Clang: get int value from clang::Expr

getIntFromExpr.cpp

/// ASTContext needed, you can get it from CompilerInstance
int64_t getIntFromExpr(const Expr *E, const ASTContext *C) {
    clang::Expr::EvalResult R;
    if (E->EvaluateAsInt(R, *C)) {
        return R.Val.getInt().getExtValue();
    }
    /// Could be that returning -1 is not safe, you should modify this
    return -1;
}