return type of bool function

Your functions mypredicate and compare are merely thin wrappers over the binary operators == and <. Operators are like functions: they take a number of arguments of a given type, and return a result of a given type.

For example, imagine a function bool operator==(int a, int b) with the following specification:

• if a equals b then return true
• otherwise return false

And a function bool operator<(int a, int b) with the following specification:

• if a is strictly lesser than b then return true
• otherwise return false.

Then you could write:

bool mypredicate (int i, int j) {
    return operator==(i, j);
}

bool compare(int a, int b){
    return operator<(a, b);
}

For convenience, most programming languages allow you to use a shorter, functionnaly equivalent syntax: i == j and a < b.