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  • Author Author: Jan Cumps
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C++ write your own stream class - part 1: output stream initial design (embedded friendly C++)

Jan Cumps

In C++ it's common to stream data. You write to a file using the << operator.

example: 

cout << "hello, world! << endl;

In this blog I'm making my own minimal output stream class. Just enough code to show that it works. This could become an object that allows you to write to a UART, SPI, CAN, ... . In this post, I called my class uartostream, although it doesn't do anything UART. Just to give it an embedded sounding name, and to show how this could look like in firmware.

I put embedded friendly in the title because this is a resource-friendly exercise. Can be used on the smallest controllers.

output stream class

You can stream c-type strings to this class, and std::string objects. 

class uartostream {
public:
    uartostream() {}    
    
    uartostream& operator << (const char* msg) {
      // write to UART
      return *this;
    }
    
    uartostream& operator << (const std::string& msg) {
      // write to UART
      return *this;
    }
};

Notice that the class doesn't do anything with them. In this first post I want to check if the code compiles. And I use a debugger to see if the strings arrive in their handlers. If I put a breakpoint in the handler methods, and the msg parameter contains the string we've streamed to it, the mechanism works.

using the class

In the example code, I'll stream a std::string and a classic c-style char string. 

#include <string>

class uartostream{ // see previous section for class definition
};

uartostream u;

int main() {
  u << std::string("5") << "5" ;
}

test

I put a breakpoint in both operator << members. The code should first halt in the one that takes a std::string reference. Then the one that takes the c string.

image

The screen capture above shows the state while streaming the first part. If you want to see what happens when the "5" is streamed: try it in your favourite C++ IDE.

Thank you for reading. To be continued.