Retrieving the type of auto in C++11 without executing the program

Question

I have some C++11 code using the auto inferred type that I have to convert to C++98. How would I go about converting the code, substituting in the actual type f

Answer 1

It is going to be a PITA, but you can declare an incomplete struct template accepting a single type parameter.

Given the variable x you want to know the type of, you can use the struct with decltype(x) and that will lead to a compiler error that will show you the inferred type.

For example:

template struct S;

int main() {
    auto x = ...;
    S();
}

Live demo

which will produce an error message in the form:

error: implicit instantiation of undefined template 'S' (clang++)
error: invalid use of incomplete type 'struct S' (g++)

with X being the inferred type. In this particular case the type is int.

Trivia: This has been recommended by Scott Meyer in one of the recent NDC 2014's videos (I don't remember which one).