如何修复“错误：'_1' 未在此范围内声明”？

Question

我目前正在尝试将函数绑定到我打算使用的算法。

#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

int main(){
    vector<int> coll{1, 2, 3, 4, 5, 6};

    vector<int>::iterator pos;
    pos = find_if (coll.begin(), coll.end(), 
                    bind(greater<int>(),_1,3)); 

    return 0;
}

并获得此错误反馈：

AlgoTesting.cpp: In function ‘int main()’:
AlgoTesting.cpp:184:41: error: ‘_1’ was not declared in this scope
                     bind(greater<int>(),_1,3)); // criterion
                                         ^~
AlgoTesting.cpp:184:41: note: suggested alternative:
In file included from algostuff.hpp:15:0,
                 from AlgoTesting.cpp:5:
/usr/include/c++/7/functional:275:34: note:   ‘std::placeholders::_1’
     extern const _Placeholder<1> _1;
                                  ^~

我正在努力理解错误日志的含义。有谁知道我在这里想念什么？

Answer 1

您需要包含<functional>和使用std::placeholders::_1

pos = find_if (coll.begin(), coll.end(), bind(greater<int>(),placeholders::_1,3));

一个更简单的选择是使用lambda：

pos = find_if(coll.begin(), coll.end(), [](int v) { return std::greater<int>{}(v, 3); });

或者

pos = find_if(coll.begin(), coll.end(), [](int v) { return 3 < v; });