How can I insert custom objects into a std::map? - c++

Question

I've read through the map::map reference at cplusplus.com and I'm still not sure how to get this to work. All I want to do is create a map like the following:
std::map<TriSpec, unsigned int> TriSpecMap;
Then I want to insert into it as follows:
result = TriSpecMap.insert(std::make_pair(triSpecObject, anUnsignedInt));
The following is a short example of my TriSpec header and .cpp:
//TriSpec.h
#ifndef TRISPEC_H
#define TRISPEC_H
class TriSpec
{
public:
TriSpec(void);
~TriSpec(void);
unsigned int m_position;
};
bool operator< (const TriSpec& lhs, const TriSpec& rhs);
#endif
//TriSpec.cpp
#include "TriSpec.h"
TriSpec::TriSpec(void){}
TriSpec::~TriSpec(void){}
bool operator< (const TriSpec& lhs, const TriSpec& rhs)
{
if (lhs.m_position < rhs.m_position) return true;
else return false;
}
Am I overloading the correct operator? Is the function/formatting I'm using correct? When I look at result.second, it is always true, even when I know the object being inserted should already exist in the map.

Answer 1

I do not see any problem with your code except that bool operator should be friend method (it does not even compile without friend keyword):
bool friend operator< (const TriSpec& lhs, const TriSpec& rhs)
{
return (lhs.m_position < rhs.m_position);
}
Then it works as expected:
int main(int argc, _TCHAR* argv[])
{
std::map<TriSpec, unsigned int> TriSpecMap;
TriSpec triSpecObject1;
triSpecObject1.m_position = 1;
TriSpec triSpecObject2;
triSpecObject2.m_position = 1;
TriSpec triSpecObject3;
triSpecObject3.m_position = 3;
std::pair<std::map<TriSpec, unsigned int>::iterator, bool> retVal =
TriSpecMap.insert(std::make_pair(triSpecObject1, 1));
retVal = TriSpecMap.insert(std::make_pair(triSpecObject2, 1));
retVal = TriSpecMap.insert(std::make_pair(triSpecObject3, 1));
return 0;
}
The result of first insertion is true, the result of second is false a the third is true again - as it should be. The map container contains two objects then - triSpecObject1 a triSpecObject3.

Answer 2

That doesn't look like it would compile - your operator< doesn't return a value in all cases. (Edit: you've fixed this, thanks.) You can simplify it quite a bit:
bool operator< (const TriSpec& lhs, const TriSpec& rhs)
{
return (lhs.m_position < rhs.m_position);
}
Since you don't show the complete code that inserts into the map, it's impossible to say why the return value .second always returns true.

Answer 3

Since C++11 you can also use a lambda expression instead of providing an operator< for your class. As a result, you can create your map with only two lines of code as follows:
int main()
{
auto comp = [](const TriSpec& t1, const TriSpec& t2) { return t1.m_position < t2.m_position; };
std::map<TriSpec, unsigned int, decltype(comp)> TriSpecMap(comp);
TriSpec t1, t2, t3;
t1.m_position = 1;
t2.m_position = 3;
t3.m_position = 5;
auto retVal = TriSpecMap.emplace(t1, 4);
retVal = TriSpecMap.emplace(t2, 2);
retVal = TriSpecMap.emplace(t3, 6);
for (auto const &kv : TriSpecMap)
std::cout << kv.first.m_position << ": " << kv.second << std::endl;
return 0
}
Output:
1: 4
3: 2
5: 6
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