c++ - 对功能库 std::bind 中的语法感到困惑

Question

#include <iostream>
#include <functional>
class Solver{
public:
    void print(){
        std::cout << "i solved" << std::endl;
    }
};

class ThingHandler{
public:
    void handleThing(Solver& solver){
        std::cout << "i handled something " << std::endl;
        solver.print();
    }
};

class CantHandle{
public:
    void needHelp(std::function<void(Solver&)> handleThing){
        handleThing(); // how do i here pass Solver as a parameter?
    }
};



int main() {
    ThingHandler thingHandler;
    CantHandle cantHandle;
    Solver solver;
    auto fp = std::bind(&ThingHandler::handleThing, thingHandler,solver);
    cantHandle.needHelp(fp);
    return 0;
}

在“CantHandle”类中，当我调用辅助函数时它要求提供一个参数。这是有道理的，因为被调用的函数是 handleThing，它将求解器的引用作为参数。但我似乎无法弄清楚类 ThingHandler 中的语法。不应该 handleThing 已经绑定(bind)了参数吗？我收到以下错误“不匹配调用‘(std::function) ()”

完整错误信息:

error: no match for call to ‘(std::function) ()’ 23 |
handleThing(); // how do i here pass Solver as a parameter? note: candidate: ‘_Res std::function<_Res(_ArgTypes ...)>::operator()(_ArgTypes ...) const [with _Res = void; _ArgTypes = {Solver&}]’ 685 | function<_Res(_ArgTypes...)>:: |
^~~~~~~~~~~~~~~~~~~~~~~~~~~~ /usr/include/c++/9/bits/std_function.h:685:5: note: candidate expects 1 argument, 0 provided gmake[3]: * [CMakeFiles/funcObj.dir/build.make:63: CMakeFiles/funcObj.dir/main.cpp.o] Error 1 gmake[2]: [CMakeFiles/Makefile2:73: CMakeFiles/funcObj.dir/all] Error 2 gmake[1]: [CMakeFiles/Makefile2:85: CMakeFiles/funcObj.dir/rule] Error 2 gmake: * [Makefile:118: funcObj] Error 2

Answer 1

needHelp 方法需要一个带一个参数的函数，但您想传递一个不带参数的函数。改变这个:

void needHelp(std::function<void(Solver&)> handleThing){

到

void needHelp(std::function<void()> handleThing){

使用 lambda 而不是 bind 通常更容易编写和阅读，并且当您可以接受任何可调用时通常不需要使用 std::function:

#include <iostream>

struct Solver{
    void print(){ 
        std::cout << "i solved" << std::endl; 
    }
};

struct ThingHandler{
    void handleThing(Solver& solver){
        std::cout << "i handled something " << std::endl;
        solver.print();
    }
};

struct CantHandle{
    template <typename F>
    void needHelp(F f){
        f();
    }
};   

int main() {
    ThingHandler thingHandler;
    CantHandle cantHandle;
    Solver solver;   
    cantHandle.needHelp(
        [&solver,&thingHandler](){
            thingHandler.handleThing(solver);
        }
    );
    return 0;
}