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