错误:非静态引用成员，无法使用默认赋值运算符

Question

我正在修改一个现有的库("Webduino"，一个用于Arduino的web服务器)来使用另一个现有的库("WiFly"，一个wifi模块)，但是遇到了一个问题。每个库都可以独立工作。Webduino库期望通过SPI使用以太网硬件模块，而WiFi模块使用串行端口(UART)。我得到的错误是：

WiFlyClient.h: In member function 'WiFlyClient& WiFlyClient::operator=(const WiFlyClient&)':
WiFlyClient.h:14:
error: non-static reference member 'WiFlyDevice& WiFlyClient::_WiFly', can't use default assignment operator
WiFlyWebServer.h: In member function 'void WebServer::processConnection(char*, int*)':
WiFlyWebServer.h:492: note: synthesized method 'WiFlyClient& WiFlyClient::operator=(const WiFlyClient&)' first required here

下面是相关的代码片段。请注意，到目前为止，我只修改了WiFlyWebServer.h (Webduino)：

// WiFlyWebServer.h (Webduino)
...
WiFlyServer m_server; // formerly EthernetServer and
WiFlyClient m_client; // EthernetClient
...
void WebServer::processConnection(char *buff, int *bufflen){
  ...
  // line 492
  m_client = m_server.available();
  ...
}



// WiFlyClient.h
class WiFlyClient : public Client {
 public:
  WiFlyClient();
  ...
private:
  WiFlyDevice& _WiFly;
  ...
}



// WiFlyClient.cpp
#include "WiFly.h"
#include "WiFlyClient.h"

WiFlyClient::WiFlyClient() :
  _WiFly (WiFly) {    // sets _wiFly to WiFly, which is an extern for WiFlyDevice (WiFly.h)
  ...
}


// WiFly.h
#include "WiFlyDevice.h"
...
extern WiFlyDevice WiFly;
...



// WiFlyDevice.h
class WiFlyDevice {
  public:
    WiFlyDevice(SpiUartDevice& theUart);
...



// WiFlyDevice.cpp
WiFlyDevice::WiFlyDevice(SpiUartDevice& theUart) : SPIuart (theUart) {
  /*

    Note: Supplied UART should/need not have been initialised first.

   */
  ...
}

问题源于m_client = m_server.available();，如果我注释掉问题就会消失(但整个事情都依赖于这一行)。实际的问题似乎是_WiFly成员不能被初始化(覆盖？)当WiFiClient对象被赋值时，但我不明白为什么它不能在这里工作，因为它不需要修改就能工作。

(是的，我知道头文件中有实现，我不知道他们为什么要这样写，不要怪我！)

有什么见解吗？

Answer 1

WiFlyClient的WiFly成员使类不可赋值。原因是赋值不能用于更改引用引用的对象。例如：

int a = 1;
int b = 2;
int &ar = a;
int &br = b;
ar = br; // changes a's value to 2, does not change ar to reference b

由于您的所有WiFlyClient都引用相同的WiFlyDevice实例，因此您可以按照编译器的建议更改WiFlyClient，以使用静态成员：

// WiFlyClient.h
class WiFlyClient : public Client {
 public:
  WiFlyClient();
  ...
private:
  static WiFlyDevice& _WiFly;
  ...
};

然后，不在构造函数中初始化它，而是在定义它的源文件中初始化它。

WiFlyDevice & WiFlyClient::_WiFly = WiFly;

Answer 2

显然，问题在于WiFlyClient是不可分配的。考虑将其包含在std::unique_ptr (在C++03中，在std::auto_ptr中)中，以便至少可以分配该类型的项。

std::unique_ptr<WiFlyClient> m_client;

...

m_client = m_server.available();

...

// convert m_client.<...> to m_client-><...>

// change m_server.available() to return std::unique_ptr<WiFlyClient>

Answer 3

尝试覆盖operator=

WiFlyClient& operator= (const WiFlyClient & wi)
{
  /* make a copy here */ 
  return *this;
}