[PATCH] Make dense_hash_map work with non-default-constructible values

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Under C++11, try to compile google::dense_hash_map<int, 
std::reference_wrapper<int>> x;
2. Observe that compilation fails due to trying to default-construct the 
reference_wrapper.

What is the expected output? What do you see instead?

The code should compile. It's a perfectly reasonable thing to do if you are 
trying to build a secondary index on top of a data structure. Please see the 
attached patch for a proposed fix.

What version of the product are you using? On what operating system?

sparsehash-2.0.2 on Ubuntu 14.04 and CentOS 6.3
Ran "make check" in non-C++11 mode with g++-4.8, and in C++11 mode with g++-4.8 
and g++-4.9.

lesha@ubuntu:~/sparsehash/patch$ cat demo1.cpp 
#include <sparsehash/dense_hash_map>

int main() {
  google::dense_hash_map<int, std::reference_wrapper<int>> x;
  return 0;
}
lesha@ubuntu:~/sparsehash/patch$ g++ -std=c++11 demo1.cpp -o demo1 && ./demo1
lesha@ubuntu:~/sparsehash/patch$ cat demo2.cpp 
#include <sparsehash/dense_hash_map>

int main() {
  int a = 5;
  google::dense_hash_map<int, std::reference_wrapper<int>> x;
  x.set_empty_value(std::make_pair(0, std::ref(a)));
  x.insert(std::make_pair(3, std::ref(a)));
  return 0;
}
lesha@ubuntu:~/sparsehash/patch$ g++ -std=c++11 demo2.cpp -o demo2 && ./demo2
lesha@ubuntu:~/sparsehash/patch$ 

Original issue reported on code.google.com by snarkmas...@gmail.com on 19 Feb 2015 at 3:55

Attachments:

• dense_hash_map_non_default_constructible.patch