How to have a set of structs in C++

Question

I have a struct which has a unique key. I want to insert instances of these structs into a set. I know that to do this the < operator has to be overloaded so that set can make a comparison in order to do the insertion.

The following does not work:

#include <iostream> #include <set> using namespace std; struct foo  {       int key; };  bool operator<(const foo& lhs, const foo& rhs) {       return lhs.key < rhs.key; }  set<foo> bar;  int main() {     foo *test = new foo;     test->key = 0;     bar.insert(test); } 

Answer 1

This might help:

struct foo {   int key; };  inline bool operator<(const foo& lhs, const foo& rhs) {   return lhs.key < rhs.key; } 

If you are using namespaces, it is a good practice to declare the operator<() function in the same namespace.

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For the sake of completeness after your edit, and as other have pointed out, you are trying to add a foo* where a foo is expected.

If you really want to deal with pointers, you may wrap the foo* into a smart pointer class (auto_ptr, shared_ptr, ...).

But note that in both case, you loose the benefit of the overloaded operator< which operates on foo, not on foo*.