std :: map使用。 这是怎么了 核心转储？ 我没有正确做些什么？

Question

What is wrong here ? core dump ? What I do not do correctly?

#include <cstdio>
#include <map>
#include <cmath>
#include <cstring>
#include <iostream>

bool compFloats (const float f1, const float f2){
  return floor (f1) < floor (f2);
};

int main (int argc, char **argv){

  std::map < float, char, decltype (&compFloats) > m;


  m.emplace (3.9, 'a');
  m.emplace (3.1, 'b');
  m.emplace (4.1, 'c');
  m.emplace (4.5, 'd');
  m.emplace (5.2, 'e');

  for (auto i = m.begin (); i != m.end (); i++)
    printf ("%c ", i->second);
  printf ("\n");


  printf ("hello world\n");

  return 0;
}

Answer 1

Starting with the assumption that your custom-built comparison operator is necessary for this code to function, the issue you're facing is that the argument being passed to the map template, decltype (&compFloats) , doesn't describe the specific function you want to use as a comparator, but rather the typename of that function. As a result, when the map attempts to perform comparisons, it doesn't have any concrete function to use, and probably dereferences a null pointer somewhere, causing the core dump.

If you actually need this kind of comparison (where the floating point values are being rounded down to the nearest integer before being saved), you'll need to define the map like this instead, using a local struct that can compare float s together:

struct CompFloat {
    bool operator()(float a, float b) const {
        return floor(a) < floor(b);
    }
};

int main() {
    std::map<float, char, CompFloat> m;
    //...
}

Alternatively, if what you need is just to use a std::map with float s as the key, and the truncation is immaterial (or unnecessary/unhelpful), just write this instead.

int main() {
    std::map<float, char> m;
    //...
}

This will do what you probably expect of it.

Answer 2

Based on the preliminary indications of @Xirema I tried an "expected syntax"

instead of:

std::map < float, char, decltype (&compFloats) > m;

I initialized it as:

std::map<float,char,decltype(&floatComp)> m(&floatComp);

I don't know where in the documents it is written but now works correctly as expected !!!

Answer 3

The code to understand better the difference between class and object of a map follows by defining two maps with the same type but different comparison functions.

#include <cstdio>
#include <map>
#include <cmath>
#include <cstring>
#include <iostream>

bool comp1(const float f1,const float f2)   {return floor(f1)<floor(f2);}
bool comp2(const float f1,const float f2) {return f1-floor(f1)<f2-floor(f2);}

typedef bool (*comp_t)(const float,const float);
typedef std::map<float,char,comp_t> M;

int main(int argc, char **argv)
{   
        M m1(&comp1);
        M m2(&comp2);

        m1.emplace(3.9,'a');m2.emplace(3.9,'a');
        m1.emplace(3.1,'b');m2.emplace(3.1,'b');
        m1.emplace(4.1,'c');m2.emplace(4.1,'c');
        m1.emplace(4.5,'d');m2.emplace(4.5,'d');
        m1.emplace(5.2,'e');m2.emplace(5.2,'e');

    for (auto i=m1.begin();i!=m1.end();i++)
        printf("%c ",i->second);
    printf("\n");

    for (auto i=m2.begin();i!=m2.end();i++)
        printf("%c ",i->second);
    printf("\n");

    return 0;
}