Custom types as key for a map - 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 like the below, everything worked.

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 making the operator overloads as friend. I am wondering why my first code is not working?

Any thoughts?

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.

Answer 2

It's probably looking for const member operators (whatever the correct name is). This works (note const):

bool operator<(const Foo& foo1) const { return foo_value < foo1.foo_value;}

EDIT: deleted operator> from my answer as it was not needed (copy/paste from question) but it was attracting comments :)

Note: I'm 100% sure that you need that const because I compiled the example.