Why does a map insert change the object member values? - c++

Question

#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
class Car {
public:
Car( float costs, float speed) : costs(costs), speed(speed){};
float getSpeed() const {
return speed;
}
float getCosts() const {
return costs;
}
private:
float costs;
float speed;
};
class CarManager {
public:
CarManager(){};
void assignCars( std::vector<Car> &cars){
float costs = 1000.0;
float inc_costs = 700.0;
float inc_speed = 50.0;
for (size_t i = 0; i <= 10; ++i) {
float speed = 10.0;
std::vector<Car*> car_vec;
for (size_t j = 0; j < 5; ++j) {
if (j >= i) {
cars.push_back(Car(costs, speed));
car_vec.push_back(&cars.back());
}
speed += inc_speed;
}
car_storage.insert( {costs , car_vec } );
costs += inc_costs;
}
}
void print(){
for (const auto & p : car_storage) {
std::cout << "map[" << p.first << "] = ";
for (const auto & s : p.second) {
std::cout << "[" << s->getSpeed() <<", " << s->getCosts() << "] , ";
}
std::cout << std::endl;
}
}
private:
std::map<float, std::vector<Car*> > car_storage;
};
int main()
{
std::vector<Car> cars;
CarManager car_manager;
car_manager.assignCars(cars);
car_manager.print();
}
The printout somehow differs from what I expect. I push a Car with costs and speed into a vector [cars.push_back(Car(costs, speed))] and later push a pointer to this Car in a local vector of Car*: car_vec.push_back(&cars.back()). Next, I insert the key (costs) and the value (car_vec) into the map. When I print the map out, the values aren't correct anymore.
In the first case for example:
first loop i = 0, inner loop j, thus (j >= i) and I push_back a Car( 1000.0, 10.0). What's contained in the map, later on, is simply a Car(0,0). For the other cars, this applies as well, but they have some strange values.
map[1000] = [0, 0] , [0, 7.41086e-38] , [0, 7.41063e-38] , [0, 7.41064e-38] , [210, 1700] ,
map[1700] = [0, 7.41092e-38] , [0, 7.41094e-38] , [0, 7.41094e-38] , [210, 1700] ,
map[2400] = [110, 2400] , [160, 2400] , [210, 2400] ,
map[3100] = [160, 3100] , [210, 3100] ,
map[3800] = [210, 3800] ,
I have no clue what is going on. Debugging showed me, that the values have been ok until they get inserted into the map at car_storage.insert( {costs , car_vec } ).

Answer 1

What happens when you write this:
cars.push_back(Car(costs, speed))
car_vec.push_back(&cars.back())
? You create a Car and push it's copy to the cars vector. On the next line you push a pointer to the copy in this vector. Never do it! On any further operation with first vector's size change like push_back, clear and so on it may reallocate all it's content. I.e. it takes new memory fragment, copies values there, deletes the old one. Old pointers to it's content become invalid.