sorting a list containing circles c++

Question

I'd like to run this simple code:

#include <cstdlib> //std :: rand ()
#include <vector> //std::vector<>
#include <list> //std::list<>
#include <iostream> //std::ostream_iterator
#include <iterator> //std:: cout std::ostream_iterator
#include <algorithm> //std::reverse, std::generate
#include <map>
#include "ColorRGB.hpp"
#include "point2d.hpp"
#include "circle.hpp"
#include <cmath>

int main(){

std::list<Circle>lk; 

    Point2d a(7.5,3.2);
    Point2d b(6.5,2.2);
    Point2d c(5.5,1.2);

    ColorRGB d(0, 0, 0);
    ColorRGB e(0, 1, 1);
    ColorRGB f(1, 1, 0);

    Circle c1(a, 2, d);
    Circle c2(b, 1, e);
    Circle c3(c, 0.4, f);

    lk.push_back(c1);
    lk.push_back(c2);
    lk.push_back(c3);

sort(lk.begin(), lk.end(), [] (Circle const& lhs,Circle const& rhs)
    ->bool{return(lhs.getrad() <= rhs.getrad())};
    return 0;
    }

but get the compile message:

Juliuss-MacBook-Pro-2:uebung4 JONG$ g++ -o 4_1 4_1.cpp
4_1.cpp:42:30: error: expected expression
    sort(lk.begin(), lk.end(), [] (Circle const& lhs,Circle const& r...
                               ^
1 error generated.

Answer 1

You can not sort a list using std::sort because std::sort requires a random access iterator and list has a bidirectional iterator only.

Use a vector instead. Alternatively, if you insist on using list, there is a member function called list.sort() . It can accept a custom comparator as well.

Related question / answers here

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Also, use an SSCCE in the question. That means getting rid of/providing the code of custom classes as much as possible.

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You also had errors in the braces. Here is a code that is self-contained and also compiles. It show the usage of both std::sort with vector and the list::sort function:

#include <list>
#include <vector>
#include <iostream>
#include <iterator>
#include <algorithm>

class Circle
{
    int r;
public:
    Circle(int r) : r(r) {}
    int getrad() const { return r; }
};

int main(){
    std::vector<Circle> vk;
    std::list<Circle> lk;

    Circle c1(2);
    Circle c2(1);
    Circle c3(3);

    vk.push_back(c1);
    vk.push_back(c2);
    vk.push_back(c3);

    lk.push_back(c1);
    lk.push_back(c2);
    lk.push_back(c3);

    std::sort(vk.begin(), vk.end(),
        [] (const Circle& lhs, const Circle& rhs) -> bool {
            return lhs.getrad() < rhs.getrad();
        });

    lk.sort(
        [] (const Circle& lhs, const Circle& rhs) -> bool {
            return lhs.getrad() < rhs.getrad();
        });

    return 0;
}

Still, I recommend vector .

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After the OPs update:

As pointed out by @nwp, use a C++11 compiler if you use lambas:

g++ -std=c++11 -Wall -Wextra filename.cpp

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As pointed out by @WhozCraig , the comparator also needed a fix.