std :: map不接受我的运算符<

Question

I have a map with a more or less custom Key-Type ( Point2f from OpenCV ) and thus need to write my own operator< . Except, my operator< doesn't get accepted.

Creation of the Map/Accessing an element by key:

using namespace cv;
void foo()
{
    map<Point2f, double> properties;

    properties[Point2f(0, 0)] = 0;
}

and this is my operator:

using namespace cv;
bool operator<(Point2f lhs, Point2f rhs)
{
    return lhs.x == rhs.x ? lhs.y < rhs.y : lhs.x < rhs.x;
}

but when I try to set a value of the map using a key like above, my compiler gives me

/usr/include/c++/4.8/bits/stl_function.h|235|error: no match for ‘operator<’ (operand types are ‘const cv::Point_<float>’ and ‘const cv::Point_<float>’)|

(gcc, IDE Code::Blocks)

I already tried

• specifying the type exactly ( cv::Point_<float> )
• putting the operator directly above the function calling it
• using const, reference or const reference for the variables passed to the operator instead of values

Nothing worked, the error keeps coming. Why does it appear and what do I need to change to make it work?

Answer 1

Adapting the example posted in the comments, so that the mock Point2f class is within the cv namespace, as the original is, reproduces the error.

namespace cv
{
struct Point2f
{
    int x, y;
    Point2f(int v1, int v2) : x(v1), y(v2) {}
};
}

Live demo

Adding a using directive following the definition above makes no difference because using namespace cv means bring everything under the cv namespace into the current scope , not everything that follows automatically gets added to the cv namespace .

Define your operator< as follows so that ADL is able to find it.

namespace cv
{
bool operator<(Point2f const& lhs, Point2f const& rhs) // pass by reference is not necessary
                                                       // but might as well
{
    std::cout << "calling custom operator <\n";
    return lhs.x == rhs.x ? lhs.y < rhs.y : lhs.x < rhs.x;
}
}

Live demo

---

Another option, to avoid adding operator overloads to someone else's namespace, is to define a comparator for Point2f objects.

struct Point2fLess
{
    bool operator()(Point2f const&lhs, Point2f const& rhs) const
    {
        std::cout << "calling custom operator <\n";
        return lhs.x == rhs.x ? lhs.y < rhs.y : lhs.x < rhs.x;
    }
};

Now define your map as

std::map<Point2f, double, Point2fLess> properties;

Live demo