初始化map <string,vector <string >>

Question

我正在初始化map<string, vector<string> >如下:

map <string, vector<string> > dict;

dict["USA"].push_back("NYC");
dict["USA"].push_back("LA");
dict["USA"].push_back("Chicago");
dict["USA"].push_back("Dallas");

dict["India"].push_back("Delhi");
dict["India"].push_back("Bombay");

dict["Australia"].push_back("Melbourne");
dict["Australia"].push_back("Sydney");
dict["Australia"].push_back("Adelaide");

我发现这很麻烦.同样的事情可以做tcl如下更清洁:

array set dict {
USA {NYC LA Chicago Dallas}
India {Delhi Bombay}
Australia {Melbourne Sydney Adelaide}
}

是否有更简洁的初始化方式C++？我的编译器是gcc 3.4.6

Answer 1

初始化在旧C++中有许多限制.事实上,你的代码根本没有初始化任何东西; 它只是在已经初始化的对象上调用了很多成员函数.

在当前的C++(C++ 11)中,您可以正确初始化地图:

std::map<std::string, std::vector<std::string>> const dict {
   { "USA", { "NYC", "LA", "Chicago" } },
   { "India", { "Delhi", "Bombay" }    }
};

Answer 2

如果您不反对使用Boost.Assign库并且您使用的是比C++ 11更早的C++,那么您可以这样做:

#include <boost/assign/list_of.hpp>
#include <boost/assign/std/vector.hpp>
#include <map>
#include <string>
#include <vector>

std::map<std::string, vector<std::string> > dict = boost::assign::map_list_of<std::string, std::vector<std::string> >
    ("USA",   boost::assign::list_of<std::string>("NYC")("LA")("Chicago")("Dallas"))
    ("India", boost::assign::list_of<std::string>("Delhi")("Bombay"))
;

Answer 3

如果C++ 11不可用,你可以这样做:

map <string, vector<string> > dict;

string usa[] = { "NYC" , "LA" , "Chicago" , "Dallas" };
dict["USA"] = std::vector<string>(usa,usa+4);

Answer 4

如果你不害怕使用一些C风格的宏和一些帮助器结构,你可能会发现这个稍微不那么烦躁; 地图的初始化是在一行中完成的; 你只需要填写数据(无论如何你必须这样做).

#include <iostream>
#include <map>
#include <vector>
#include <string>
#include <utility>

using namespace std;

struct entry {
    string  key;
    string* vals;
    size_t  nvals;
};
#define NVAL(x) (sizeof(x)/sizeof(x[0]))

int main( void ) {
    // Create your data 
    string      usa[]      = {"NYC", "LA"};
    string      india[]    = {"Delhi", "Mumbai"};
    entry       dd[] = {
                          {"USA", usa, NVAL(usa)},
                          {"India", india, NVAL(india)}
                        };
    map<string, vector<string> > dict;

    // initialize the map
    for(unsigned int i=0; i<NVAL(dd); i++)
        dict.insert( make_pair(dd[i].key, vector<string>(dd[i].vals, dd[i].vals+dd[i].nvals)) );

    // Verify
    for( map<string,vector<string> >::const_iterator ptr=dict.begin();
         ptr!=dict.end(); ptr++) {
        cout << ptr->first << ": ";
        for( vector<string>::const_iterator eptr=ptr->second.begin();
             eptr!=ptr->second.end(); eptr++)
                cout << *eptr << " ";
        cout << endl;
    }
    return 0;
}