如何使用函数指针定义多重集? [英] How to define a multiset using a function pointer?

问题描述

我在进行 C ++ Primer 5th Edition 练习时陷入困境，就像

I'm stuck when doing an exercise from C++ Primer 5th Edition ,which goes like

练习11.11:不使用decltype重新定义书店.

Exercise 11.11: Redefine bookstore without using decltype.

以下是本书中的相关代码:

Below is the relevant codes in this book:

multiset<Sales_data, decltype(compareIsbn)*> bookstore(compareIsbn);

Sales_data类的代码在此处有些冗长，因此我编写了一个简单的代码，并以相同的样式定义了多集，如下所示.它编译没有任何错误.

The code for class Sales_data is a little bit verbose to post here,so I wrote a simpler one and defined the multiset in the same style, as shown below. It compiled without any error.

class A
{
    int lenth;
public:
    int getLenth() const {return lenth;}
};

bool compareA(const A &a1, const A &a2)
{
    return a1.getLenth() < a2.getLenth();
}

int main()
{
    std::multiset<A, decltype(compareA)*> m1(compareA);
    return 0;
}

我猜想本练习希望读者回顾一下函数指针的知识，因此我尝试了另一种定义它的方法.但这是行不通的.下面是我被困住的地方

I guess this exercise want readers to review knowledge of function pointers, so I tried another way to define it. But it doesn't work. Below is where I'm stuck

int main()
{
    bool (*fp) (const A &a1, const A &a2);
    fp = &compareA;

    std::multiset<A, fp*> m1(fp);

    return 0;
}

产生了三个错误:

error: template argument 2 is invalid
error: invalid type in declaration before '(' token
error: invalid conversion from 'bool (*)(const A&, const A&)' to 'int' [-fpermissive]

我有什么问题?如何解决?

What's my problem?How to fix it?

推荐答案

您很近，但是需要在<> 括号中而不是在指针中使用类型争论

You were close, but needed a type arguement in the open <> brackets rather than a pointer

typedef bool (*fp) (const A &a1, const A &a2);
int main() {    
    std::multiset<A, fp> m1(&compareA);

    return 0;
}

或

bool (*fp) (const A &a1, const A &a2) = compareA;
int main() {    
    std::multiset<A, bool (*) (const A &, const A &)> m1(fp);

    return 0;
}

使用更新的编译器，您可以通过使用高阶函数来内联定义比较器:

With a more recent compiler, you can define the comparator inline through the use of higher-order functions:

using comparator = std::function<bool(const A&, const A&)>;

auto main() -> int {    
    std::multiset<A, comparator> m1([](const A& a1, const A& a2) -> bool {
         return a1.getLenth() < a2.getLenth();
    });
    return 0;
}

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