std::bind: Each argument may either be bound to a value or be a placeholder:
(1)、If bound to a value, calling the returned function object will always use that value as argument;
(2)、If a placeholder, calling the returned function object forwards an argument passed to the call (the one whose order number is specified by the placeholder).
std::bind receives a pointer to a function (it also can be a lambda expression or a functor) and receives a list of parameters that pass it to the function. As result, bind returns a new function object with a different prototype because all the parameters of the function were already specified.
A placeholder is an object that specifies the number of parameter in the bound function that will be used to enter this parameter.
std::bind is a Standard Function Objects that acts as a Functional Adaptor i.e. it takes a function as input and returns a new function Object as an output with with one or more of the arguments of passed function bound or rearranged.
std::bind函数是用来绑定函数调用的某些参数的。std::bind它可以预先把指定可调用实体的某些参数绑定到已有的变量，产生一个新的可调用实体。它绑定的参数的个数不受限制，绑定的具体哪些参数也不受限制，由用户指定。
std::bind：(1)、将函数、成员函数和闭包转成function函数对象；(2)、将多元(n>1)函数转成一元函数或者(n-1)元函数。
std::bind本身是一种延迟计算的思想，它本身可以绑定普通函数、全局函数、静态函数、类静态函数甚至是类成员函数。无法使用std::bind绑定一个重载函数的参数，必须显式地指出需要绑定的重载函数的版本。成员函数区别于普通函数的一个特殊之处在于，其第一个参数必须是该类型的一个对象(或对象的指针或引用)。
std::bind接受一个函数(或者函数对象，或者任何你可以通过"(…)"符号调用的事物)，生成一个其有某一个或多个函数参数被”绑定”或重新组织的函数对象。绑定的参数将会以值传递的方式传递给具体函数，占位符将会以引用传递。
std::bind需要注意事项：
(1)、bind预先绑定的参数需要传具体的变量或值进去，对于预先绑定的参数，是pass-by-value的；
(2)、对于不事先绑定的参数，需要传std::placeholders进去，从_1开始，依次递增。placeholder是pass-by-reference的；
(3)、bind的返回值是可调用实体，可以直接赋给std::function对象；
(4)、对于绑定的指针、引用类型的参数，使用者需要保证在可调用实体调用之前，这些参数是可用的；
(5)、类的this可以通过对象或者指针来绑定。
下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：
#include "bind.hpp"
#include <iostream>
#include <string>
#include <functional> // std::bind
#include <random>
#include <memory>
#include <algorithm>
//////////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/utility/functional/bind
static void f(int n1, int n2, int n3, const int& n4, int n5)
{
std::cout << n1 << ' ' << n2 << ' ' << n3 << ' ' << n4 << ' ' << n5 << '\n';
}
static int g(int n1)
{
return n1;
}
struct Foo_bind {
void print_sum(int n1, int n2)
{
std::cout << n1 + n2 << '\n';
}
int data = 10;
};
int test_bind1()
{
using namespace std::placeholders; // for _1, _2, _3...
// demonstrates argument reordering and pass-by-reference
int n = 7;
// (_1 and _2 are from std::placeholders, and represent future
// arguments that will be passed to f1)
auto f1 = std::bind(f, _2, _1, 42, std::cref(n), n);
n = 10;
f1(1, 2, 1001); // 1 is bound by _1, 2 is bound by _2, 1001 is unused
// makes a call to f(2, 1, 42, n, 7)
// nested bind subexpressions share the placeholders
auto f2 = std::bind(f, _3, std::bind(g, _3), _3, 4, 5);
f2(10, 11, 12);
// common use case: binding a RNG with a distribution
std::default_random_engine e;
std::uniform_int_distribution<> d(0, 10);
std::function<int()> rnd = std::bind(d, e); // a copy of e is stored in rnd
for (int n = 0; n<10; ++n)
std::cout << rnd() << ' ';
std::cout << '\n';
// bind to a pointer to member function
Foo_bind foo;
auto f3 = std::bind(&Foo_bind::print_sum, &foo, 95, _1);
f3(5);
// bind to a pointer to data member
auto f4 = std::bind(&Foo_bind::data, _1);
std::cout << f4(foo) << '\n';
// smart pointers can be used to call members of the referenced objects, too
//std::cout << f4(std::make_shared<Foo_bind>(foo)) << '\n'
// << f4(std::make_unique<Foo_bind>(foo)) << '\n';
return 0;
}
///////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/functional/bind/
// a function: (also works with function object: std::divides<double> my_divide;)
double my_divide(double x, double y) { return x / y; }
struct MyPair {
double a, b;
double multiply() { return a*b; }
};
int test_bind2()
{
using namespace std::placeholders; // adds visibility of _1, _2, _3,...
// binding functions:
auto fn_five = std::bind(my_divide, 10, 2); // returns 10/2
std::cout << fn_five() << '\n'; // 5
auto fn_half = std::bind(my_divide, _1, 2); // returns x/2
std::cout << fn_half(10) << '\n'; // 5
auto fn_invert = std::bind(my_divide, _2, _1); // returns y/x
std::cout << fn_invert(10, 2) << '\n'; // 0.2
auto fn_rounding = std::bind<int>(my_divide, _1, _2); // returns int(x/y)
std::cout << fn_rounding(10, 3) << '\n'; // 3
MyPair ten_two{ 10, 2 };
// binding members:
auto bound_member_fn = std::bind(&MyPair::multiply, _1); // returns x.multiply()
std::cout << bound_member_fn(ten_two) << '\n'; // 20
auto bound_member_data = std::bind(&MyPair::a, ten_two); // returns ten_two.a
std::cout << bound_member_data() << '\n'; // 10
return 0;
}
/////////////////////////////////////////////////
// reference: https://oopscenities.net/2012/02/24/c11-stdfunction-and-stdbind/
static void show(const std::string& a, const std::string& b, const std::string& c)
{
std::cout << a << "; " << b << "; " << c << std::endl;
}
int test_bind3()
{
using namespace std::placeholders;
// Thanks to the placeholders, you can change the order of the arguments passed as parameters to a bound function.
auto x = bind(show, _1, _2, _3);
auto y = bind(show, _3, _1, _2);
auto z = bind(show, "hello", _2, _1);
x("one", "two", "three");
y("one", "two", "three");
z("one", "two");
return 0;
}
/////////////////////////////////////////////////////
// reference: http://*.com/questions/22422147/why-is-stdbind-not-working-without-placeholders-in-this-example-member-functi
static void f_(int a, int b, int c)
{
fprintf(stdout, "a = %d, b = %d, c = %d\n", a, b, c);
}
struct foo_bind
{
void f() const { fprintf(stdout, "foo_bind\n"); };
};
int test_bind4()
{
using namespace std::placeholders;
// The first parameter of std::bind() is the function to be called, and the rest are the arguments of the call.
std::function<void()> f_call = std::bind(f_, 1, 2, 3);
f_call(); //Equivalent to f_(1,2,3)
// If you need to specify that some parameters have to be specified at the call point,
// you have to use placeholders to represent that parameters
std::function<void(int, int, int)> f_call2 = std::bind(f_, _1, _2, _3);
f_call2(1, 2, 3); //Same as f_(1,2,3)
// Note that the numbers of the placeholders specify the number of the parameter at the call point.
// The first parameter of the call point is identified by _1, the second by _2, and so on.
// This could be used to specify parameters in different ways, reordering the parameters of a function call, etc.
std::function<void(int, int)> f_call3 = std::bind(f_, _1, 2, _2);
f_call3(1, 3); //Equivalent to f_( 1 , 2 , 3 );
std::function<void(int, int, int)> reordered_call = std::bind(f_, _3, _2, _1);
reordered_call(3, 2, 1); //Same as f_( 1 , 2 , 3 );
// std::bind() could be used to bind a member function to the object used to call it
foo_bind myfoo;
std::function<void()> f = std::bind(&foo_bind::f, std::cref(myfoo));
f();
return 0;
}
/////////////////////////////////////////////////
// reference: https://msdn.microsoft.com/en-us/library/bb982702.aspx
static void square(double x)
{
std::cout << x << "^2 == " << x * x << std::endl;
}
static void product(double x, double y)
{
std::cout << x << "*" << y << " == " << x * y << std::endl;
}
int test_bind5()
{
using namespace std::placeholders;
double arg[] = { 1, 2, 3 };
std::for_each(&arg[0], arg + 3, square);
std::cout << std::endl;
std::for_each(&arg[0], arg + 3, std::bind(product, _1, 2));
std::cout << std::endl;
std::for_each(&arg[0], arg + 3, std::bind(square, _1));
return (0);
}