Introduce gdb::make_function_view

This adds gdb::make_function_view, which lets you create a function
view from a callable without specifying the function_view's template
parameter.  For example, this:

    auto lambda = [&] (int) { ... };
    auto fv = gdb::make_function_view (lambda);

instead of:

    auto lambda = [&] (int) { ... };
    gdb::function_view<void (int)> fv = lambda;

It is particularly useful if you have a template function with an
optional function_view parameter, whose type depends on the function's
template parameters.  Like:

    template<typename T>
    void my_function (T v, gdb::function_view<void(T)> callback = nullptr);

For such a function, the type of the callback argument you pass must
already be a function_view.  I.e., this wouldn't compile:

    auto lambda = [&] (int) { ... };
    my_function (1, lambda);

With gdb::make_function_view, you can write the call like so:

    auto lambda = [&] (int) { ... };
    my_function (1, gdb::make_function_view (lambda));

Unit tests included.

Tested by building with GCC 9.4, Clang 10, and GCC 4.8.5, on x86_64
GNU/Linux, and running the unit tests.

Change-Id: I5c4b3b4455ed6f0d8878cf1be189bea3ee63f626

1 files changed, 81 insertions, 1 deletions

diff --git a/gdb/unittests/function-view-selftests.c b/gdb/unittests/function-view-selftests.c
index 7af0245c57..726c22323a 100644
--- a/gdb/unittests/function-view-selftests.c
+++ b/gdb/unittests/function-view-selftests.c
@@ -61,7 +61,7 @@ struct plus_one_int_func_obj
 };
 
 static void
-run_tests ()
+test_function_view ()
 {
   /* A simple lambda.  */
   auto plus_one_lambda = [] (int val) { return ++val; };
@@ -168,6 +168,86 @@ run_tests ()
   SELF_CHECK (!check_op_eq_null);
 }
 
+/* A template function where the function_view type is dependent on a
+   template parameter.  */
+
+template<typename T>
+static int
+tmpl_func (T val, gdb::function_view<T (T)> callback)
+{
+  return callback (val) + 1;
+}
+
+static int
+make_fv_test_func (int val)
+{
+  return val + 1;
+}
+
+/* A function object with const operator().  */
+
+struct func_obj_const_op
+{
+  int operator() (int val) const
+  {
+    return val + 1;
+  }
+};
+
+/* A function object with non-const operator().  */
+
+struct func_obj_non_const_op
+{
+  int operator() (int val)
+  {
+    return val + 1;
+  }
+};
+
+static void
+test_make_function_view ()
+{
+  /* Function reference.  */
+  SELF_CHECK (3 == tmpl_func (1, gdb::make_function_view (make_fv_test_func)));
+
+  /* Function pointer.  */
+  SELF_CHECK (3 == tmpl_func (1, gdb::make_function_view (&make_fv_test_func)));
+
+  /* Reference to const and non-const function pointers.  */
+  typedef int (*func_ptr) (int);
+  func_ptr ptr = make_fv_test_func;
+  const func_ptr cptr = make_fv_test_func;
+  SELF_CHECK (3 == tmpl_func (1, gdb::make_function_view (ptr)));
+  SELF_CHECK (3 == tmpl_func (1, gdb::make_function_view (cptr)));
+
+  /* Lambdas.  */
+
+  auto lambda = [] (int val) -> int { return val + 1; };
+
+  /* This wouldn't compile, since tmpl_func is a template and its
+     function_view argument's callable type is a dependent type.  The
+     passed argument must be of the exact type of the function's
+     parameter.  */
+  // SELF_CHECK (3 == tmpl_func (1, lambda));
+
+  SELF_CHECK (3 == tmpl_func (1, gdb::make_function_view (lambda)));
+
+  /* Regular function objects.  */
+
+  func_obj_non_const_op fobj;
+  SELF_CHECK (3 == tmpl_func (1, gdb::make_function_view (fobj)));
+
+  func_obj_const_op cfobj;
+  SELF_CHECK (3 == tmpl_func (1, gdb::make_function_view (cfobj)));
+}
+
+static void
+run_tests ()
+{
+  test_function_view ();
+  test_make_function_view ();
+}
+
 } /* namespace function_view */
 } /* namespace selftests */