我有一个使用pimpl习语的界面，但界面需要是可重入的。但是，调用线程不需要知道锁定。这是四部分问题和一部分无偿设计的C ++ 11示例（示例包括解决我遇到的几个类似FAQ的问题：locking，pimpl，rvalue和C ++ 11的答案在质量上有些不确定。）
在标题中，example.hpp：
#ifndef EXAMPLE_HPP
#define EXAMPLE_HPP
#include <memory>
#include <string>
#ifndef BOOST_THREAD_SHARED_MUTEX_HPP
# include <boost/thread/shared_mutex.hpp>
#endif
namespace stackoverflow {
class Example final {
public:
typedef ::boost::shared_mutex shared_mtx_t;
typedef ::boost::shared_lock< shared_mtx_t > shared_lock_t;
typedef ::boost::unique_lock< shared_mtx_t > unique_lock_t;
Example();
Example(const std::string& initial_foo);
~Example();
Example(const Example&) = delete; // Prevent copying
Example& operator=(const Example&) = delete; // Prevent assignment
// Example getter method that supports rvalues
std::string foo() const;
// Example setter method using perfect forwarding & move semantics. Anything
// that's std::string-like will work as a parameter.
template<typename T>
bool foo_set(T&& new_val);
// Begin foo_set() variants required to deal with C types (e.g. char[],
// char*). The rest of the foo_set() methods here are *NOT* required under
// normal circumstances.
// Setup a specialization for const char[] that simply forwards along a
// std::string. This is preferred over having to explicitly instantiate a
// bunch of const char[N] templates or possibly std::decay a char[] to a
// char* (i.e. using a std::string as a container is a Good Thing(tm)).
//
// Also, without this, it is required to explicitly instantiate the required
// variants of const char[N] someplace. For example, in example.cpp:
//
// template bool Example::foo_set<const char(&)[6]>(char const (&)[6]);
// template bool Example::foo_set<const char(&)[7]>(char const (&)[7]);
// template bool Example::foo_set<const char(&)[8]>(char const (&)[8]);
// ...
//
// Eww. Best to just forward to wrap new_val in a std::string and proxy
// along the call to foo_set<std::string>().
template<std::size_t N>
bool foo_set(const char (&new_val)[N]) { return foo_set(std::string(new_val, N)); }
// Inline function overloads to support null terminated char* && const
// char* arguments. If there's a way to reduce this duplication with
// templates, I'm all ears because I wasn't able to generate a templated
// versions that didn't conflict with foo_set<T&&>().
bool foo_set(char* new_val) { return foo_set(std::string(new_val)); }
bool foo_set(const char* new_val) { return foo_set(std::string(new_val)); }
// End of the foo_set() overloads.
// Example getter method for a POD data type
bool bar(const std::size_t len, char* dst) const;
std::size_t bar_capacity() const;
// Example setter that uses a unique lock to access foo()
bool bar_set(const std::size_t len, const char* src);
// Question #1: I can't find any harm in making Impl public because the
// definition is opaque. Making Impl public, however, greatly helps with
// implementing Example, which does have access to Example::Impl's
// interface. This is also preferre, IMO, over using friend.
class Impl;
private:
mutable shared_mtx_t rw_mtx_;
std::unique_ptr<Impl> impl_;
};
} // namespace stackoverflow
#endif // EXAMPLE_HPP
然后在实施中：
#include "example.hpp"
#include <algorithm>
#include <cstring>
#include <utility>
namespace stackoverflow {
class Example;
class Example::Impl;
#if !defined(_MSC_VER) || _MSC_VER > 1600
// Congratulations!, you're using a compiler that isn't broken
// Explicitly instantiate std::string variants
template bool Example::foo_set<std::string>(std::string&& src);
template bool Example::foo_set<std::string&>(std::string& src);
template bool Example::foo_set<const std::string&>(const std::string& src);
// The following isn't required because of the array Example::foo_set()
// specialization, but I'm leaving it here for reference.
//
// template bool Example::foo_set<const char(&)[7]>(char const (&)[7]);
#else
// MSVC workaround: msvc_rage_hate() isn't ever called, but use it to
// instantiate all of the required templates.
namespace {
void msvc_rage_hate() {
Example e;
const std::string a_const_str("a");
std::string a_str("b");
e.foo_set(a_const_str);
e.foo_set(a_str);
e.foo_set("c");
e.foo_set(std::string("d"));
}
} // anon namespace
#endif // _MSC_VER
// Example Private Implementation
class Example::Impl final {
public:
// ctors && obj boilerplate
Impl();
Impl(const std::string& init_foo);
~Impl() = default;
Impl(const Impl&) = delete;
Impl& operator=(const Impl&) = delete;
// Use a template because we don't care which Lockable concept or LockType
// is being used, just so long as a lock is held.
template <typename LockType>
bool bar(LockType& lk, std::size_t len, char* dst) const;
template <typename LockType>
std::size_t bar_capacity(LockType& lk) const;
// bar_set() requires a unique lock
bool bar_set(unique_lock_t& lk, const std::size_t len, const char* src);
template <typename LockType>
std::string foo(LockType& lk) const;
template <typename T>
bool foo_set(unique_lock_t& lk, T&& src);
private:
// Example datatype that supports rvalue references
std::string foo_;
// Example POD datatype that doesn't support rvalue
static const std::size_t bar_capacity_ = 16;
char bar_[bar_capacity_ + 1];
};
// Example delegating ctor
Example::Impl::Impl() : Impl("default foo value") {}
Example::Impl::Impl(const std::string& init_foo) : foo_{init_foo} {
std::memset(bar_, 99 /* ASCII 'c' */, bar_capacity_);
bar_[bar_capacity_] = '\0'; // null padding
}
template <typename LockType>
bool
Example::Impl::bar(LockType& lk, const std::size_t len, char* dst) const {
BOOST_ASSERT(lk.owns_lock());
if (len != bar_capacity(lk))
return false;
std::memcpy(dst, bar_, len);
return true;
}
template <typename LockType>
std::size_t
Example::Impl::bar_capacity(LockType& lk) const {
BOOST_ASSERT(lk.owns_lock());
return Impl::bar_capacity_;
}
bool
Example::Impl::bar_set(unique_lock_t &lk, const std::size_t len, const char* src) {
BOOST_ASSERT(lk.owns_lock());
// Return false if len is bigger than bar_capacity or the values are
// identical
if (len > bar_capacity(lk) || foo(lk) == src)
return false;
// Copy src to bar_, a side effect of updating foo_ if they're different
std::memcpy(bar_, src, std::min(len, bar_capacity(lk)));
foo_set(lk, std::string(src, len));
return true;
}
template <typename LockType>
std::string
Example::Impl::foo(LockType& lk) const {
BOOST_ASSERT(lk.owns_lock());
return foo_;
}
template <typename T>
bool
Example::Impl::foo_set(unique_lock_t &lk, T&& src) {
BOOST_ASSERT(lk.owns_lock());
if (foo_ == src) return false;
foo_ = std::move(src);
return true;
}
// Example Public Interface
Example::Example() : impl_(new Impl{}) {}
Example::Example(const std::string& init_foo) : impl_(new Impl{init_foo}) {}
Example::~Example() = default;
bool
Example::bar(const std::size_t len, char* dst) const {
shared_lock_t lk(rw_mtx_);
return impl_->bar(lk, len , dst);
}
std::size_t
Example::bar_capacity() const {
shared_lock_t lk(rw_mtx_);
return impl_->bar_capacity(lk);
}
bool
Example::bar_set(const std::size_t len, const char* src) {
unique_lock_t lk(rw_mtx_);
return impl_->bar_set(lk, len, src);
}
std::string
Example::foo() const {
shared_lock_t lk(rw_mtx_);
return impl_->foo(lk);
}
template<typename T>
bool
Example::foo_set(T&& src) {
unique_lock_t lk(rw_mtx_);
return impl_->foo_set(lk, std::forward<T>(src));
}
} // namespace stackoverflow
我的问题是：
-O4启用Link-Time Optimization时，链接器应该可以绕过std::unique_ptr的取消引用开销。有人核实过吗？foo_set("asdf")并正确显示示例链接？我们在确定const char[6]的正确显式模板实例化时遇到了问题。现在我已经设置了一个模板专门化，它创建了一个std::string对象并代理了对foo_set（）的调用。考虑到所有事情，这似乎是最好的前进方式，但我想知道如何传递“asdf”和std::decay结果。关于锁定策略，由于以下几个原因，我对此产生了明显的偏见：
我已经读过ACE也使用这种类型的锁定策略，但我欢迎来自ACE用户或其他人的一些现实批评：将锁定作为界面的必需部分传递。< / p>
为了完整起见，这里有一个example_main.cpp供人们去咀嚼。
#include <sysexits.h>
#include <cassert>
#include <iostream>
#include <memory>
#include <stdexcept>
#include "example.hpp"
int
main(const int /*argc*/, const char** /*argv*/) {
using std::cout;
using std::endl;
using stackoverflow::Example;
{
Example e;
cout << "Example's foo w/ empty ctor arg: " << e.foo() << endl;
}
{
Example e("foo");
cout << "Example's foo w/ ctor arg: " << e.foo() << endl;
}
try {
Example e;
{ // Test assignment from std::string
std::string str("cccccccc");
e.foo_set(str);
assert(e.foo() == "cccccccc"); // Value is the same
assert(str.empty()); // Stole the contents of a_str
}
{ // Test assignment from a const std::string
const std::string const_str("bbbbbbb");
e.foo_set(const_str);
assert(const_str == "bbbbbbb"); // Value is the same
assert(const_str.c_str() != e.foo().c_str()); // Made a copy
}
{
// Test a const char[7] and a temporary std::string
e.foo_set("foobar");
e.foo_set(std::string("ddddd"));
}
{ // Test char[7]
char buf[7] = {"foobar"};
e.foo_set(buf);
assert(e.foo() == "foobar");
}
{ //// And a *char[] & const *char[]
// Use unique_ptr to automatically free buf
std::unique_ptr<char[]> buf(new char[7]);
std::memcpy(buf.get(), "foobar", 6);
buf[6] = '\0';
e.foo_set(buf.get());
const char* const_ptr = buf.get();
e.foo_set(const_ptr);
assert(e.foo() == "foobar");
}
cout << "Example's bar capacity: " << e.bar_capacity() << endl;
const std::size_t len = e.bar_capacity();
std::unique_ptr<char[]> buf(new char[len +1]);
// Copy bar in to buf
if (!e.bar(len, buf.get()))
throw std::runtime_error("Unable to get bar");
buf[len] = '\0'; // Null terminate the C string
cout << endl << "foo and bar (a.k.a.) have different values:" << endl;
cout << "Example's foo value: " << e.foo() << endl;
cout << "Example's bar value: " << buf.get() << endl;
// Set bar, which has a side effect of calling foo_set()
buf[0] = 'c'; buf[1] = buf[2] = '+'; buf[3] = '\0';
if (!e.bar_set(sizeof("c++") - 1, buf.get()))
throw std::runtime_error("Unable to set bar");
cout << endl << "foo and bar now have identical values but only one lock was acquired when setting:" << endl;
cout << "Example's foo value: " << e.foo() << endl;
cout << "Example's bar value: " << buf.get() << endl;
} catch (...) {
return EX_SOFTWARE;
}
return EX_OK;
}
构建使用C++11和libc++的说明：
clang++ -O4 -std=c++11 -stdlib=libc++ -I/path/to/boost/include -o example.cpp.o -c example.cpp
clang++ -O4 -std=c++11 -stdlib=libc++ -I/path/to/boost/include -o example_main.cpp.o -c example_main.cpp
clang++ -O4 -stdlib=libc++ -o example example.cpp.o example_main.cpp.o /path/to/boost/lib/libboost_exception-mt.dylib /path/to/boost/lib/libboost_system-mt.dylib /path/to/boost/lib/libboost_thread-mt.dylib
作为一个小小的奖励，我更新了这个例子，使用foo_set()方法中的右值引用包含完美转发。虽然不完美，但是花费的时间比我预期的要长，以使模板实例化正确，这在链接时是一个问题。这还包括C基本类型的适当重载，包括：char*，const char*，char[N]和const char[N]。
答案 0 :(得分：1)
对于问题1，我想要做的一件事是使用SFINAE来限制传入的锁定类型LockType允许shared_lock_t或unique_lock_t。
即：
template <typename LockType>
typename std::enable_if<
std::is_same< LockType, shared_lock_t > || std::is_same< LockType, unique_lock_t >,
size_t
>::type
bar_capacity(LockType& lk) const;
...但确实有点冗长。
这意味着传入错误类型的Lock会给出“无匹配”错误。另一种方法是使两个不同的bar_capacity公开，shared_lock_t和unique_lock_t公开，私人bar_capacity认为它们采用模板LockType。
正如所写，任何具有.owns_lock()方法的类型都返回可转换为bool的类型，这是一个有效的参数......
答案 1 :(得分：1)
使用Pimpl习语，互斥体应该是实现的一部分。这将使您在锁定启动时掌握。
BTW，为什么在lock_guard足够的时候使用unique_lock？
我认为没有任何优势可以公开。
std :: unique_ptr应该与大多数现代编译器的指针一样高效。但未经过验证。
我会将const char [N] foo_set转发为
template<std::size_t N>
bool foo_set(const char (&new_val)[N]) { return foo_set(std::string(new_val, N)); }
但是喜欢
template<std::size_t N>
bool foo_set(const char (&new_val)[N]) { return foo_set(N, new_val); }
这样可以避免在头文件上创建字符串，并让实现执行任何需要的操作。