在if语句中测量时间c ++

Question

Well... since 2 days ago I have been working on a project which records the clicks you make and repeats that over and over(just like a bot), the problem is at the moment that is recording the time between clicks because when measuring time with the " steady_clock::now " on a if statement, it is only declared in the if statement and if I try to make it a global variable with a NULL value, the compiler throws me an error because the " auto " variable type has to have a default value.

#include<chrono>

using namespace std::chrono;

auto start = NULL; //this is an error

int main()
{
     if (!GetAsyncKeyState(VK_LBUTTON))
    {
        auto start = steady_clock::now();
    }
     else if (GetAsyncKeyState(VK_LBUTTON))
    {
        auto end = steady_clock::now();

        std::chrono::duration<double> elapsed = end - start;   //here the compiler throws me an error because start is not declared
    }

}

I will really appreciate if someone responds my question.

Sorry for my english...

Answer 1

This is where decltype is useful:

decltype(steady_clock::now()) start;
// ...
start = steady_clock::now();

It is the type of the expression inside the parenthesis.

Answer 2

First of all, you are trying to set up "start" twice and are also trying to use auto in an incorrect manner. What is the compiler supposed to assume when it sees auto start = NULL ? So kindly look up how it used.

Meanwhile,I think what you are looking for is something like this :

#include<chrono>

std::chrono::steady_clock::time_point start; // You can mention the type of start and end explicitly,.
std::chrono::steady_clock::time_point end;

int main()
{
    if (!GetAsyncKeyState(VK_LBUTTON))
    {
        start = std::chrono::steady_clock::now();
    }
    else // don't think you needed an else-if here.
    {
        end = std::chrono::steady_clock::now();
        std::chrono::duration<double> elapsed = end - start; 
    }
}