如何在 C++ 中使用关键字“auto”声明和使用变量？

Question

As the title, how to I could declare a variable using key word "auto" in C++ I've been tried, but failed

#include <iostream>
 using namespace std;
 int main(int argc, char const *argv[])
 {
      auto v;
      cin >> v;
      cout << v << endl;
      return 0;
  }

Answer 1

You can use it when getting results from a function if you don't know the exact type it returns.

auto test = someFunc();
cout << a;

For example, of what type is the result of std::vector<int>().begin() ? No need to worry about this:

std::vector<int> myvector = {1, 2, 3};

for (std::vector<int>::iterator it = myvector.begin() ; it != myvector.end(); ++it)
std::cout << ' ' << *it;

You can simplify it to the following:

for (auto it = myvector.begin() ; it != myvector.end(); ++it)
std::cout << ' ' << *it;

As you can see, you don't really need to know the type of it , now it's your compiler's job.

Answer 2

the reason is that auto doesnt behave in the same way as php and javascript's var do. auto is simply "make it the same type as the data which i am assigning to it". It's not a data type on its own.

As you can see, you are not assigning any data when you create the variable in your example, so the compiler cannot tell what the type is supposed to be.