在模板实例化中使用运行时值

Question

Please consider class A , and the function object A_less . A_less compares two A pointers, depending on the result of A::getvalue() .

class A {
    int getvalue(const string &Parameter);
};

struct A_less : public binary_function<A *, A *, bool> {
    A_less(const string &P) : Parameter(P) { }
    bool operator()(const A *lhs, const A *rhs) const {
        return A->getvalue(Parameter) < rhs->getvalue(Parameter);
    }
    string Parameter;
}

How do I go about declaring/creating sorted containers (sets, priority_queues, ...), of A pointers, sorted by A_less depending on specific (run-time) values of Parameter ?

Answer 1

Like this:

std::string p = ...;
std::set<A,A_less> m(A_less(p));

You have to specify the template parameter Compare (which is 2nd for set ). When constructing a map, you need to give the comparison function object to the constructor of map .