C++ invoke apply visit

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Callable對象，是任何可以調用的東西。函數指標，函數，重載了operator()的對象，lamda

#include <iostream>#include <stdlib.h>#include <stdio.h>#include <functional>struct S{    int f1(char c){        std::cout << "S::f1 called\n";    }    void operator()(char c){        std::cout << "S::operator() called\n";        }};int main(){    int (S::*fptr)(char c) = &S::f1;        S obj;    (obj.*fptr)(‘a‘);        S* pobj = &obj;    (pobj->*fptr)(‘a‘);        obj(‘a‘);}

在模板編程實踐中，經常做調用轉寄，例如：

template< class Callable, typename ... Args>void dosomething( Callable f, Args... args){    f(args...);  }void foo(char c){    std::cout << "foo called\n" ;}int main(){    dosomething(foo,‘c‘);  //調用了 foo(‘c‘);}

但是，F(args...)只能應付普通函數，和重載operator()的對象。如果F是函數指標，就掛了。std::invoke就是一個utility類，協助解決這個大問題的。

template< class Callable, typename ... Args>void dosomething( Callable f, Args... args){    //f(args...);   //應用範圍太窄, 對於成員函數指標，需要 (arg1.*f)(args...); 其中，arg1應從args中提取首個參數    std::invoke(f,args...); //ok 已經處理好一切}void foo(char c){    std::cout << "foo called\n" ;}int main(){    int (S::*fptr)(char c) = &S::f1;    S  obj;        dosomething(fptr, obj, ‘a‘);}

 參考：

//https://kelvinh.github.io/blog/2014/03/27/cpp-tutorial-pointer-to-member-function/

https://stackoverflow.com/questions/46388524/when-to-use-stdinvoke-instead-of-simply-calling-the-invokable

std::apply功能上類似std::invoke，Callable參數由tuple提供: 

#include <iostream>#include <functional>int add(int a,int b){    return a+b;}int main(){    std::cout << std::apply(add, std::make_pair(1, 2)) << ‘\n‘;}

參考：http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0376r0.html

 

C++ invoke apply visit