为什么编译器不让我向前声明类型定义?
假设这是不可能的,那么保持我的包含树小的最佳实践是什么?
为什么编译器不让我向前声明类型定义?
假设这是不可能的,那么保持我的包含树小的最佳实践是什么?
当前回答
正如Bill Kotsias所指出的,保持点的typedef细节为私有并向前声明的唯一合理方法是继承。不过,使用c++ 11可以做得更好一些。考虑一下:
// LibraryPublicHeader.h
class Implementation;
class Library
{
...
private:
Implementation* impl;
};
// LibraryPrivateImplementation.cpp
// This annoyingly does not work:
//
// typedef std::shared_ptr<Foo> Implementation;
// However this does, and is almost as good.
class Implementation : public std::shared_ptr<Foo>
{
public:
// C++11 allows us to easily copy all the constructors.
using shared_ptr::shared_ptr;
};
其他回答
在c++(而不是普通C)中,对一个类型定义两次是完全合法的,只要两个定义完全相同:
// foo.h
struct A{};
typedef A *PA;
// bar.h
struct A; // forward declare A
typedef A *PA;
void func(PA x);
// baz.cc
#include "bar.h"
#include "foo.h"
// We've now included the definition for PA twice, but it's ok since they're the same
...
A x;
func(&x);
另一个解决方案是将forward声明和typedefs放在一个单独的头文件中,并包括:
// ForwardDeclarations.h
#pragma once
namespace Foo
{
struct Bar;
typedef Bar Baz;
}
// SomeFile.h
#include "ForwardDeclarations.h"
Foo::Baz baz;
当然,这实际上并没有减少要包含的文件数量,编译器仍然必须从磁盘读取这个文件,但至少内容比完整的定义更简单。您可以在同一个文件中添加更多的前向声明,并将其包含在相关位置。
正如Bill Kotsias所指出的,保持点的typedef细节为私有并向前声明的唯一合理方法是继承。不过,使用c++ 11可以做得更好一些。考虑一下:
// LibraryPublicHeader.h
class Implementation;
class Library
{
...
private:
Implementation* impl;
};
// LibraryPrivateImplementation.cpp
// This annoyingly does not work:
//
// typedef std::shared_ptr<Foo> Implementation;
// However this does, and is almost as good.
class Implementation : public std::shared_ptr<Foo>
{
public:
// C++11 allows us to easily copy all the constructors.
using shared_ptr::shared_ptr;
};
你可以使用正向类型定义。但是要做
typedef A B;
你必须先向前申报A:
class A;
typedef A B;
像@BillKotsias一样,我使用了继承,而且它对我很有效。
我改变了这个混乱(这需要所有的boost头在我的声明*.h)
#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics.hpp>
#include <boost/accumulators/statistics/stats.hpp>
#include <boost/accumulators/statistics/mean.hpp>
#include <boost/accumulators/statistics/moment.hpp>
#include <boost/accumulators/statistics/min.hpp>
#include <boost/accumulators/statistics/max.hpp>
typedef boost::accumulators::accumulator_set<float,
boost::accumulators::features<
boost::accumulators::tag::median,
boost::accumulators::tag::mean,
boost::accumulators::tag::min,
boost::accumulators::tag::max
>> VanillaAccumulator_t ;
std::unique_ptr<VanillaAccumulator_t> acc;
在这个声明中(*.h)
class VanillaAccumulator;
std::unique_ptr<VanillaAccumulator> acc;
实现(*.cpp)是
#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics.hpp>
#include <boost/accumulators/statistics/stats.hpp>
#include <boost/accumulators/statistics/mean.hpp>
#include <boost/accumulators/statistics/moment.hpp>
#include <boost/accumulators/statistics/min.hpp>
#include <boost/accumulators/statistics/max.hpp>
class VanillaAccumulator : public
boost::accumulators::accumulator_set<float,
boost::accumulators::features<
boost::accumulators::tag::median,
boost::accumulators::tag::mean,
boost::accumulators::tag::min,
boost::accumulators::tag::max
>>
{
};