How to make std chrono output in milliseconds or nanoseconds? [duplicate] - c++

Question

This question already has answers here:
How to get duration, as int milli's and float seconds from <chrono>?
(5 answers)
Closed 13 days ago.
#include <iostream>
#include <chrono>
using namespace std;
class Time
{
chrono::steady_clock::time_point start;
public:
Time()
{
start = chrono::steady_clock::now();
}
~Time()
{
auto end = chrono::steady_clock::now();
std::chrono::duration<double> time = end-start;
cout << "time " << time.count() << endl;
}
};
void revstr(char* input)
{
int length = strlen(input);
int last_pos = length-1;
for(int i = 0; i < length/2; i++)
{
char tmp = input[i];
input[i] = input[last_pos - i];
input[last_pos - i] = tmp;
}
}
int main()
{
Time t;
char str[] = "abcd";
cout << str << endl;
revstr(str);
cout << str << endl;
}
The above outputs:
time 7.708e-06
How should we turn that in milliseconds, nanoseconds or seconds?
I tried (replace std::chrono::duration<double> time = end-start;)
std::chrono::duration<std::chrono::nanoseconds> time = end-start;
But it says:
A duration representation cannot be a duration.

Answer 1

Use std::chrono::duration_cast():
#include <chrono>
#include <iostream>
int main()
{
std::chrono::duration<double> time{7.708e-06};
auto nanos = std::chrono::duration_cast<std::chrono::nanoseconds>(time);
std::cout << nanos.count() << '\n';
}