1. Before explaining std::bind, let's review std::function first.
std::function is a "callable object" wrapper, a class template that can accommodate all callable objects except class member function pointers, which can handle functions, function objects, function pointers, and Allows saving and delaying their execution.
What is a callable object?
(1) Function pointer
(2) Class object with operator() member function (legendary functor), lambda expression
(3) Class object that can be converted into a function pointer
(4) Class member (function) pointer
( 5) bind expressions or other function objects
for example:
// 普通函数
int add(int a, int b){return a+b;}
// lambda表达式
auto mod = [](int a, int b){ return a % b;}
// 函数对象类
struct divide{
int operator()(int denominator, int divisor){
return denominator/divisor;
}
};Save the above types with std::function:
std::function<int(int ,int)> a = add;
std::function<int(int ,int)> b = mod ;
std::function<int(int ,int)> c = divide(); Two, std::bind
The std::bind function can be thought of as a generic function adapter that takes a "callable object" and generates a new callable object that "fits" the argument list of the original object.
std::bind mainly has the following two functions:
(1) Bind the callable object and its parameters into an anti-function;
(2) Only bind some parameters, reducing the parameters passed in by the callable object.
1. Binding common functions
double divide (double x, double y) {
return x/y;
}
auto divide_half = std::bind (my_divide,_1,2);
std::cout << fn_half(10) << std::endl; // 输出5(1) The first parameter of bind is the function name. When an ordinary function is used as an actual parameter, it will be implicitly converted into a function pointer. So std::bind (divide,_1,2) is equivalent to std::bind (÷,_1,2);
(2) _1 represents a placeholder, located in <functional>, std::placeholders::_1;
2. Binding member functions
#include <functional>
#include <iostream>
using namespace std;
struct Foo {
void sum(int n1, int n2)
{
std::cout << n1+n2 << std::endl;
}
static int StaticMember(int a) {
return 66 + a;
}
int data = 10;
};
int main() {
Foo foo;
auto f = std::bind(&Foo::sum, &foo, 95, std::placeholders::_1);
f(5); // 输出100
}(1) When bind binds a class member function, the first parameter represents the pointer of the member function of the object, and the second parameter represents the address of the object.
(2) Must explicitly specify &Foo::sum, because the compiler will not implicitly convert the member function of the object into a function pointer, so you must add & before Foo::sum; (3) When using the pointer of the object member
function , it is necessary to know which object the pointer belongs to, so the second parameter is the address of the object &foo;
3. Binding static member functions
auto fun2 = &Foo::StaticMember;
cout<<fun2(3)<<endl;4. Binding member variables
#include <functional>
#include <iostream>
using namespace std;
class TestClass4 {
public:
TestClass4() : m_a(100)
{}
public:
int m_a;
};
int main() {
TestClass4 test4;
auto fun4 = std::bind(&TestClass4::m_a, std::placeholders::_1);
int var = fun4(test4);
std::cout<<var<<std::endl;
}5. Binding functors
#include <functional>
#include <iostream>
using namespace std;
class TestClass5 {
public:
TestClass5() = default;
TestClass5(const TestClass5& obj) {
std::cout<<"TestClass5 copy construct."<<std::endl;
}
void operator()(int a) {
std::cout<<a<<std::endl;
}
};
int main(){
TestClass5 test5;
auto fun5 = test5;
fun5(2018);
}6. When binding member functions, which methods will call the "copy constructor" first?
#include <functional>
#include <iostream>
using namespace std;
class TestClass {
public:
TestClass(int a):m_a(a){}
TestClass(const TestClass& obj) {
m_a = obj.m_a + 100;
std::cout<<"copy construct."<<std::endl;
}
int ClassMember(int a) {
std::cout<<" this:"<<this<<" :"<<&m_a<<" "<<m_a<<std::endl;
return 55 + a;
}
int ClassMember2(int a,char ch,float f) {
std::cout <<ch <<" "<< f << " "<<a<<std::endl;
return 55 + a;
}
static int StaticMember(int a) {
return 66 + a;
}
public:
int m_a;
};;
int main(){
TestClass test(67);
std::cout<<"&test "<<&test<<" "<<test.m_a<<" &test.m_a "<<&test.m_a<<std::endl;
std::cout<<"---------------------------------------------------------------------"<<std::endl;
auto fun4 = std::bind(&TestClass::ClassMember, test, std::placeholders::_1); // 调用拷贝构造函数
fun4(4);
std::cout<<"---------------------------------------------------------------------"<<std::endl;
auto fun5 = std::bind(&TestClass::ClassMember, &test, std::placeholders::_1); // 不调用拷贝构造函数
fun5(5);
std::cout<<"---------------------------------------------------------------------"<<std::endl;
auto fun6 = &TestClass::StaticMember; // 不调用拷贝构造函数
std::cout<<fun6(6)<<std::endl;
std::cout<<"---------------------------------------------------------------------"<<std::endl;
auto fun7 = std::bind(&TestClass::m_a, std::placeholders::_1); // 不调用拷贝构造函数
std::cout<<fun7(test)<<" "<<test.m_a<<std::endl;
}Results of the:
7. The problem of passing reference parameters
When you need to pass the object to the parameter in bind, you need to use ref or cref.
#include<iostream>
#include<functional>
using namespace std;
using namespace placeholders;
void alter(int &a, int b) {
a = b;
}
int main() {
int a = 5;
auto f1 = bind(alter, a, _1); //此处bind中的参数a只是a的一个拷贝，而不是a的引用
f1(9);
cout << a << endl; //所以此处a仍为5
auto f2 = bind(alter, ref(a), _1); //使用ref可以将a的引用传递给bind
f2(9);
cout << a << endl; //所以此处a变成了9
return 0;
}
8. Pointer to member function
#include <iostream>
struct Foo {
int value;
void f1() { std::cout << "f1(" << this->value << ")\n"; }
void f2() { std::cout << "f2(" << this->value << ")\n"; }
};
void apply(Foo* foo1, Foo* foo2, void (Foo::*fun)()) {
(foo1->*fun)(); // call fun on the object foo1
(foo2->*fun)(); // call fun on the object foo2
}
int main() {
Foo foo1{1};
Foo foo2{2};
apply(&foo1, &foo2, &Foo::f1);
apply(&foo1, &foo2, &Foo::f2);
}(1) Definition of member function pointer: void (Foo::*fun)(), actual parameter passed when calling: &Foo::f;
(2) fun is a class member function pointer, so the member function *fun must be obtained by dereferencing when calling, that is, (foo1->*fun)().
reference:
(1) std::function and std::bind in C++11