Use of 'auto func(int)' before deduction of 'auto' in C++14

Question

I have compiled following program in GCC using C++14.

#include 
using namespace std;

auto func(int i);

int main() 
{
    auto         


        

Answer 1

In your example, there is really no reason that you could not just move the implementation of the function before main():

#include 
using namespace std;  // you should avoid this, too

auto func(int i)
{
  if (i == 1)
    return i;
  else
    return func(i-1) + i;
}

int main()
{
    auto ret = func(5);
    return 0;
}

Otherwise you just can't use the auto keyword. In particular, you can't use auto in a recursive function which does not return anything. You have to use void. And that applies to lambda functions. For example:

int main()
{
    auto f = [](int i)
    {
        // ... do something with `i` ...
        if(i > 0)
        {
            f(i - 1);   // <-- same error here
        }
    }

    auto ret(func(5));
    return 0;
}

The call f(i - 1) has a problem. To fix it you have to replace the auto by the actual type:

int main()
{
    typedef std::function func_t;
    func_t f = [](int i)
    {
    ...

If you really want a function which support varying return types you want to use a template anyway, not auto. This is really only to help you with less typing, not so much as a way to allow "any type".