+++ /dev/null
-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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