Using unique_ptr with vector - c++

Question

I'm trying to convert written code used by shared_ptr to unique_ptr because in the code of its usage seems shared_ptr unnecessary and it will be an exercise with smart pointers. As far as I can detect with debugger the problem is on v.at(i)->push_back(t);. So, as soon as a value entered, the program crashes.
Working code with shared_ptr:
#include <iostream>
#include <iomanip>
#include <memory> // For smart pointers
#include <vector> // For vector container
#include <locale> // For toupper()
using std::vector;
using std::shared_ptr;
int main()
{
vector <shared_ptr<vector<double>>>records; // Temperature records by days
size_t day{ 1 }; // Day number
char answer{}; // Response to prompt
double t{}; // A temperature
while (true) // Collect temperatures by day
{ // Vector to store current day's temperatures created on the heap
auto pDay = std::make_shared<vector<double>>();
records.push_back(pDay); // Save pointer in records vector
std::cout << "Enter the temperatures for day " << day++
<< " separated by spaces. Enter 1000 to end:\n";
while (true)
{ // Get temperatures for current day
std::cin >> t;
if (t == 1000.0) break;
pDay->push_back(t);
}
std::cout << "Enter another day's temperatures (Y or N)? ";
std::cin >> answer;
if (toupper(answer) == 'N') break;
}
double total{};
size_t count{};
day = 1;
std::cout << std::fixed << std::setprecision(2) << std::endl;
for (auto record : records)
{
std::cout << "\nTemperatures for day " << day++ << ":\n";
for (auto temp : *record)
{
total += temp;
std::cout << std::setw(6) << temp;
if (++count % 5 == 0) std::cout << std::endl;
}
std::cout << "\nAverage temperature: " << total / count << std::endl;
total = 0.0;
count = 0;
}
}
Output:
23 34 29 36 1000
Enter another day's temperatures (Y or N)? y
Enter the temperatures for day 2 separated by spaces. Enter 1000 to end:
34 35 45 43 44 40 37 35 1000
Enter another day's temperatures (Y or N)? y
Enter the temperatures for day 3 separated by spaces. Enter 1000 to end:
44 56 57 45 44 32 28 1000
Enter another day's temperatures (Y or N)? n
Temperatures for day 1:
23.00 34.00 29.00 36.00
Average temperature: 30.50
Temperatures for day 2:
34.00 35.00 45.00 43.00 44.00
40.00 37.00 35.00
Average temperature: 39.13
Temperatures for day 3:
44.00 56.00 57.00 45.00 44.00
32.00 28.00
Average temperature: 43.71
The converted code with unique_ptr:
#include <iostream>
#include <vector>
#include <memory>
#include <iomanip>
using std::vector;
int main()
{ // Function scope starts here
// a vector(outside) holding unique_ptrs to a vector(inside) which type is double
vector<std::unique_ptr<vector<double>>> v;
size_t day{ 1 };
char answer{};
double t{};
while (true)
{
size_t i{};
auto pDay = std::unique_ptr<vector<double>>();
v.push_back(std::move(pDay));
std::cout << "Enter the temperatures for day " << day++
<< " separated by spaces. Enter 1000 to end:\n";
while (true)
{
std::cin >> t;
if (t >= 1000.0) break;
v.at(i)->push_back(t);
++i;
}
//std::cout << v.at(0)->at(0) << std::endl;
std::cout << "Enter another day's temperatures (Y or N)? ";
std::cin >> answer;
if (toupper(answer) == 'N') break;
}
double total{};
size_t count{};
day = 1;
std::cout << std::fixed << std::setprecision(2) << std::endl;
for (auto const& record : v)
{
std::cout << "\nTemperatures for day " << day++ << ":\n";
for (auto temp : *record)
{
total += temp;
std::cout << std::setw(6) << temp;
if (++count % 5 == 0) std::cout << std::endl;
}
std::cout << "\nAverage temperature: " << total / count << std::endl;
total = 0.0;
count = 0;
}
} // Function scope ends here

Answer 1

Your std::unique_ptr<std::vector<double>> doesn't point at anything. You'd need to initialize it with a pointer to a vector.