错误：没有可行的重载运算符[]

Question

This is a bit of my code:

#include "pugi/pugixml.hpp"

#include <iostream>
#include <string>
#include <map>
int main() {
    pugi::xml_document doca, docb;
    std::map<std::string, pugi::xml_node> mapa, mapb;

    if (!doca.load_file("a.xml") || !docb.load_file("b.xml"))
        return 1;

    for (auto& node: doca.child("site_entries").children("entry")) {
        const char* id = node.child_value("id");
        mapa[new std::string(id, strlen(id))] = node;
    }

    for (auto& node: docb.child("site_entries").children("entry"))
        const char* idcs = node.child_value("id");
        std::string id = new std::string(idcs, strlen(idcs));
        if (!mapa.erase(id)) {
            mapb[id] = node;
        }
    }

When compiling I get this error:

src/main.cpp:16:13: error: no viable overloaded operator[] for type 'std::map<std::string, pugi::xml_node>'
        mapa[new std::string(id, strlen(id))] = node;

Answer 1

You have a type mismatch. mapa is of type:

std::map<std::string, pugi::xml_node> mapa,
         ^^^^^^^^^^^^

But you're doing:

mapa[new std::string(id, strlen(id))] = node;
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         string*

std::map has two overloads of operator[] :

T& operator[](const Key& );
T& operator[](Key&& );

In your case, Key is std::string . But you're trying to pass in std::string* , for which there is no conversion to std::string - hence you get an error for "no viable overloaded operator[] ".

What you meant to do was:

mapa[id] = node;

Same comment for this line:

std::string id = new std::string(idcs, strlen(idcs));

C++ is not Java, you just do:

std::string id(idcs, strlen(idcs));

or simply:

std::string id = idcs;