C++ error: 'unordered_map' does not name a type

Question

I am doing everything correctly as far as I can tell and I have gotten the error message:

error: \'unordered_map\' does not name a type
error: \'mymap\' does         


        

Answer 1

Compile with g++ -std=c++11 (my gcc version is gcc 4.7.2) AND

#include <unordered_map>
#include <string>

using namespace std;

//global variable
unordered_map<string,int> mymap;

int main() {
  mymap.reserve(7000); // <-- try putting it here
  return 0;
}

Answer 2

You can't execute arbitrary expressions at global scope, so you should put

mymap.reserve(7000);

inside main.

This is also true for other STL containers like map and vector.

Answer 3

If you want to support <unordered_map> for versions older than c++11 use
#include<tr1/unordered_map> and declare your maps in the form :- std::tr1::unordered_map<type1, type2> mymap
which will use the technical report 1 extension for backward compatibility.