C++ templates error: instantiated from 'void TestClass::testMethod(T, std::map) [with T = SomeClass]

Question

Hello there,

I've some problem in my code I cannot deal with:

#ifndef TESTCLASS_H_
#define TESTCLASS_H_

#include <map>

using namespace std;
template <typename T>
class TestClass
{
public:
  TestClass(){};
  virtual ~TestClass(){};
  void testMethod(T b,std::map<T,int> m);
};

template <typename T>
void TestClass<T>::testMethod(T b,std::map<T,int> m){
  m[b]=0;
}
#endif /*TESTCLASS_H_*/

int main(int argc, char* argv[]) {
  SomeClass s;
  TestClass<SomeClass> t;
  map<SomeClass,int> m;
  t.testMethod(s,m);
}  

Compiler gives me following error in line m[b]=0; :

instantiated from 'void TestClass::testMethod(T, std::map) [with T = SomeClass]

Could you help find the problem??

Thanks in advance

Answer 1

Not even seeing the error, I can tell you one thing that you probably are doing wrong. Here:

void TestClass<T>::testMethod(T b,std::map<T,int> m){

You are aware, that you're taking a whole map by value?

That however, is not the source of the error. Map is a sorted associative container, and expects keys that can be sorted. Built in types have their sorting via the < operator. For your own class, you need to either provide an operator, or initialize the map with a custom sorting functor.

Hence either an operator:

bool operator<(const SomeClass&,const SomeClass&)
{ 
     return ...;
} 

...or a functor:

struct CompareSomeClass {
  bool operator()(const SomeClass&,const SomeClass&) 
  {
      return ...;
  }
};

Answer 2

This

#include <map>

class SomeClass {};

bool operator<(const SomeClass&,const SomeClass&);

class in {};

template <typename T>
class TestClass
{
public:
  TestClass(){};
  virtual ~TestClass(){};
  void testMethod(T b,std::map<T,int> m);
};

template <typename T>
void TestClass<T>::testMethod(T b,std::map<T,int> m){
  m[b]=0;
}

int main() {
  SomeClass s;
  TestClass<SomeClass> t;
  std::map<SomeClass,int> m;
  t.testMethod(s,m);
}

compiles just fine for me using VC10 and Comeau.

Answer 3

Most likely, SomeClass does not have operator< defined for it. Define that and you should be good to go:

bool operator<(const SomeClass &sc1, const SomeClass sc2)
{
    ...
}

Answer 4

You're right. Overloading operator < in SomeClass solved problem:

bool operator<(const SomeClass &s1) const{
    ....
 }

Thanks a lot!