ref

对于函数式编程（如 std::bind），是对参数直接拷贝，而不是引用，std::ref的引入就是为了解决一些传参问题

ref是个函数模板，用来构建一个reference_wrapper对象并返回，该对象拥有传入的elem变量的引用。如果参数本身是一个reference_wrapper类型的对象，则创建该对象的一个副本，并返回。
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使用

#include <iostream>
#include <functional>

void f(int& n1, int& n2, const int& n3)
{
    std::cout << "In function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
    ++n1;
    ++n2;
}

int main()
{
    int n1 = 1, n2 = 2, n3 = 3;
    std::function<void()> bound_f = std::bind(f, n1, std::ref(n2), std::cref(n3));
    n1 = 10;
    n2 = 11;
    n3 = 12;
    std::cout << "Before function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
    bound_f();
    std::cout << "After function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
}

运行结果：

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std::bind 使用的是参数的拷贝而不是引用，所以在执行 std::bind 后，在函数 f() 中 n1 的值仍然是 1，n2 和 n3 改成了修改的值，因此必须使用 std::ref 来进行引用绑定。

与 std::bind 类似，std::thread 也是必须通过 std::ref 来绑定引用进行传参，否则，形参的引用声明是无效的。

#include <iostream>
#include <thread>

void threadFunc(std::string &str, int a)
{
    str = "change by threadFunc";
    a = 13;
}

int main()
{
    std::string str("hello");
    int a = 1;
    std::thread th(threadFunc, std::ref(str), a);

    th.join();

    std::cout<<"str = " << str << std::endl;
    std::cout<<"a = " << a << std::endl;

    return 0;
}

运行结果：

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ref 和引用的区别

#include <iostream>
#include <functional>
#include<vector>

using namespace std;

void f(int& a, int& b, int& c)
{
    cout << "in function a = " << a << "  b = " << b << "  c = " << c << endl;
    cout << "in function a = " << &a << "  b = " << &b << "  c = " << &c << endl;
    a += 1;
    b += 10;
    c += 100;
}

int main() {

    int n1 = 1, n2 = 10, n3 = 100;
    int& r1 = n1;
    int& r2 = n2;

    function<void()> f1 = bind(f, r1, r2, ref(n3));		

    f1();
    cout << "out function a = " << n1 << "  b = " << n2 << "  c = " << n3 << endl;
    cout << "out function a = " << &n1 << "  b = " << &n2 << "  c = " << &n3 << endl;
    f1();
    cout << "out function a = " << n1 << "  b = " << n2 << "  c = " << n3 << endl;
    cout << "out function a = " << &n1 << "  b = " << &n2 << "  c = " << &n3 << endl;
    return 0;
}