Does auto deduce types in compile time or run time in C++11?

Question

I googled and found a solution in stackoverflow C++ 11 auto compile time or runtime?. It is an duplicate but not entirely, the answer stated there was compile time. Is it really compile time? Let us consider the below example.

auto give_something(bool mybool)
{
    if (mybool)
        return string("auto");
    return 6.66f;
}

int main()
{
    bool mybool = (rand() % 2) ? true : false;
    auto x = give_something(mybool); // how type of x is deduced?
    return 0;
}

Here the type of x and return type of give_something can't be deduced while compiling (my assumption). It should be deduced only at run time. So auto compile time or run time or both?

`Does auto deduce types in compile time or run time in C++11?`: **At compile time**, it follows the rules of template argument deduction with a few exceptions. — 101010, Apr 18 '16 at 13:46
All types must be known by the compiler at time of compilation. — Some programmer dude, Apr 18 '16 at 13:47

score 4 · Accepted Answer · answered Apr 18 '16 at 13:47

4

auto works at compile time.

You are correct that the return type of give_something can't be deduced at compile-time. In such a case, your code will not compile. Clang gives this error message for your example:

main.cpp:8:5: error: 'auto' in return type deduced as 'float' here but deduced 
                      as 'std::__cxx11::basic_string<char>' in earlier return 
                      statement
    return 6.66f;

answered Apr 18 '16 at 13:47

TartanLlama

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I didn't compiled and check just my thought. – jblixr Apr 18 '16 at 13:51
2

@jblixr I'd suggest attempting to compile the code next time, as your compiler would essentially give you the answer. – TartanLlama Apr 18 '16 at 13:52
1

So using `auto` I cannot return two different types from a function. – jblixr Apr 18 '16 at 13:54
1

@jblixr Nope. You can't return different types from a function at all. – TartanLlama Apr 18 '16 at 13:57

Does auto deduce types in compile time or run time in C++11?

1 Answers1