How can I use a custom type as key for a map in C++?

Question

I am trying to assign a custom type as a key for std::map. Here is the type which I am using as key:

struct Foo {     Foo(std::string s) : foo_value(s){}      bool operator<(const Foo& foo1) {   return foo_value < foo1.foo_value;  }      bool operator>(const Foo& foo1) {   return foo_value > foo1.foo_value;  }          std::string foo_value; }; 

When used with std::map, I am getting the following error:

error C2678: binary '<' : no operator found which takes a left-hand operand of type 'const Foo' (or there is no acceptable conversion) c:\program files\microsoft visual studio 8\vc\include\functional 143 

If I change the struct to the one below, everything works:

struct Foo {     Foo(std::string s) : foo_value(s)   {}      friend bool operator<(const Foo& foo,const Foo& foo1) { return foo.foo_value < foo1.foo_value;  }      friend bool operator>(const Foo& foo,const Foo& foo1) { return foo.foo_value > foo1.foo_value;  }          std::string foo_value; }; 

Nothing changed, except that the operator is overloaded as friend. Why does my first code not work?

Answer 1

I suspect you need

bool operator<(const Foo& foo1) const; 

Note the const after the arguments, this is to make "your" (the left-hand side in the comparison) object constant.

The reason only a single operator is needed is that it is enough to implement the required ordering. To answer the abstract question "does a have to come before b?" it is enough to know whether a is less than b.