priority_queue constant expression

Question

In code looks like this:

comparision algorithm

class PathComp{
public:
virtual bool betterThan(const PathInfo& path1, const PathInfo& path2) const{
//returns true if path1 is shorther than path2
{
};

class with redefined operator ()

class QueueComp{
private:
PathComp* comp;
public:
QueueComp(PathComp* pc);
bool operator () (const pair<PathInfo, string>& item1, const pair<PathInfo, string> item2);
};

QueueComp::QueueComp(PathComp* pc):comp(pc){}
bool QueueComp::operator () (const pair<PathInfo, string>& item1, const pair<PathInfo, string>& item2){
    return comp->betterThan(item1.first, item2.first);
}

function using priority queue

list<string> Graph::shortestPath(const string& from, const string& to, PathComp* pc) const{
    const QueueComp comp(pc);
    std::priority_queue<pair<PathInfo, string>, set<pair<PathInfo, string> >, comp> q;
}

the compilator shows error messages : 'comp' cannot appear in a constant-expression, template argument 3 is invalid, invalid type in declaration before ; token

Does anybody know where's the problem? Thanks for all help.

Answer 1

The compiler already says what the problem is: a non- constexpr cannot be a template argument. You probably meant to write

std::priority_queue<std::pair<PathInfo, std::string>,
                    std::vector<std::pair<PathInfo, std::string> >,
                    QueueComp>
    q(comp);

The immediate issue was that comp is an object but the template expects a type for the comparison function. Once this is fixed the next problem would be that std::set<...> isn't a viable container to be used with std::priorit_queue<...> but std::vector<...> is.