C++, Pass function to algorithm vs Lambda expression

#customobject #lambda #passfunction #algorithms

If we need to use standard functions with custom objects (our defined struct or class) before C++11, we can only pass functions to algorithms.

struct City {
    string name;
    double population;
};
// compare City by population
bool cmpfnc(const City& a, const City& b) {
    return a.population < b.population;
}
// then we can use it
vector<City> vCity = { {"city 1", 140000}, {"city 2", 100000},{"city 3", 700000},{"city 4", 500000} };
sort(vCity.begin(), vCity.end(), cmpfnc); //pass cmpfnc function  to standard stl sort
for (auto c : vCity) {
     cout << "City name: " << c.name << " population: " << c.population << endl;
}

from C++11, Lambda expression makes it simple

vector<City> vCity = { {"city 1", 140000}, {"city 2", 100000},{"city 3", 700000},{"city 4", 500000} };
//lambda expression
sort(vCity.begin(), vCity.end(), [](const City& a, const City& b) {return a.population < b.population; });
for (auto c : vCity) {
   cout << "City name: " << c.name << " population: " << c.population << endl;
}

But sometimes, I also forget the ";" in the code of a lambda expression [](const City& a, const City& b) {return a.population < b.population;} so don't forget it.