。

如果由于某种原因不想使用指针形式，则可以使用 std :: ref（） 以防止创建该参数的副本：

If, for any reason, you don't want to use the pointer form, you can use std::ref() to prevent a copy of the argument from being created:

auto L = std::bind(&Foo::get, std::ref(foo), 3);

这篇关于类成员函数的std :: bind的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！