C++ STL：使用派生的虚拟 class 作为 std::sort() 的“严格弱排序”

Question

I hit a wall using std::sort(). I have a pure virtual class (named Compare ) that the caller of a method derives from (named MyComp ). I use the pure virtual class for my API's prototype:

void Object::DoSort(Compare &comp) {
    std::sort(this->mKeys.begin(),this->mKeys.end(), comp);
}

the caller:

class MyComp: public Compare {
    bool operator()(const Row *r1, const Row *r2)  { ... }
} cmp;
...
obj->DoSort(cmp);

The g++ compiler on Linux complains that: "cannot allocate an object of type 'Compare' since type 'Compare' has abstract virtual functions"

Even if I modify Compare to be simply virtual (not pure), the std::sort() still calls the Compare::operator() code instead of the MyComp::operator() .

Calling cmp(r1,r2) compiles fine and return the right result.

I must do something wrong, or I do not get it. Help please!

Answer 1

std::sort (and other STL functions) take comparator objects by value , so your object is being copied, but the derived part (including its vtbl) is being "sliced away".

You could wrap your object in a proxy:

class Proxy
{
private:
    Compare &cmp;
public:
    Proxy(Compare &cmp) : cmp(cmp) {}
    bool operator()(const Row *r1, const Row *r2) { return cmp(r1, r2); }
};


...

MyCompare cmp = MyCompare();

std::sort(x.begin(), x.end(), Proxy(cmp));