C++ <<运算符重载和Arduino模板

Question

我正在开发一个arduino项目,我使用以下模板使用<<运算符打印各种数据类型

template<class T>
inline Print &operator <<(Print &obj, T arg)
{ obj.print(arg); return obj; }

在尝试处理使用Arduinos P宏存储的char数组之前一直没有问题,后者将数据存储在flash而不是ram中

//params stored in flash using P() from webduino library
P(CT_PLAIN) = "text/plain\n";
server << CT_PLAIN;

这导致编译器错误

httpServer.h : : In function 'Print& operator<<(Print&, T) [with T = const prog_uchar*]':
httpServer.cpp : instantiated from here
httpServer.h : call of overloaded 'print(const prog_uchar*&)' is ambiguous

虽然以下编译

//params stored in flash using P() from webduino library
P(CT_PLAIN) = "text/plain\n";
server.printP(CT_PLAIN);

我试图创建一个<<运算符重载,但我不完全理解语法和方法,我已经研究了几个小时无济于事,并将高度感谢任何反馈.

 WebServer &operator <<(WebServer &server,const prog_uchar *str)
 { server.printP(str); }

 template<class T>
 inline Print &operator <<(Print &obj, T arg)
 { obj.print(arg); return obj; }

虽然我仍然得到相同的编译错误.

WebServer :: printP的声明是

 void printP(const prog_uchar *str);

任何反馈和帮助将受到高度赞赏!

完整的编译器错误:

 Compiling 'webapp' for 'Arduino Mega 2560 or Mega ADK'
 httpServer.h : : In function 'Print& operator<<(Print&, T) [with T = const prog_uchar*]':
 httpServer.cpp : instantiated from here
 httpServer.h : call of overloaded 'print(const prog_uchar*&)' is ambiguous
 Print.h : print(const String&) <near match>
 Print.h : print(const char*) <near match>
 Print.h : print(char) <near match>
 Print.h : print(unsigned char, int) <near match>
 Print.h : print(int, int) <near match>
 Print.h : print(unsigned int, int) <near match>
 Print.h : print(long int, int) <near match>
 Print.h : print(long unsigned int, int) <near match>
 Error compiling

另外还有WebServer :: printP的定义

 void WebServer::printP(const prog_uchar *str)
 {
 // copy data out of program memory into local storage, write out in
 // chunks of 32 bytes to avoid extra short TCP/IP packets
   uint8_t buffer[32];
   size_t bufferEnd = 0;

   while (buffer[bufferEnd++] = pgm_read_byte(str++))
   {
     if (bufferEnd == 32)
     {
       m_client.write(buffer, 32);
       bufferEnd = 0;
     }
   }

   // write out everything left but trailing NUL
   if (bufferEnd > 1)
     m_client.write(buffer, bufferEnd - 1);
 }

Answer 1

事情是在你的第一个例子中,你正在调用printP(const prog_uchar*)一个const prog_uchar[12]可以轻易转换为的类型的参数const prog_uchar*.

在使用时operator<<,您使用print()类型对象调用函数,const prog_uchar[12]但是没有print适合此类型的任何重载.因此,编译器会查找不同的print重载并采用最合适的重载.但是,在您的情况下,不仅有一个"最合适的"重载,而且其中有8个,因此最终会有详细的错误消息.

解决您的问题的方法是明确地将您要打印的内容转换为const char*:

P(CT_PLAIN) = "text/plain\n";
server << (const char*) CT_PLAIN;

另一种解决方案是提供print过载const prog_uchar*.