sorting a vector of objects c++

#include <vector>
#include <algorithm>

using namespace std;

vector< MyStruct > values;

sort( values.begin( ), values.end( ), [ ]( const MyStruct& lhs, const MyStruct& rhs )
{
   return lhs.key < rhs.key;
});

struct MyStruct
{
    int key;
    std::string stringValue;

    MyStruct(int k, const std::string& s) : key(k), stringValue(s) {}
};

struct less_than_key
{
    inline bool operator() (const MyStruct& struct1, const MyStruct& struct2)
    {
        return (struct1.key < struct2.key);
    }
};

std::vector < MyStruct > vec;

vec.push_back(MyStruct(4, "test"));
vec.push_back(MyStruct(3, "a"));
vec.push_back(MyStruct(2, "is"));
vec.push_back(MyStruct(1, "this"));

std::sort(vec.begin(), vec.end(), less_than_key());