求助大佬, Lambda 表达式可以直接转成 Functor 吗?

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我想在 std::map 构造的时候直接使用 Lambda 表达式指定排序方式, 但是编译器报错了

class Person {
public:
    std::string name;
    int age;
    Person(const std::string & n , int a) {
        name = n;
        age = a;
    }
    class Compare {
    public:
        bool operator()(const Person & lhs, const Person & rhs) {
            return lhs.age < rhs.age;
        }
    };
};

    auto lambda = [](const Person & lhs, const Person & rhs){
        return lhs.age < rhs.age;
    };
    auto wapper = [lambda]() {
        return lambda;
    };


    bool a = Person::Compare()(Person("p1", 1), Person("p2", 2));
    bool b = wapper()(Person("p1", 1), Person("p2", 2));

    // Success
    std::set<Person, Person::Compare> sp; 

    // E: Template argument for template type parameter must be a type
    std::set<Person, wapper> sp;

12 replies • 2019-08-22 00:42:15 +08:00

noli

Aug 21, 2019

std::bind

sl0000

Aug 21, 2019

@noli 请问需要怎么实现

ysc3839

Aug 21, 2019 via Android

std::set<Person, decltype(lambda)> sp(lambda);

noli

Aug 21, 2019

@sl0000 #2

https://www.jianshu.com/p/621fc81a1dc1

wisan

Aug 21, 2019 via Android

lambda 要用 std::move，不然不能拷贝。。更别说当参数了。。

noli

Aug 21, 2019

@ysc3839 #3

使用 std::function 代替 decltype<lambda> 是否更通用灵活一些？

ysc3839

Aug 21, 2019 via Android

@noli 我觉得不，因为还要多引入一个头文件。

geelaw

Aug 21, 2019 via iPhone

说得够明白了：代替 Person::Compare 的必须是一个类型，你的 wapper 不是一个类型，而是一个对象。

你可以这样

template <typename TElement, typename TCompare>
std::set<TElement, TCompare> make_set(TCompare &&cmp)
{
return {};
}

auto my_set = make_set<Person>([](Person const &a, Person const &b)
{
return a.age < b.age;
});

blinue

Aug 21, 2019

Compare 在 set 里被默认构造用于比较，lambda 对象的默认构造函数被删除，decltype(lambda)在 C++20 前行不通，只能定义新的类。在 C++20 里，不捕获的 lamda 对象的类型具有默认构造函数，就可以这样了：set<Person, decltype(lambda)>。

sl0000

Aug 21, 2019

@geelaw 这个编译不能通过呀, 提示
````
In file included from ~/Desktop/learnC++/LearnC++/main.cpp:11:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/set:458:19: error: no matching constructor for initialization of 'std::__1::set<Person, (lambda at ~/Desktop/learnC++/LearnC++/main.cpp:36:32), std::__1::allocator<Person> >::value_compare' (aka '(lambda at /Users/cosmojulis/Desktop/learnC++/LearnC++/main.cpp:36:32)')
: __tree_(value_compare()) {}
^
~/Desktop/learnC++/LearnC++/main.cpp:34:12: note: in instantiation of member function 'std::__1::set<Person, (lambda at ~/Desktop/learnC++/LearnC++/main.cpp:36:32), std::__1::allocator<Person> >::set' requested here
return {};
^
~/Desktop/learnC++/LearnC++/main.cpp:36:15: note: in instantiation of function template specialization 'make_set<Person, (lambda at ~/Desktop/learnC++/LearnC++/main.cpp:36:32)>' requested here
auto my_set = make_set<Person>([](const Person & a, const Person & b)
^
~/Desktop/learnC++/LearnC++/main.cpp:36:32: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 0 were provided
auto my_set = make_set<Person>([](const Person & a, const Person & b)
^
~/Desktop/learnC++/LearnC++/main.cpp:36:32: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 0 were provided
````

sl0000

Aug 21, 2019

感谢各位大佬, 今天学到特别多, 我睡了

geelaw

Aug 22, 2019

@sl0000 #10 Oops 你需要对 TCompare 删除 cv qualifiers 并且使用 std::forward。