在模板参数中传递向量

Question

I want to define a compare function so that it can be passed to std::sort. The comparision needs to be done based on the ordering of the vector x as demonstrated in 'compare_by_x' function below.

template <std::vector<double> x>
bool compare_by_x(int i, int j){
  return x[i] <= x[j];
}

I want to pass the compare_by_x function as follows. This is not working.

std::sort(some_index_vector.begin(), some_index_vector.end(), compare_by_x<x>);

Answer 1

You cannot pass object references to template or to function. But you can pass them to structs.

Here is working example:

#include <iostream>
#include <vector>
#include <algorithm>

struct compare_by_x
{
    std::vector<double>& x;
    compare_by_x(std::vector<double>& _x) : x(_x) {}

    bool operator () (int i, int j)
    {
        return x[i] <= x[j];
    }
};

int main(int argc, const char *argv[])
{
    std::vector<double> some_index_vector;
    some_index_vector.push_back(0);
    some_index_vector.push_back(1);
    some_index_vector.push_back(2);
    std::vector<double> x;
    x.push_back(3);
    x.push_back(1);
    x.push_back(2);

    std::sort(some_index_vector.begin(), some_index_vector.end(), compare_by_x(x));

    for (std::vector<double>::const_iterator it = some_index_vector.begin(); it != some_index_vector.end(); ++it)
    {
        std::cout << *it << ' ';
    }
    std::cout << std::endl;

    return 0;
}

Answer 2

You simply cannot do that – templates are for types and a few compile-time constants only.

You need to take a look at the documentation of std::sort , which explains what kind of comparison function is expected as the third argument. Yours wouldn't work even if the template did miraculously compile.

Luckily for you, the solution to your problem has already been posted on Stack Overflow .