std::set custom compare function for double values. insert() does not work

Question

I created a set via

bool(*fn_pt)(const double&, const double&) = comp_double;
std::set<double, bool(*)(const double&, const double&) > values(fn_pt);

where my comp_double function is given by

bool comp_double (const double& p1, const double& p2)
{
return (std::abs(p1-p2)<1e-05);
}

I inserted two dummy elements, values.insert(0.01) and values.insert(0.02), but the size of my set is still zero. There are no compiler warnings or errors, so I guess the problem lies within my comp_double function. Any input would be appreciated!

Best

Answer 1

First, we should remember that a std::set is an associative container (that contains only the "keys") . Yes, we can provide our own comparison operation for an associative container. This operation must conform to strict weak ordering (we can think of it as a "less than" ), means to have the following properties:

• Two keys cannot be both "less than" each other. (Also, a key cannot yield "less" with itself)
• The comparison has to be transitive. If key1 < key2 , key2 < key3 , then key1 < key3
• If we have two keys and neither of these is "less than" the other, they must be equivalent.

Consider this case:

key1 = 0.000001;
key2 = 0.000002;

abs(key1 - key2) == 0.000001; // which is less than 0.00001, true
abs(key2 - key1) == 0.000001; // same, true too

Each of them is "less" than the other, means they violate the first constraint. Also, a key here will yield true for "less" with itself .

You may want to take a look at: Does using epsilon in comparison of floating-point break strict-weak-ordering?

For more on the requirements on a Compare check the documentation .