Why does a variable need to be declared as auto?

Question

The "auto" keyword is used to make a variable automatic, but even without using the keyword, a variable becomes automatic by default. In the following program, the variable inside two blocks of statements is automatic and works the same way.

#include <stdio.h>

int main()
{
    int a = 1;

    printf("Value of 'a' is %d\n", a);

    {
        auto int a = 2;
        printf("Value of 'a' is %d\n", a);
    }

    printf("Value of 'a' is %d\n", a);

    {
        int a = 3;
        printf("Value of 'a' is %d\n", a);
    }

    printf("Value of 'a' is %d", a);

    return 0;
}

Output:

Value of 'a' is 1

Value of 'a' is 2

Value of 'a' is 1

Value of 'a' is 3

Value of 'a' is 1

Answer 1

The auto keyword is now used to let the compiler deduce what the type should be.

What compiler are you using that lets you compile:

auto int a = 2;

?

I receive these errors:

E0084 invalid combination of type specifiers

C3530 'auto' cannot be combined with any other type-specifier

Answer 2

In C++, the auto keyword lets you define a variable with the type based on its initialization expression. something similar to var keyword in C#, though auto has much higher scope.

See the program you mentioned modified with to see the correct usage of auto.

#include <stdio.h>

int main()
{
    int a = 1;

    printf("Value of 'a' is %d\n", a);

    {
//correct declaration of auto variable. 
        auto a = 2;
        printf("Value of 'a' is %d\n", a);
    }

    printf("Value of 'a' is %d\n", a);

    {
        int a = 3;
        printf("Value of 'a' is %d\n", a);
    }

    printf("Value of 'a' is %d", a);

    return 0;
}

The auto can be used even as a function return type. see the following example.

#include <iostream>
// Create a function
auto myFunction() {
  return 3 * 2;
}

int main() {
  printf("Value of 'a' is %d", myFunction());
  return 0;
}

I recommend you to refer the following article for further details.

https://learn.microsoft.com/en-us/cpp/cpp/auto-cpp?view=msvc-170

Hope this helps