Print types of arbitrary C++ expressions

Question

I'm interested in writing a tool for teaching purposes that evaluates C++ expressions and prints their types. Essentially, my thinking is that my students could type in any expression, and the program would echo back the type of the expression. Is there an existing tool that already does this? If not, is there a pretty easy way to do it by integrating with an existing compiler and calling into its debugger or API? I've been told, for example, that Clang has a fairly complete compiler API, perhaps there's some way to just pass a string into Clang along with the appropriate include directives and have it spit out a type?

I realize that this is potentially a huge project if there's nothing close to this existing today. I just thought it would have significant educational value, so it seemed like it was worth checking.

Answer 1

I came up with an answer inspired by Ben Voigt's comments. Just make a bug and let the compiler tell you the type which caused it:

template <typename T> void foo(T); // No definition

int main() {
  foo(1 + 3.0);
}

Result:

In function `main':
prog.cpp:(.text+0x13): undefined reference to `void foo<double>(double)'

Also, since you execute nothing but the compiler, you're pretty safe. No sandboxing needed, really. If you get anything other than "undefined reference to void foo<T>(T)", it wasn't an expression.

[edit] How would you put this into a tool? Simple, with macro's

// TestHarness.cpp
// Slight variation to make it a compile error
template <typename T> void foo(T) { typename T::bar t = T::bar ; }

int main() {
  foo(EXPR);
}

Now compile with $(CC) /D=(EXPR) TestHarness.cpp. Saves you from rebuilding the input file every time.