[PATCH 3/9] selftest.h: add temp_override fixture

[David Malcolm <dmalcolm at redhat dot com>]

We have a lot of global state in our code.  Ideally we'd reduce the
amount of such global state, but a prerequisite for sane refactoring
is having automated testing in place to ensure that the refactoring
doesn't break anything.

However, the global state itself makes it hard to write such automated
testing.

To break this Catch-22, this patch introduces a class temp_override,
for temporarily assigning a value to a global variable, saving the old
value, and then restoring that old value in a dtor.

gcc/ChangeLog:
	* selftest.h (selftest::temp_override): New class.
---
 gcc/selftest.h | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/gcc/selftest.h b/gcc/selftest.h
index e5f5c60..4c50217 100644
--- a/gcc/selftest.h
+++ b/gcc/selftest.h
@@ -153,6 +153,35 @@ for_each_line_table_case (void (*testcase) (const line_table_case &));
 
 extern char *read_file (const location &loc, const char *path);
 
+/* A fixture for temporarily overriding a global variable with a new
+   value.  The original value of the variable is captured in the ctor,
+   and restored in the dtor.  */
+
+template <typename T>
+class temp_override
+{
+ public:
+  temp_override (T& var, T new_value)
+  : m_var (var),
+    /* Record the current value of VAR.  */
+    m_old_value (var)
+  {
+    /* Set the var to the new value.  */
+    m_var = new_value;
+  }
+
+  ~temp_override ()
+  {
+    /* Restore the value of the variable to that stored in the
+       ctor.  */
+    m_var = m_old_value;
+  }
+
+ private:
+  T& m_var;
+  T m_old_value;
+};
+
 /* Declarations for specific families of tests (by source file), in
    alphabetical order.  */
 extern void bitmap_c_tests ();
-- 
1.8.5.3