Adding upstream version 14.2.21.upstream/14.2.21upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+This page discusses the design of new Google Mock features.
+
+
+
+# Macros for Defining Actions #
+
+## Problem ##
+
+Due to the lack of closures in C++, it currently requires some
+non-trivial effort to define a custom action in Google Mock.  For
+example, suppose you want to "increment the value pointed to by the
+second argument of the mock function and return it", you could write:
+
+```
+int IncrementArg1(Unused, int* p, Unused) {
+  return ++(*p);
+}
+
+... WillOnce(Invoke(IncrementArg1));
+```
+
+There are several things unsatisfactory about this approach:
+
+  * Even though the action only cares about the second argument of the mock function, its definition needs to list other arguments as dummies.  This is tedious.
+  * The defined action is usable only in mock functions that takes exactly 3 arguments - an unnecessary restriction.
+  * To use the action, one has to say `Invoke(IncrementArg1)`, which isn't as nice as `IncrementArg1()`.
+
+The latter two problems can be overcome using `MakePolymorphicAction()`,
+but it requires much more boilerplate code:
+
+```
+class IncrementArg1Action {
+ public:
+  template <typename Result, typename ArgumentTuple>
+  Result Perform(const ArgumentTuple& args) const {
+    return ++(*tr1::get<1>(args));
+  }
+};
+
+PolymorphicAction<IncrementArg1Action> IncrementArg1() {
+  return MakePolymorphicAction(IncrementArg1Action());
+}
+
+... WillOnce(IncrementArg1());
+```
+
+Our goal is to allow defining custom actions with the least amount of
+boiler-plate C++ requires.
+
+## Solution ##
+
+We propose to introduce a new macro:
+```
+ACTION(name) { statements; }
+```
+
+Using this in a namespace scope will define an action with the given
+name that executes the statements.  Inside the statements, you can
+refer to the K-th (0-based) argument of the mock function as `argK`.
+For example:
+```
+ACTION(IncrementArg1) { return ++(*arg1); }
+```
+allows you to write
+```
+... WillOnce(IncrementArg1());
+```
+
+Note that you don't need to specify the types of the mock function
+arguments, as brevity is a top design goal here.  Rest assured that
+your code is still type-safe though: you'll get a compiler error if
+`*arg1` doesn't support the `++` operator, or if the type of
+`++(*arg1)` isn't compatible with the mock function's return type.
+
+Another example:
+```
+ACTION(Foo) {
+  (*arg2)(5);
+  Blah();
+  *arg1 = 0;
+  return arg0;
+}
+```
+defines an action `Foo()` that invokes argument #2 (a function pointer)
+with 5, calls function `Blah()`, sets the value pointed to by argument
+#1 to 0, and returns argument #0.
+
+For more convenience and flexibility, you can also use the following
+pre-defined symbols in the body of `ACTION`:
+
+| `argK_type` | The type of the K-th (0-based) argument of the mock function |
+|:------------|:-------------------------------------------------------------|
+| `args`      | All arguments of the mock function as a tuple                |
+| `args_type` | The type of all arguments of the mock function as a tuple    |
+| `return_type` | The return type of the mock function                         |
+| `function_type` | The type of the mock function                                |
+
+For example, when using an `ACTION` as a stub action for mock function:
+```
+int DoSomething(bool flag, int* ptr);
+```
+we have:
+| **Pre-defined Symbol** | **Is Bound To** |
+|:-----------------------|:----------------|
+| `arg0`                 | the value of `flag` |
+| `arg0_type`            | the type `bool` |
+| `arg1`                 | the value of `ptr` |
+| `arg1_type`            | the type `int*` |
+| `args`                 | the tuple `(flag, ptr)` |
+| `args_type`            | the type `std::tr1::tuple<bool, int*>` |
+| `return_type`          | the type `int`  |
+| `function_type`        | the type `int(bool, int*)` |
+
+## Parameterized actions ##
+
+Sometimes you'll want to parameterize the action.   For that we propose
+another macro
+```
+ACTION_P(name, param) { statements; }
+```
+
+For example,
+```
+ACTION_P(Add, n) { return arg0 + n; }
+```
+will allow you to write
+```
+// Returns argument #0 + 5.
+... WillOnce(Add(5));
+```
+
+For convenience, we use the term _arguments_ for the values used to
+invoke the mock function, and the term _parameters_ for the values
+used to instantiate an action.
+
+Note that you don't need to provide the type of the parameter either.
+Suppose the parameter is named `param`, you can also use the
+Google-Mock-defined symbol `param_type` to refer to the type of the
+parameter as inferred by the compiler.
+
+We will also provide `ACTION_P2`, `ACTION_P3`, and etc to support
+multi-parameter actions.  For example,
+```
+ACTION_P2(ReturnDistanceTo, x, y) {
+  double dx = arg0 - x;
+  double dy = arg1 - y;
+  return sqrt(dx*dx + dy*dy);
+}
+```
+lets you write
+```
+... WillOnce(ReturnDistanceTo(5.0, 26.5));
+```
+
+You can view `ACTION` as a degenerated parameterized action where the
+number of parameters is 0.
+
+## Advanced Usages ##
+
+### Overloading Actions ###
+
+You can easily define actions overloaded on the number of parameters:
+```
+ACTION_P(Plus, a) { ... }
+ACTION_P2(Plus, a, b) { ... }
+```
+
+### Restricting the Type of an Argument or Parameter ###
+
+For maximum brevity and reusability, the `ACTION*` macros don't let
+you specify the types of the mock function arguments and the action
+parameters.  Instead, we let the compiler infer the types for us.
+
+Sometimes, however, we may want to be more explicit about the types.
+There are several tricks to do that.  For example:
+```
+ACTION(Foo) {
+  // Makes sure arg0 can be converted to int.
+  int n = arg0;
+  ... use n instead of arg0 here ...
+}
+
+ACTION_P(Bar, param) {
+  // Makes sure the type of arg1 is const char*.
+  ::testing::StaticAssertTypeEq<const char*, arg1_type>();
+
+  // Makes sure param can be converted to bool.
+  bool flag = param;
+}
+```
+where `StaticAssertTypeEq` is a compile-time assertion we plan to add to
+Google Test (the name is chosen to match `static_assert` in C++0x).
+
+### Using the ACTION Object's Type ###
+
+If you are writing a function that returns an `ACTION` object, you'll
+need to know its type.  The type depends on the macro used to define
+the action and the parameter types.  The rule is relatively simple:
+| **Given Definition** | **Expression** | **Has Type** |
+|:---------------------|:---------------|:-------------|
+| `ACTION(Foo)`        | `Foo()`        | `FooAction`  |
+| `ACTION_P(Bar, param)` | `Bar(int_value)` | `BarActionP<int>` |
+| `ACTION_P2(Baz, p1, p2)` | `Baz(bool_value, int_value)` | `BazActionP2<bool, int>` |
+| ...                  | ...            | ...          |
+
+Note that we have to pick different suffixes (`Action`, `ActionP`,
+`ActionP2`, and etc) for actions with different numbers of parameters,
+or the action definitions cannot be overloaded on the number of
+parameters.
+
+## When to Use ##
+
+While the new macros are very convenient, please also consider other
+means of implementing actions (e.g. via `ActionInterface` or
+`MakePolymorphicAction()`), especially if you need to use the defined
+action a lot.  While the other approaches require more work, they give
+you more control on the types of the mock function arguments and the
+action parameters, which in general leads to better compiler error
+messages that pay off in the long run.  They also allow overloading
+actions based on parameter types, as opposed to just the number of
+parameters.
+
+## Related Work ##
+
+As you may have realized, the `ACTION*` macros resemble closures (also
+known as lambda expressions or anonymous functions).  Indeed, both of
+them seek to lower the syntactic overhead for defining a function.
+
+C++0x will support lambdas, but they are not part of C++ right now.
+Some non-standard libraries (most notably BLL or Boost Lambda Library)
+try to alleviate this problem.  However, they are not a good choice
+for defining actions as:
+
+  * They are non-standard and not widely installed.  Google Mock only depends on standard libraries and `tr1::tuple`, which is part of the new C++ standard and comes with gcc 4+.  We want to keep it that way.
+  * They are not trivial to learn.
+  * They will become obsolete when C++0x's lambda feature is widely supported.  We don't want to make our users use a dying library.
+  * Since they are based on operators, they are rather ad hoc: you cannot use statements, and you cannot pass the lambda arguments to a function, for example.
+  * They have subtle semantics that easily confuses new users.  For example, in expression `_1++ + foo++`, `foo` will be incremented only once where the expression is evaluated, while `_1` will be incremented every time the unnamed function is invoked.  This is far from intuitive.
+
+`ACTION*` avoid all these problems.
+
+## Future Improvements ##
+
+There may be a need for composing `ACTION*` definitions (i.e. invoking
+another `ACTION` inside the definition of one `ACTION*`).  We are not
+sure we want it yet, as one can get a similar effect by putting
+`ACTION` definitions in function templates and composing the function
+templates.  We'll revisit this based on user feedback.
+
+The reason we don't allow `ACTION*()` inside a function body is that
+the current C++ standard doesn't allow function-local types to be used
+to instantiate templates.  The upcoming C++0x standard will lift this
+restriction.  Once this feature is widely supported by compilers, we
+can revisit the implementation and add support for using `ACTION*()`
+inside a function.
+
+C++0x will also support lambda expressions.  When they become
+available, we may want to support using lambdas as actions.
+
+# Macros for Defining Matchers #
+
+Once the macros for defining actions are implemented, we plan to do
+the same for matchers:
+
+```
+MATCHER(name) { statements; }
+```
+
+where you can refer to the value being matched as `arg`.  For example,
+given:
+
+```
+MATCHER(IsPositive) { return arg > 0; }
+```
+
+you can use `IsPositive()` as a matcher that matches a value iff it is
+greater than 0.
+
+We will also add `MATCHER_P`, `MATCHER_P2`, and etc for parameterized
+matchers.
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