I expected the auto keyword to deduce the type of a variable from an intializer once and keep that type throughout the code. To my surprise, my compiler (g 9.3.0) allows me to change it's type and it still works. This is sort of acceptable to me when I first use the variable as an int and then as a float. But when I use the auto keyword to declare a string and then assign a float value to that variable, the compiler doesn't throw an error but also doesn't print the value after the float assignment. Can someone explain why it allows assigning a float value to a string variable in the first place? Does the compiler just accept the new assignment each time? Or does it throw some kind of exception that I am not able to catch? Code below -

#include <iostream>

int main()
{
 auto x = '2';
 
 std::cout << x << std::endl;
 
 x = 3.0; // Why is this allowed?
 
 std::cout << x << std::endl; // This won't print
 
 return 0;
}

CodePudding user response：

To show you wha't going on I extended the example with some compile time type checks:

#include <type_traits>
#include <iostream>

int main()
{
    auto x = '2';

    // decltype(x) is the type for variable x
    // compare it at compile time with char type
    static_assert(std::is_same_v<decltype(x), char>);

    std::cout << x << std::endl;

    x = 3.0; // Why is this allowed? because it does a narrowing conversion from double to char
    // my compiler even gives this warning :
    // main.cpp(11,6): warning C4244: '=': conversion from 'double' to 'char', possible loss of data
    
    // type of x is still the same and it is still a char
    static_assert(std::is_same_v<decltype(x), char>);

    std::cout << static_cast<int>(x) << std::endl; // the cast is here to be able to print the actual value of the char

    return 0;
}

CodePudding user response：

But when I use the auto keyword to declare a string

You are confused. This code declares a variable of type char:

auto x = '2';

If you do want to declare a string you have to use double quotes:

auto x = "2";

The subsequent assignment of 3.0 will not compile with this change.