error in compilation while looping over map - c++

Question

got compilation error while trying to compile simple cpp program
i have tried the following compilation lines
1) gcc hello.cpp -std=gnu++11
2) g++ hello.cpp -std=gnu++11
this is my cpp file
#include <iostream>
#include <map>
#include <string>
using namespace std;
static map<string,int> create_map()
{
map<string,int> mymap = {
{ "alpha", 10 },
{ "beta", 20 },
{ "gamma", 30 } };
return mymap;
}
map<string,int> m= create_map();
int main()
{
cout << "Hello, World!";
for (map<string,int> x: m)
cout << x.first << ": " << x.second << '\n';
return 0;
}
the output of gcc contain a lot of linkage error like
hello.cpp:(.text+0x135): undefined reference to `std::cout'
hello.cpp:(.text+0x13a): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)
and g++ output was shorter
hello.cpp: In function 'int main()':
hello.cpp:19:29: error: conversion from 'std::pair<const std::basic_string<char>, int>' to non-scalar type 'std::map<std::basic_string<char>, int>' requested
hello.cpp:20:18: error: 'class std::map<std::basic_string<char>, int>' has no member named 'first'
hello.cpp:20:37: error: 'class std::map<std::basic_string<char>, int>' has no member named 'second'
the code is correct for 99.99%. i don't know really how to compile it properly

Answer 1

There are some very different kind of errors you see.
gcc hello.cpp -std=gnu++11
This tries to compile the code using the GNU c-compiler.
C++ source files are recognized and compiled regarding the c++ syntax, but the linker uses c standard libraries, and the c++ standard libraries aren't automatically linked.
There must be some difference with your source, since I can't reproduce the linker errors you're claiming with this test.
In the other case
g++ hello.cpp -std=gnu++11
your code is compiled using the c++ compiler, and
for (map<string,int> x: m)
is wrong regarding the type for x in the range based loop (which actually is pair<const string,int>).
You should simply change that to
for (const auto& x: m)
to get the code working.
x will be the type of the container element type, that is iterated over from m.
Here's a working online demo.

Answer 2

Replace the loop with:
for (auto const& x: m)
cout << x.first << ": " << x.second << '\n';
Alternatively, you could use:
for (std::pair<string, int> x: m)
or
for (const std::pair<string, int>& x: m)
const because you do not want to change x. & to prevent a copy of the pair into x.
In the range based loop for an std::map, the variable that iterates over the container needs to be std::pair<const Key, T>. This is defined by the value_type for the container's Member types.

Answer 3

Since the type of x is std::pair<const string,int> you need to use instead (after comments):
for (const std::pair<const string,int>& x: m)
const & is added so that redundant pair copies are avoided (in the newest compilers they are probably avoided any way).
for (const auto& x: m)
may alternatively be used!