pointer to std::map fails to insert

Question

I am very confused. Why does this work:

double doubleValue = 20;
NcVar variable = {some process obtaining an instance}
map<NcVar,double> th;
th.insert(std::make_pair(variable, doubleValue));

and this fails:

double doubleValue = 20;
NcVar variable = {some process obtaining an instance}
map<NcVar,double> *th = new map<NcVar,double>();
th->insert(std::make_pair(variable, doubleValue));

That means, the first variant ends up with one key/value-pair, while the second leaves the map unchanged (0 entries)?

How do you know that it fails? There's nothing wrong with the code you've posted here, so I assume the error is elsewhere. — templatetypedef, Jun 17 '13 at 19:10
You will get better help if you post a complete program and describe what behavior you expect and what behavior you observe. — Johan Råde, Jun 17 '13 at 19:13
I know it fails because in the first case the map has an entry. In the second code it winds up empty. I do check this in the debugger. The rest of the code is really of no consequence, just parsing some command line parameters. Nothing spectacular. — Jürgen Simon, Jun 17 '13 at 19:17
Yes, it is std::map. I am using boost (among other things), but that should not interfere with std::map afaik. — Jürgen Simon, Jun 17 '13 at 19:18
If you are so sure it fails, post some simple code that shows this. It is hard to see how it could be possible. — juanchopanza, Jun 17 '13 at 19:21
Does not work for me. I extirpated the code from the larger program it was embedded in into a standalone. Same trouble. See here: http://ideone.com/XiAetN PS: you will not be able to run this without a NetCDF-File that is CF-Metadata compliant. This is why I was trying to avoid posting it here. — Jürgen Simon, Jun 17 '13 at 19:59

score 1 · Answer 1 · answered Jun 17 '13 at 20:03

Works for me:

#include <map>
#include <iostream>
using namespace std;
int main(){
  typedef map<int,float> mapp;
  mapp map1;
  map1.insert(make_pair(1,1.1));

  mapp * mp2 = new mapp();
  mp2->insert(make_pair(2,2.2));
  cout << map1.begin()->second << endl;
  cout << mp2->begin()->second <<endl;
return 0;

}

And output:

$g++ map_test.cpp 
$ ./a.out 
1.1
2.2

score 0 · Answer 2 · answered Jun 18 '13 at 10:01

Thanks for the help, guys. I feel kinda stupid now. The assumption that the map was empty was based on the appearance in the debugger. I am using XCode as IDE and when using a pointer to map, it would simply mess up and display the map as empty. Using cout revealed the truth.

pointer to std::map fails to insert

2 Answers2