C++ - 不完整的类声明(incomplete class declarations ) 问题 和 解答

不完整的类声明(incomplete class declarations ) 问题 和 解答

 
两个类放入一个文件时, 如果包含内联函数, 及友元(friend)类, 则函数放置的顺序尤为重要, 类的前置声明(class forward declaration)也需要注意.
否则会产生编译错误: 不完整的类声明(incomplete class declarations)
错误: "error: return type 'class StrBlobPtr' is incomplete"
错误: "error: invalid use of incomplete type 'class StrBlobPtr'"
则需要修改类的成员函数的放置位置, 一般来说, 正确的写法为:class B;
class A {
friend class B;
......
};
class B {
......
};
... A::fcn {
......
}
... B::fcn {
......
}
即 声明在前, 实现在后, 如果是内联(inline)函数使用B类, 则有可能找不到B类的成员;
则会出现"不完整的类声明(incomplete class declarations);
如以下示例(C++ Primer, Eclipse CDT (GCC 4.8.1));/*
* CppPrimer.cpp
*
* Created on: 2013.11.6
* Author: Caroline
*/

/*eclipse cdt*/

#include <iostream>

#include <vector>
#include <string>
#include <initializer_list>

#include <memory>

#include <cstddef>
#include <stdexcept>

using namespace std;

class StrBlobPtr;

class StrBlob {
friend class StrBlobPtr;
public:
typedef std::vector<std::string>::size_type size_type;
StrBlob();
StrBlob(std::initializer_list<std::string> il);
size_type size() const{ return data->size(); }
bool empty() const { return data->empty(); }
void push_back(const std::string& t) { data->push_back(t); }
void pop_back();
std::string& front() ;
std::string& back() ;
StrBlobPtr begin();
StrBlobPtr end();
private:
std::shared_ptr<std::vector<std::string> > data;
void check(size_type i, const std::string &msg) const;
};

class StrBlobPtr {
public:
StrBlobPtr() : curr(0) {}
StrBlobPtr(StrBlob &a, std::size_t sz = 0) : wptr(a.data), curr(sz) {}
std::string& deref() const;
StrBlobPtr& incr();
private:
std::shared_ptr<std::vector<std::string> >
check(std::size_t, const std::string&) const;
std::weak_ptr<std::vector<std::string> > wptr;
std::size_t curr;
};

StrBlob::StrBlob():data(std::make_shared<std::vector<std::string>>()) {}
StrBlob::StrBlob(std::initializer_list<std::string> il):
data(std::make_shared< std::vector<std::string> >(il)) {}
void StrBlob::check(size_type i, const std::string &msg) const {
if(i >= data->size())
throw out_of_range(msg);
}
std::string& StrBlob::front () {
check(0, "front on empty StrBlob");
return data->front();
}
std::string& StrBlob::back () {
check(0, "back on empty StrBlob");
return data->back();
}
void StrBlob::pop_back () {
check(0, "pop_back on empty StrBlob");
data->pop_back();
}
StrBlobPtr StrBlob::begin() {
return StrBlobPtr(*this);
}
StrBlobPtr StrBlob::end() {
auto ret = StrBlobPtr(*this, data->size());
return ret;
}

std::shared_ptr<std::vector<std::string> >
StrBlobPtr::check(std::size_t i, const std::string& msg) const {
auto ret = wptr.lock();
if (!ret)
throw std::runtime_error("unbound StrBlobPtr");
if (i >= ret->size())
throw std::out_of_range(msg);
return ret;
}
std::string& StrBlobPtr::deref() const {
auto p = check(curr, "dereference past end");
return (*p)[curr];
}
StrBlobPtr& StrBlobPtr::incr() {
check(curr, "increment past end of StrBlobPtr");
++curr;
return *this;
}

int main (void) {

StrBlob b1;
//{}释放b2
{
StrBlob b2 = {"baby", "girl", "lady", "woman", "female"};
b1 = b2;
b2.push_back("pretty");
}
std::cout << "b1 size = " << b1.size() << std::endl;
//b2已经被释放了
std::cout << "b1.begin() = " << b1.begin().deref() << std::endl;
std::cout << "b1.begin().incr().deref() = "
<< b1.begin().incr().deref() << std::endl;

return 0;

}