rtc::FunctionView improvements: accept function pointers and nullptr
BUG=webrtc:5801
Review-Url: https://codereview.webrtc.org/2375023004
Cr-Original-Commit-Position: refs/heads/master@{#14483}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 607d9d7f604f7d8cc4a427de38db06eb674482b9
diff --git a/base/function_view.h b/base/function_view.h
index 6a7c910..861bccf 100644
--- a/base/function_view.h
+++ b/base/function_view.h
@@ -14,16 +14,27 @@
#include <type_traits>
#include <utility>
+#include "webrtc/base/checks.h"
+
// Just like std::function, FunctionView will wrap any callable and hide its
// actual type, exposing only its signature. But unlike std::function,
// FunctionView doesn't own its callable---it just points to it. Thus, it's a
// good choice mainly as a function argument when the callable argument will
// not be called again once the function has returned.
//
-// TODO(kwiberg): FunctionView doesn't work with function pointers, just with
-// lambdas. It's trivial to work around this by wrapping the function pointer
-// in a stateless lambda, but it's tedious so it'd be nice to not have to do
-// it.
+// Its constructors are implicit, so that callers won't have to convert lambdas
+// and other callables to FunctionView<Blah(Blah, Blah)> explicitly. This is
+// safe because FunctionView is only a reference to the real callable.
+//
+// Example use:
+//
+// void SomeFunction(rtc::FunctionView<int(int)> index_transform);
+// ...
+// SomeFunction([](int i) { return 2 * i + 1; });
+//
+// Note: FunctionView is tiny (essentially just two pointers) and trivially
+// copyable, so it's probably cheaper to pass it by value than by const
+// reference.
namespace rtc {
@@ -33,36 +44,85 @@
template <typename RetT, typename... ArgT>
class FunctionView<RetT(ArgT...)> final {
public:
- // This constructor is implicit, so that callers won't have to convert
- // lambdas and other callables to FunctionView<Blah(Blah, Blah)> explicitly.
- // This is safe because FunctionView is only a reference to the real
- // callable.
- //
- // We jump through some template metaprogramming hoops to ensure that this
- // constructor does *not* accept FunctionView arguments. That way, copy
- // construction, assignment, swap etc. will all do the obvious thing (because
- // they use the implicitly-declared copy constructor and copy assignment),
- // and we will never get a FunctionView object that points to another
- // FunctionView.
- template <typename F,
- typename std::enable_if<!std::is_same<
- FunctionView,
- typename std::remove_cv<typename std::remove_reference<
- F>::type>::type>::value>::type* = nullptr>
+ // Constructor for lambdas and other callables; it accepts every type of
+ // argument except those noted in its enable_if call.
+ template <
+ typename F,
+ typename std::enable_if<
+ // Not for function pointers; we have another constructor for that
+ // below.
+ !std::is_function<typename std::remove_pointer<
+ typename std::remove_reference<F>::type>::type>::value &&
+
+ // Not for nullptr; we have another constructor for that below.
+ !std::is_same<std::nullptr_t,
+ typename std::remove_cv<F>::type>::value &&
+
+ // Not for FunctionView objects; we have another constructor for that
+ // (the implicitly declared copy constructor).
+ !std::is_same<FunctionView,
+ typename std::remove_cv<typename std::remove_reference<
+ F>::type>::type>::value>::type* = nullptr>
FunctionView(F&& f)
- : f_(&f), call_(Call<typename std::remove_reference<F>::type>) {}
+ : call_(CallVoidPtr<typename std::remove_reference<F>::type>) {
+ f_.void_ptr = &f;
+ }
+
+ // Constructor that accepts function pointers. If the argument is null, the
+ // result is an empty FunctionView.
+ template <
+ typename F,
+ typename std::enable_if<std::is_function<typename std::remove_pointer<
+ typename std::remove_reference<F>::type>::type>::value>::type* =
+ nullptr>
+ FunctionView(F&& f)
+ : call_(f ? CallFunPtr<typename std::remove_pointer<F>::type> : nullptr) {
+ f_.fun_ptr = reinterpret_cast<void (*)()>(f);
+ }
+
+ // Constructor that accepts nullptr. It creates an empty FunctionView.
+ template <typename F,
+ typename std::enable_if<std::is_same<
+ std::nullptr_t,
+ typename std::remove_cv<F>::type>::value>::type* = nullptr>
+ FunctionView(F&& f) : call_(nullptr) {}
+
+ // Default constructor. Creates an empty FunctionView.
+ FunctionView() : call_(nullptr) {}
RetT operator()(ArgT... args) const {
+ RTC_DCHECK(call_);
return call_(f_, std::forward<ArgT>(args)...);
}
+ // Returns true if we have a function, false if we don't (i.e., we're null).
+ explicit operator bool() const { return !!call_; }
+
private:
+ union VoidUnion {
+ void* void_ptr;
+ void (*fun_ptr)();
+ };
+
template <typename F>
- static RetT Call(void* f, ArgT... args) {
- return (*static_cast<F*>(f))(std::forward<ArgT>(args)...);
+ static RetT CallVoidPtr(VoidUnion vu, ArgT... args) {
+ return (*static_cast<F*>(vu.void_ptr))(std::forward<ArgT>(args)...);
}
- void* f_;
- RetT (*call_)(void* f, ArgT... args);
+ template <typename F>
+ static RetT CallFunPtr(VoidUnion vu, ArgT... args) {
+ return (reinterpret_cast<typename std::add_pointer<F>::type>(vu.fun_ptr))(
+ std::forward<ArgT>(args)...);
+ }
+
+ // A pointer to the callable thing, with type information erased. It's a
+ // union because we have to use separate types depending on if the callable
+ // thing is a function pointer or something else.
+ VoidUnion f_;
+
+ // Pointer to a dispatch function that knows the type of the callable thing
+ // that's stored in f_, and how to call it. A FunctionView object is empty
+ // (null) iff call_ is null.
+ RetT (*call_)(VoidUnion, ArgT...);
};
} // namespace rtc
diff --git a/base/function_view_unittest.cc b/base/function_view_unittest.cc
index 3ca2df4..c769fe6 100644
--- a/base/function_view_unittest.cc
+++ b/base/function_view_unittest.cc
@@ -19,21 +19,28 @@
namespace {
int CallWith33(rtc::FunctionView<int(int)> fv) {
- return fv(33);
+ return fv ? fv(33) : -1;
+}
+
+int Add33(int x) {
+ return x + 33;
}
} // namespace
-// Test the main use case of FunctionView: implicitly converting a lambda
-// function argument.
+// Test the main use case of FunctionView: implicitly converting a callable
+// argument.
TEST(FunctionViewTest, ImplicitConversion) {
EXPECT_EQ(38, CallWith33([](int x) { return x + 5; }));
+ EXPECT_EQ(66, CallWith33(Add33));
+ EXPECT_EQ(-1, CallWith33(nullptr));
}
TEST(FunctionViewTest, IntIntLambdaWithoutState) {
auto f = [](int x) { return x + 1; };
EXPECT_EQ(18, f(17));
rtc::FunctionView<int(int)> fv(f);
+ EXPECT_TRUE(fv);
EXPECT_EQ(18, fv(17));
}
@@ -41,12 +48,34 @@
int x = 13;
auto f = [x]() mutable { return ++x; };
rtc::FunctionView<int()> fv(f);
+ EXPECT_TRUE(fv);
EXPECT_EQ(14, f());
EXPECT_EQ(15, fv());
EXPECT_EQ(16, f());
EXPECT_EQ(17, fv());
}
+TEST(FunctionViewTest, IntIntFunction) {
+ rtc::FunctionView<int(int)> fv(Add33);
+ EXPECT_TRUE(fv);
+ EXPECT_EQ(50, fv(17));
+}
+
+TEST(FunctionViewTest, IntIntFunctionPointer) {
+ rtc::FunctionView<int(int)> fv(&Add33);
+ EXPECT_TRUE(fv);
+ EXPECT_EQ(50, fv(17));
+}
+
+TEST(FunctionViewTest, Null) {
+ // These two call constructors that statically construct null FunctionViews.
+ EXPECT_FALSE(rtc::FunctionView<int()>());
+ EXPECT_FALSE(rtc::FunctionView<int()>(nullptr));
+
+ // This calls the constructor for function pointers.
+ EXPECT_FALSE(rtc::FunctionView<int()>(reinterpret_cast<int(*)()>(0)));
+}
+
// Ensure that FunctionView handles move-only arguments and return values.
TEST(FunctionViewTest, UniquePtrPassthrough) {
auto f = [](std::unique_ptr<int> x) { return x; };
@@ -111,8 +140,8 @@
}
// Ensure that when you copy-construct a FunctionView, the new object points to
-// the same function as the old one, as opposed to the new object pointing to
-// the old one.
+// the same function as the old one (as opposed to the new object pointing to
+// the old one).
TEST(FunctionViewTest, CopyConstructorChaining) {
auto f17 = [] { return 17; };
rtc::FunctionView<int()> fv1(f17);
@@ -126,14 +155,14 @@
}
// Ensure that when you assign one FunctionView to another, we actually make a
-// copy as opposed to making the second FunctionView point to the first one.
+// copy (as opposed to making the second FunctionView point to the first one).
TEST(FunctionViewTest, CopyAssignmentChaining) {
auto f17 = [] { return 17; };
rtc::FunctionView<int()> fv1(f17);
- auto f3 = [] { return 3; };
- rtc::FunctionView<int()> fv2(f3);
+ rtc::FunctionView<int()> fv2;
+ EXPECT_TRUE(fv1);
EXPECT_EQ(17, fv1());
- EXPECT_EQ(3, fv2());
+ EXPECT_FALSE(fv2);
fv2 = fv1;
EXPECT_EQ(17, fv1());
EXPECT_EQ(17, fv2());
