Overloading operator < - c++

Question

I experts, once again while practicing online, i encountered another problem. This is regarding function template. I am able to create the template but i am not sure how to overload appropriate operator. Please advise.
Question
A function template largestOfTree to return the largest of 3 elements of the same parameterised type. To what class can the function template be applied? Write a class trainEngine with fields for name,model,mass. Overload the appropriate operator so the largestOfThree function template can be applied to three trainEngine objects.
So far ?
template<class T>
bool largestOfThree(T t1, T t2, T t3){
if(t1<t2&&t2<t3){
return true;
}else{
return false;
}
}
trainEngine
class trainEngine {
private:
string name;
string model;
string mass;
public:
friend bool operator<(trainEngine const& lhs) {
if (lhs.name<lhs.model&&lhs.model<lhs.mass){
return true;
}
};

Answer 1

A friend operator< is going to be non-member and should thus be binary. Moreover, you forgot the return type. You likely want:
friend bool operator<(trainEngine const& lhs, trainEngine const& rhs) {
// implementation
}
This would go where your operator< declaration is currently.
Here is a list of idiomatic signatures for operator overloads, and a more elaborate explanation of what juanchopanza mentioned in the comments. Note that the non-member operators can be defined within the class body if they are marked as friends. (They are still non-member functions if you do this.)

Answer 2

If you overload operator '<', you should also overload operator '>'
and you have to write return type bool also.
friend bool operator<(trainEngine const& obj1, trainEngine const& obj2)
In fact, it’s convention in most code to prefer the usage of < over > But more generally, always overload the complete set of related operators; in your case, this would probably also be ==, !=, <= and >=.

Answer 3

Let me note that currently your implementation only depends on the right-hand-side (which you call lhs!). It is surely not what you want.
I think you wanted something like this:
bool operator<(trainEngine const& rhs) {
if(name!=rhs.name)
return(name<rhs.name);
if(model!=rhs.model)
return (model<rhs.model);
return (mass<rhs.mass);
}
or the friend version:
//within the class
friend bool operator<(trainEngine const& lhs, trainEngine const& rhs);
//outside the class
bool operator<(trainEngine const& lhs, trainEngine const& rhs) {
if(lhs.name!=rhs.name)
return(lhs.name<rhs.name);
if(lhs.model!=rhs.model)
return (lhs.model<rhs.model);
return (lhs.mass<rhs.mass);
}