c++ - find_if 的 Lambda 表达式

Question

我目前正试图在 vector V 中找到一个元素。但我得到了很多错误。

bool VNS::remove(const HostName& name){ ^在/usr/lib/gcc/x86_64-pc-cygwin/4.8.2/include/c++/algorithm:62:0 包含的文件中， 来自 vns.cc:2:.....

 bool VNS::remove(const HostName& name){
        auto it=find_if(v.begin(),v.end(),[](const HostName& a, const HostName& b){return a==b;});
        //code that will remove the elem.
        if(it!=v.end()){
            return true;
        }else{
            return false;
        }
    }
HeaderFile:
class VNS:public NameServerInterface{
    public:
        /*
         * Insert a name/address pair. Does not check if the name
         * or address already exists.
         */
        virtual void insert(const HostName&, const IPAddress&);

        /*
         * Remove the pair with the specified host name. Returns true
         * if the host name existed and the pair was removed, false
         * otherwise.
         */
        virtual bool remove(const HostName&);

        /*
         * Find the IP address for the specified host name. Returns
         * NON_EXISTING_ADDRESS if the host name wasn't in the name
         * server.
         */
        virtual IPAddress lookup(const HostName&) const;

    private:
        std::vector<std::pair<HostName,IPAddress> > v;
};

接口(interface):

/*
 * Interface NameServerInterface -- all name server implementations must
 * implement this interface.
 */
#ifndef NAME_SERVER_INTERFACE_H
#define NAME_SERVER_INTERFACE_H

#include <string>

using HostName = std::string;
using IPAddress = unsigned int;
const IPAddress NON_EXISTING_ADDRESS = 0;

class NameServerInterface {
public:
    virtual ~NameServerInterface() = default;

    /*
     * Insert a name/address pair. Does not check if the name
     * or address already exists.
     */
    virtual void insert(const HostName&, const IPAddress&) = 0;

    /*
     * Remove the pair with the specified host name. Returns true
     * if the host name existed and the pair was removed, false
     * otherwise.
     */
    virtual bool remove(const HostName&) = 0;

    /*
     * Find the IP address for the specified host name. Returns
     * NON_EXISTING_ADDRESS if the host name wasn't in the name
     * server.
     */
    virtual IPAddress lookup(const HostName&) const = 0;
};

#endif

我的 lambda exp 有两个参数。编译器如何知道应该如何用正确的值替换它们。

Answer 1

您的错误消息转录遗漏了最有趣的部分:错误消息!

然而，问题似乎很明显:find_if 函数只需要一个参数，因此您需要捕获name，这就是[] 用于:

auto it=find_if(v.begin(),v.end(),
    [&name](const HostName& a){return a==name;});

这reference page很好地解释了 lambda。