C++ bind 和 ref

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#include <functional>
#include <iostream>
 
void f(int& n1, int& n2, const int& n3)
{
    std::cout << "In function: " << n1 << ‘ ‘ << n2 << ‘ ‘ << n3 << ‘\n‘;
    ++n1; // increments the copy of n1 stored in the function object
    ++n2; // increments the main()‘s n2
    // ++n3; // compile error
}
 
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‘;
}

Output:
Before function: 10 11 12
In function: 1 11 12
After function: 10 12 12

在上面的例子中，如果不加ref，bind引用的是值的拷貝，而不是值的傳遞，所以當我們需要傳遞值的話，加上ref或cref就可以

C++ bind 和 ref