一、std::function
1、概念
std::function是一个函数包装器模板,最早来自boost库,对应其boost::function函数包装器。在c++0x11中,将boost::function纳入标准库中。该函数包装器模板能包装任何类型的可调用元素(callable element),例如普通函数和函数对象。包装器对象可以进行拷贝,并且包装器类型仅仅只依赖于其调用特征(call signature),而不依赖于可调用元素自身的类型。
转载:https://www.cnblogs.com/heartchord/p/5017071.html
- std::function 是一个可调用对象包装器,是一个类模板,可以容纳除了类成员函数指针之外的所有可调用对象,它可以用统一的方式处理函数、函数对象、函数指针,并允许保存和延迟它们的执行。
- 定义格式:std::function<函数类型>。
- std::function可以取代函数指针的作用,因为它可以延迟函数的执行,特别适合作为回调函数使用。它比普通函数指针更加的灵活和便利。
2、包装不同函数
①、包装普通函数
#include <iostream> #include <functional> using namespace std; int g_Minus(int i, int j) { return i - j; } int main() { function<int(int, int)> f = g_Minus; cout << f(1, 2) << endl; // -1 return 1; }
②、包装模板函数
#include <iostream> #include <functional> using namespace std; template <class T> T g_Minus(T i, T j) { return i - j; } int main() { function<int(int, int)> f = g_Minus<int>; cout << f(1, 2) << endl; // -1 return 1; }
③、包装lambda表达式
#include <iostream> #include <functional> using namespace std; auto g_Minus = [](int i, int j){ return i - j; }; int main() { function<int(int, int)> f = g_Minus; cout << f(1, 2) << endl; // -1 return 1; }
④、包装函数对象
非模板类型:
#include <iostream>
#include <functional>
using namespace std;
struct Minus
{
int operator() (int i, int j)
{
return i - j;
}
};
int main()
{
function<int(int, int)> f = Minus();
cout << f(1, 2) << endl; // -1
return 1;
}
模板类型:
#include <iostream> #include <functional> using namespace std; template <class T> struct Minus { T operator() (T i, T j) { return i - j; } }; int main() { function<int(int, int)> f = Minus<int>(); cout << f(1, 2) << endl; // -1 return 1; }
⑤、包装类静态成员函数
非模板类型:
#include <iostream> #include <functional> using namespace std; class Math { public: static int Minus(int i, int j) { return i - j; } }; int main() { function<int(int, int)> f = &Math::Minus; cout << f(1, 2) << endl; // -1 return 1; }
模板类型:
#include <iostream> #include <functional> using namespace std; class Math { public: template <class T> static T Minus(T i, T j) { return i - j; } }; int main() { function<int(int, int)> f = &Math::Minus<int>; cout << f(1, 2) << endl; // -1 return 1; }
⑥、包装类对象成员函数
非模板类型:
#include <iostream> #include <functional> using namespace std; class Math { public: int Minus(int i, int j) { return i - j; } }; int main() { Math m; function<int(int, int)> f = bind(&Math::Minus, &m, placeholders::_1, placeholders::_2); cout << f(1, 2) << endl; // -1 return 1; }
模板类型:
#include <iostream> #include <functional> using namespace std; class Math { public: template <class T> T Minus(T i, T j) { return i - j; } }; int main() { Math m; function<int(int, int)> f = bind(&Math::Minus<int>, &m, placeholders::_1, placeholders::_2); cout << f(1, 2) << endl; // -1 return 1; }
转载:https://www.cnblogs.com/heartchord/p/5017071.html
二、std::bind
1、概念
可将std::bind函数看作一个通用的函数适配器,它接受一个可调用对象,生成一个新的可调用对象来“适应”原对象的参数列表。
std::bind将可调用对象与其参数一起进行绑定,绑定后的结果可以使用std::function保存。
概括:设置可调用对象(函数)的某些参数为可变变量,将某些参数设置为固定值,然后将整个调用对象返回成std::funciton类型
2、举例
#include <random> #include <iostream> #include <memory> #include <functional> void f(int n1, int n2, int n3, const int& n4, int n5) { std::cout << n1 << ' ' << n2 << ' ' << n3 << ' ' << n4 << ' ' << n5 << ' '; } int g(int n1) { return n1; } struct Foo { void print_sum(int n1, int n2) { std::cout << n1+n2 << ' '; } int data = 10; }; int main() { using namespace std::placeholders; // for _1, _2, _3... std::cout << "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, 42, _1, 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, 42, 1, n, 7) std::cout << "achieving the same effect using a lambda: "; auto lambda = [ncref=std::cref(n), n=n](auto a, auto b, auto /*unused*/) { f(b, 42, a, ncref, n); }; lambda(1, 2, 1001); // same as a call to f1(1, 2, 1001) std::cout << "nested bind subexpressions share the placeholders: "; auto f2 = std::bind(f, _3, std::bind(g, _3), _3, 4, 5); f2(10, 11, 12); // makes a call to f(12, g(12), 12, 4, 5); std::cout << "common use case: binding a RNG with a distribution: "; std::default_random_engine e; std::uniform_int_distribution<> d(0, 10); auto 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 << ' '; std::cout << "bind to a pointer to member function: "; Foo foo; auto f3 = std::bind(&Foo::print_sum, &foo, 95, _1); f3(5); std::cout << "bind to a pointer to data member: "; auto f4 = std::bind(&Foo::data, _1); std::cout << f4(foo) << ' '; std::cout << "use smart pointers to call members of the referenced objects: "; std::cout << f4(std::make_shared<Foo>(foo)) << ' ' << f4(std::make_unique<Foo>(foo)) << ' '; }